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LIBRARY
Michigan State
University

 

 

 

This is to certify that the

thesis entitled

Synthesis and Hydrolysis Studies
of APS Modified Imogolite

presented by

Leighta Maureen Johnson

has been accepted towards fulfillment
of the requirements for

Mdegree in Chemistry

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Date—MIME.—

O—7639 MS U is an Affirmative Action/Equal Opportunity Institution

PLACE ll RETURN BOX to move this checkout from your mood.
TO AVOID FINES mun on or bdoro 6‘. duo.

DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MSU Is An Affirmative Action/Equal Opponunlty Institution

SYNTHESIS AND HYDROLYSIS STUDIES OF APS MODIFIED IMOGOLITE

BY

Leighta Maureen Johnson

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

Master of Science

Department of Chemistry

1988

00544—1

ABSTRACT
SYNTHESIS AND HYDROLYSIS STUDIES OF APS MODIFIED IMOGOLITE
by

Leighta Maureen Johnson

The silylation of the tubular aluminosilicate imogolite,
was undertaken in an attempt to synthesize a new phase
transfer catalyst. The Al-OH outer surface of imogolite may
be rendered more hydrophobic through treatment with the
organosilane gamma-aminopropyltriethoxysilane (APS). The
solubility properties of the tubular particles are modified
without altering their desirable characteristics, such as a
large surface area and the capability of accommodating small
ions in the inner cavity of the tube. Synthesis of APS
modified imogolite was accomplished, but the material was
found to be unstable toward hydrolysis. A time dependent
dialysis experiment was conducted to compare the rate of
hydrolysis of APS from the surface of imogolite with that of
the hydrolysis rate from gamma-alumina, an amorphous material
possessing planar surface Al-OH sites. The results showed
that APS modified imogolite was more stable toward hydrolysis
than APS modified gamma-alumina. Future work will utilize

surfactants for the surface modification of imogolite.

To my family and Phil

ii

ACKNOWLEDGMENTS

I am deeply grateful for the guidance and support of Dr.
T.J. Pinnavaia in the pursuance of this work. Through his
insightful direction, he shaped the results that were
obtained and helped me to follow the path that became the
focus of this thesis. May he continue to broaden my
perspective in science throughout work on my dissertation.
In addition to his contagious optimism and invaluable
insight, the financial support which he was able to provide
was greatly appreciated.

My fiance, Philip Lyman, who has endured untold
hardships as a result of my decision to undertake this
endeavor, has nonetheless assisted me unfailingly in the
technical aspects of developing this thesis into its final
form. In addition to his technical assistance, I would like
to express my sincere gratitude for his support and undying
faith in me.

Tribute must also be given to Dr. Ahmad Moini. Through
his efforts, many potential crises were thwarted and many
seemingly unsolvable problems were surmounted. I would like
to express my thanks for those efforts and for the many words
of wisdom that enabled me to successfully "deal with" the

situations at hand.

iii

I would also like to thank my friends and family for
their patience and encouragement. The Chemistry department
staff, especially Kermit Johnson for his assistance with the
NMR and the library personnel, are greatly appreciated for

their efforts.

iv

LIST OF
LIST OF

Chapter

Chapter
A.
B.
C.
D.
E.
F.
G.
H.

Chapter

E.

F.

Table of Contents

TABLES
FIGURES
1 Introduction

Research Objectives

 

Imogolite

Coupling Agents

 

2 Experimental Methods

Imogolite Synthesis

 

APS-imogolite Synthesis

 

APS-alumina Synthesis

 

Sodium Lauryl Sulfate-imogolite Synthesis

Fourier Transform Infrared Spectroscopy
MAS 2951 NMR
Time Dependent Dialysis Experiment

Elemental Analysis

 

3 Results and Discussion

Imogolite Synthesis

 

Characterization of Imogolite
APS-imogolite Synthesis
Characterization of APS-imogolite

Time Dependent Dialysis Experiment

 

Conclusions

 

Page
vii

viii

14

22
23
23
24
24
25
25

26

27
28
31
33
52

64

Chapter 4 Future Studies

Imogolite Synthesis

 

APS-imogolite Synthesis

 

SLS-imogolite Synthesis and Catalysis

Analytical Techniques

 

 

vi

65

66

66

67

LIST OF TABLES

Iéhls Page
1 IR active vibrations and their frequencies 36

2 Si-O-Si stretching frequency vs. APS concentration 45

vii

Figure

10

LIST OF FIGURES

Structure of Imogolite (a) Mode in which the

orthosilicate group is attached to the face of
a gibbsite sheet. (b) Cross sectional view of
the imogolite structure.

 

Sequence of events leading to a coupling
reaction.

 

IR spectra of a) Dialyzed imogolite.
b) Non-dialyzed imogolite.

 

IR spectra of a) Air-dried APS. b) Air-dried
imogolite.

 

IR spectra of APS-imogolite samples containing
the following APS to A1 mole ratios:
a) 0.5 to 1.0 b) 1.0 to 1.0.

 

IR spectra of APS-imogolite samples containing
the following APS to Al mole ratios:
a) 2.0 to 1.0 b) 2.5 to 1.0.

 

IR spectra resulting from the subtraction of a
pure imogolite spectrum from the spectra in

figure 5. APS to Al mole ratios a) 0.5 to 1.0
b) 1.0 to 1.0.

 

IR spectra resulting from the subtraction of a
pure imogolite spectrum from the spectra in

figure 6. APS to Al mole ratios a) 2.0 to 1.0
b) 2.5 to 1.0.

2951 MAS NMR of a) Air-dried dialyzed imogolite
and b) Air-dried APS, with chemical shifts
reported with respect to TMS.

 

 

Structural models illustrating the molecular
bonding at the silane/silica3interface.
a) Q site b) Q site c) Q site.

 

viii

16

30

35

41

42

43

44

48

49

11

12

13

14

15

16

17

29Si MAS NMR of air-dried APS-imogolite with
mole ratio 0.5APS to 1.0A1.

 

IR Spectra of a) Gamma-alumina b) air-dried
imogolite.

 

IR spectra of initial Time Dependent Dialysis
Experiment samples, 1.0 to 1.0 mole ratio APS
to A1, a) APS-alumina b) APS-imogolite.

IR spectra of Time Dependent Dialysis
Experiment samples after twelve hours,
a) APS-alumina b) APS-imogolite.

 

IR spectra of Time Dependent Dialysis
Experiment samples after fifty hours,
a) APS-alumina b) APS-Imogolite.

 

Plot of percent APS versus dialysis time.

Plot of the log(%APS) versus dialysis time.

ix

53

55

56

58

59

61

Chapter 1
INTRODUCTION
A. Research Objectives

The family of aluminosilicate minerals is comprised of a
wide variety of structurally dissimilar materials. A three
dimensional porous network system is adopted in zeolites.
Clay minerals generally take the form of layered compounds
containing accessible space between the layers. Halloysite
is composed of a single sheet which has been wound into a
roll. Hollow spherical particles constitute the structure of
allophane. Imogolite, as a member of this diverse group also
possesses its own fundamental features. Its structure is
that of a hollow tube.

The outer surface of imogolite consists of an aluminum
hydroxide layer and silanols are exposed on the inner
surface. These unique structural characteristics give rise
to the particular behavior exhibited by imogolite in
solution. The multitude of surface hydroxyls provide a
wealth of potential reactive sites. These hydroxyls also
give imogolite a small but significant cation exchange
capacity (CEC). Because of the nature of the CEC, its
magnitude varies with the type of cation and the pH of the
solution. The tubular structure is also responsible for

imogolite’s unusually high surface area.

1

The combination of its shape, CEC, accessible cavity
within the tube, high surface area and water solubility
characteristics make imogolite an attractive candidate for
applications in catalysis and other areas of materials
chemistry. The purpose of the present investigation is both
to improve the reliability of the synthesis of imogolite and
to modify the surface of imogolite. Surface modification of
these inorganic tubes may prove to be useful in various
catalytic applications. Toward this end, the first goal is
to find a way to render the surface hydrophobic. This was to
be accomplished by attaching long chain hydrocarbon moieties
to the outside surface of the tubes through the use of
organosilane coupling agents. In this way, an inorganic
compound could be made to behave very much like an organic
compound. The imogolite would subsequently be extractable by
organic solvents.

If the functionalized imogolite retains a significant
affinity for water on its inner surfaces, a phase transfer
catalyst with very unique activity in an organic-aqueous
system would be expected. The long chain organic
functionalities will be too large to fit into the cavity and
thus interact with the silanols on the inside of the tubes.
Therefore the inner surface of the tube can be used to adsorb
small organic or inorganic cations, making the imogolite a
sort of "ion shuttle" between the organic and inorganic

phases of a phase transfer system.

Imogolite also has an extensive surface area on which a
reaction may occur. This leads to another possible
application for the successfully surface modified tubes.
Chelating agents coupled to reactive silane functionalities
are readily available. Affixing compounds of this nature to
the imogolite surface would allow metal cations to be
complexed by the Chelating functionality. A system complexed
with a specific metal may be useful catalytically, or the
system may be used to concentrate metal ions from solutions
for subsequent quantitative analysis of their composition.

In order to synthesize functionalized imogolite, a
reproducible synthesis and suitable purification techniques
must be found. To accomplish this, concentration and pH
limits were explored, and purification techniques were
developed. Thus, a method for reliably obtaining well
dispersed, well-formed imogolite tubes was documented.

Surface modification efforts centered around the
coupling agent gamma-aminopropyltriethoxysilane (APS). APS
was chosen because of the large quantity of information
available documenting its reactivity and application to a
wide variety of surfaces. It is also readily hydrolyzable
and well behaved in solution. Developing an understanding of
this system would provide the groundwork necessary for the
pursuit of more poorly understood coupling agents that might
impart the desired characteristics to imogolite for use in

the aforementioned applications.

Preliminary experiments to achieve coupling of APS to
imogolite proved inconclusive. Through further study, it was
determined that under the silylation conditions utilized,
bonding is achieved between APS and imogolite. It was also
found that the Si-O-Al bond is hydrolytically unstable. In
fact, all of the coupling agent can be removed from imogolite
by dialyzing the solution for two to three days. Although
this discovery was a temporary setback to the goal of surface
modification, it also revealed a way in which the relative
stabilities of the Si-O-Al bond could be compared in
different systems. A time dependent dialysis experiment was
conducted to compare the stability of APS on imogolite with
that of APS on alumina. The relative stabilities of the two
systems were easily determined from the data collected in
this experiment.

Further efforts continue to focus on the modification
of the imogolite surface in a less reversible manner.
Preliminary success has resulted from the treatment of
imogolite with the surfactant sodium lauryl sulfate. Future
studies will be directed toward understanding and controlling
the interaction between imogolite and sodium lauryl sulfate

and the behavior of this new system.

B. Imogolite
The empirical formula for the tubular aluminosilicate

imogolite is (HO)3A1203SiOH. The formula is written so that

it corresponds to the ordering of atoms along a path from the
outside to the inside of the tube. The inner wall of the
tube is made up of orthosilicate units which have been
condensed to one side of a single gibbsite sheet which
defines the outer wall. (See figure 1).1 The strain
introduced by the coupling of tetrahedrally coordinated
silicate units to the octahedrally coordinated aluminum
hydroxide sheet gives rise to curvature, allowing the edges
to be conjoined, resulting in tube formation. Evidence for
the strain is provided by the shortening of the O-O distance
around the vacant site in the gibbsite sheet to which the
SiO4 tetrahedron is condensed. This shortening of oxygen
contacts reduces the repeat distance along the tube to 8.4A
compared with the gibbsite repeat distance of 8.6A. Electron
diffraction patterns of non-oriented tube samples give
reflections at 1.4, 2.1, 2.3 (broad), 3.3 (broad), 3.7, 4.1,
5.7 (broad), 7.8 (broad), 11.8 (broad) and 21-23A.2 The
11.8, 7.8 and 5.7A reflections indicate the 8.4A repeat unit
along the tube unit axis. The 4.1, 3.7, 3.3, 3.1 and 2.3A
reflections arise from the lateral arrangement of the tube
unit with interaxial separations of 21 to 23A.3 Other
supporting evidence for this structure include transmission
electron microscopy, which shows the tubular morphology,
infrared spectroscopy, which indicates the presence of

1 1

orthosilicate units (peaks at 930cm- and 995cm- ), x-ray

diffraction patterns, which corroborate the electron

Ibl

Gfibbsito 2
imogolite 217/0

e- ."1

 

 

r 1r
: 02 I (0102’!
0 C
"3’
I I
e g e . e g e
I..- .. . -_..I .JL
0 ' SI
00 AI
00 O
.0 on

 

Figure 1 Structure of Imogolite (a) Mode in which the
orthosilicate group is attached to the face of a gibbsite

sheet.

(b) Cross sectional view of the imogolite structure.

diffraction patterns, and thermal analysis, which shows the
loss of water associated with the tubes.

Natural imogolite can be found as a component of
weathered volcanic ash and pumice beds. It was first
identified in soil samples of this type by Yoshinaga and
Aomine4 in 1962. Structural determination was slow because
of difficulties encountered in the separation of imogolite
from the spherically shaped, amorphous aluminosilicate
allophane, with which it is found. An unequivocal structure
of imogolite was presented in 1972 by Cradwick3 et a1. using
electron and x-ray diffraction data. In addition, a chemical
procedure for differentiating between silicate anions of
different degrees of polymerization was applied to imogolite.
This procedure involves converting the silicate anion present
in the sample to its corresponding trimethylsilyl ether and

5’6 For

identifying the ether or ethers by gas chromatography.
imogolite, it was found that 95% of the silicate anions are
derived from orthosilicate units and only 5% are from
pyrosilicate units. The ratio of ortho to pyrosilicate
obtained for imogolite was higher than that obtained for
other orthosilicates by this method. These results required
that the structure of imogolite contain only orthosilicate
units. The chain structure proposed by Wada and Yoshinaga2
in 1969 was no longer acceptable because of its 2.80A repeat
distance and the fact that it contained linked SiO

4
tetrahedra.

Once the structure of imogolite was known, a synthetic
route to its synthesis could be sought. The preparation of

7 et al. in 1977. Imogolite

imogolite was described by Farmer
was synthesized through the growth of a proto-imogolite
structure, made by the reaction of orthosilicic acid and
hydroxyaluminium ions in aqueous solution. Once the product

imogolite begins to form, a decrease in pH is observed. This

is believed to be due to the release of H+ during imogolite

formation according to the following scheme:8

Si St
on I
ass/'2 \. /' x a"\ 49
A1 + , Al 4 A1 A . a
é \on2 uzo’ ‘ \ If 1‘

When the proper conditions have not been met, nucleation and
growth is inhibited and the pH of the solution either stays
the same or increases slightly.

A comparison of synthetic and natural imogolite reveals
some fundamental differences. Natural imogolite has an outer
diameter of 20A, whereas the outer diameter of synthetic
imogolite is 23A. The internal diameter is 9-11A. This
requires that natural imogolite form tubes with 12 gibbsite
unit cells and synthetic imogolite form tubes with 14 units.
The lengths of the tubes vary from hundreds of angstroms to
several microns in both natural and synthetic materialsg.
Natural imogolite gives several higher order reflections that

are unobtainable in electron diffraction patterns of the'

synthetic material. This is due to the fact that naturally
formed tubes tend toward more regular packing in the solid
state than the synthetic imogolite, which arrange themselves
in a more disordered fashion.

Natural and synthetic imogolite, being formed in
aqueous solution, have very distinct colloid behavior
depending upon the pH and imogolite concentration. At very
low concentrations, 30mM Al or less, in acidic aqueous
solution imogolite is completely soluble and forms a crystal
clear solution. At a pH of about 7, aggregation of the tubes
begins, turning the solution slightly cloudy. As the pH
continues to increase, gelation begins. This behavior can be
attributed to the change in surface charge on the imogolite
tubes with changing pH. Below pH 5.5 the surface aluminum
hydroxides are all protonated, according to a study by Inoue

1° The study was based on the premise that the

and Wada.
titration of imogolite gave a near neutral reaction at the
equivalence point. The like charged tubes remain separated
by their electrostatic repulsions. Above pH 8 the surface
aluminum hydroxides are completely deprotonated and the inner
silicon hydroxide sites begin to deprotonate. In the absence
of electrostatic repulsions, the tubes are able to interact
and therefore flocculation begins. Aggregation of the tubes
begins to occur at pH 7, so the aggregation process itself

can be expected to interfere with the ability to determine

the state of ionization on the surface at these pH values.

10

Electrophoretic mobility measurements on imogolite were

11 These measurements

measured in 0.001M NaCl by Horikawa.
determine that the pH at which the surface of imogolite
becomes neutral is about 9. It is best to view the results
involving basic solutions as guidelines to flocculation
behavior, rather than absolute descriptions of the imogolite
surface charge. The other factor influencing the degree of
aggregation is imogolite concentration. Imogolite
suspensions corresponding to aluminum concentrations greater
than 30mM, will begin to flocculate, even in acidic solution.
As more and more water is removed from the solution, a gel
begins to form. Aggregation of imogolite particles by either
means (by pH or by concentration variation) is never
completely reversible. Aggregated tubes may be partially
redispersed by sonification in acidic aqueous solution. The
most effective redispersion is achieved when freeze dried
imogolite is used. Even then, a cloudy mixture is obtained.
In addition to the unique solution and flocculation
behavior of imogolite, its tubular nature lends itself to
several distinct physical properties. One of these is
microporosity. Microporosity was first invoked to explain
the discrepancy in density measurements between two methods

used by Wade and Yoshinaga.2

One set of density measurements
on imogolite was collected with a pycnometer using water as
the displacement liquid. A secOnd data set was obtained

using a float-sink test involving incremental addition of

11

acetylene tetrabromide (CZH Br4) to a sample. The results

2
from the liquid displacement method gave values of 2.53-2.70
g/cm3. Those from the float-sink method gave consistently
lower values of 1.70-1.97g/cm3. This discrepancy cannot be
explained simply by the fact that water may be adsorbing onto
the imogolite surface in the liquid displacement method. The
different results are obtained because water molecules can
penetrate into the small micropores in imogolite, but the
larger CszBr4 molecules cannot. The porosity in imogolite
is due to both the spaces inside the tubes and to those
formed between the tubes in their solid state packing
arrangement. The porosity was estimated at 55% of the total
volume of air-dried imogolite. As might be expected from its
porous nature, imogolite has been found to have a very large

2 These values were

surface area, from 1100-1200m2/g.1
obtained using the EGME (ethylene glycol monoethylether)
method, the lower value being a sample dried over P205 in a
vacuum, the higher value being an air dried sample. It is
believed that drying under vacuum may cause the tubes to
collapse, reducing their surface area. The properties of the
imogolite surface are determined in part by the presence of
hydroxyl groups. The availability of these surface hydroxyls
gives rise to surface acidity. This acidity has been
measured by observing the color change of Hammett indicators

13

of known pKa adsorbed on the surface. Each indicator pKa

is matched with the weight percent of sulfuric acid necessary

12

to establish the pH at its equivalence point for reference.
The acidity of imogolite was measured for samples
equilibrated in atmospheres at different relative humidities.
It was found that under very dry conditions, <5% relative
humidity, imogolite is very acidic, corresponding to the

acidity of 71% H2804. Below 20% relative humidity its

acidity is reduced to that of 2X10-2% HZSO Above 20%

4.
relative humidity the acidity is reduced to an equivalent

HZSO4 concentration of 8X10-8%. This latter value is

comparable to that of gibbsite at any moisture level. A

representation of the structure of the acid sites under

changes in relative humidity is as follows:13

./9” '°\.#” 9
m:s‘ \°\T Q + ”:0 O/SK "co/H20
or/ I.“ " ‘—_—H,o o'H’otfmo
Al/ Al’
(0') (6')
-H,o I —H’I In?
G
I-D\ OH Ho\ /0
/5'< ’s'\o I0 on
o o>Al' ’0 0H: |'<I
‘IH/OIIC’) I w
AI

In the presence of water, weak acidity is explained by proton
donation from SiOH and A10H2+ groups, such as are shown from
species b' to species c’. In the absence of water the acid

strength increases as the above scheme is followed in reverse

from b' to a' and then from a' to d'. The same explanation

13

applies for the decrease in pH as the material is heated.
Species a’ represents a strong Bronsted acid site and species
d’ represents a strong Lewis acid site.

As a result of the surface charge, there is a CEO and an
ABC associated with imogolite. The CEC was determined to be
30meq/100913 for an oven-dried sample by determining Na
retention from 0.05N NaCH3COO at pH 7. The ABC, measured
from adsorption of Cl from 0.05M NaCl, was found to be
15meq/100g at pH 7. The CEO and ABC values are both highly
pH dependent. They also depend on the particular cation or

anion being measured. The ABC values increase in the order

_ .. _ 2..
NO3 <C1 <CH3COO <<SO4 .

Na+<K+<Mg2+<Ba2+. This trend is due to the net charge and

The CEO values increase as

the size of the hydration sphere of each ion. When imogolite
is titrated with n-butylamine, the amounts of n-butylamine
absorbed are nearly equal to the CEC. This suggests that the
acid sites may also be the cation-exchange sites. This
conclusion would be erroneous if the n-butylamine were able
to adsorb onto the anion exchange sites as well as the acid
sites. The CEC of imogolite is found to increase 30-50% upon
heating to 105°C. It may be recalled that the acidity of
imogolite increases in this manner also. The CEC also shows
a dependence on background salt concentration. The
exchangeable cations adsorbed on imogolite are readily

hydrolyzed during dialysis against deionized water.

14

The stability of imogolite can also be understood
through the examination of its structure. Upon heating,
imogolite first undergoes dehydration at temperatures between
50 and 150°C. At 250-40000 dehydroxylation occurs and above

900°C mullite or gamma-alumina can be detected in samples.14

The surface area and CEC decreases with dry grinding.15
Changes in the IR and alteration of morphology as seen in
transmission electron micrographs offer direct evidence of
the low stability of imogolite under dehydrated conditions.
Low levels of phosphate perturb nucleation, growth and
formation of imogolite by retarding the polymerization of

16

proto-imogolite during synthesis. Imogolite exhibits

excellent stability in acidic environments, but it begins to

dissolve above pH 12.15

C. Coupling Agents

Coupling agents are compounds used in the modification
of inorganic surfaces, typically oxides and glasses.17’18
These materials often have desirable bulk properties but
their surface properties may be undesirable. The use of
coupling agents can create a surface with hydrophobic
characteristics. For,example, a flat meniscus and the
complete transferability of liquids from a pipet can be
achieved by coating the glass surface with a silane coupling

agent. A coupling agent may also provide a suitable

interphase region between two components in a composite to

15

enable it to perform as a single bulk material. One such
application is in the use of glass fibers to reinforce
organic polymers. Coating the fibers with a coupling agent
increases the flexural properties of the material and
prevents fracture between the fibers.

Organofunctional silanes of the form RnSiX are

(4-D)
typically used in these types of applications. The R group
is alkyl in nature but it commonly contains different
functionalities. The x group is usually a halogen, alkoxy,
acyloxy or amine. This group undergoes hydrolysis early in
the coupling reaction. Hydrolysis occurs fastest when X is a
halogen. When the hydrolyzable group is an alkoxy group, the
rate of hydrolysis decreases as the number of carbons in the
alkoxy and the value of n increase. The alkoxy silanes are
more convenient to work with than the halogen forms, although
hydrolysis occurs more slowly, because they do not form acids
as hydrolysis products. Trialkoxy coupling agents are better
than the disubstituted or monosubstituted compounds because
they are able to provide more effective coverage of the
surface, making them better able to alter the characteristics
of inorganic surfaces.

Different silanes are applied as coupling agents in much
the same manner. The steps involved in the coupling reaction

are shown in figure 2.19

First, there is a prehydrolysis
step, taking up to one hour, to transform the hydrolyzable

groups to hydroxyl groups. Water for hydrolysis may be

16
HYDROLYSIS

 

R--Si(OR)3 + 3H O—)R-—-Si-(OH)3 «I» 312011

2

I .
CONDENSATION

OH OH OH
I I I
R-Si-(OHl3——) R-Sli-O-Si-O-S‘i-OH + H20
I
OH R R

H-BONDING T0 SUBSTRATE

 

R R R

HO-Si-O-S is O-Si OH

I I A
0'0 0.. .
H. \I i( It a "H
\ / /

O O O

'41 5'1 '1
// Sub/st/r.£Z//
////

COVALENT BOND FORMATION

 

:0

I I
HO'Si' O-Si‘ O-S i-OH

///////'///

Figure 2 Sequence of events leading to a coupling reaction.

~0—

17

derived from the atmosphere, substrate or generated in the
course of the reaction for some preparations. Next is the
condensation step in which the trisilanols begin the react to
form oligomeric siloxanes. These siloxane oligomers then
hydrogen bond to the hydroxyl groups present on the inorganic
surface whereupon they condense to form Si-O-M bonds. Here,
M is the metal or semimetal belonging to the inorganic oxide
of interest. The degree of coverage obtained can be
monolayer or multilayer depending upon the amount of silane
and the number of free hydroxyls present on the surface.
Because the extent of oligomerization is difficult to
control, surface coverages are often irreproducible.
Condensation generally takes only a matter of minutes at room
temperature even for multilayer coverages. The final step is
the drying or curing process. Air—drying is often
sufficient, but heating can also give desirable results.
Heating may, however, lead to increased crosslinking of the
alkyl siloxane on the surface weakening its interaction with
the substrate. The surface modifications actually obtained
will be a function of the substrate, actual method of
application, quantity and type of silane used. Several
different methods have been used to study the interactions at
the silane-surface interface including Fourier Transform and
Diffuse Reflectance Infrared Spectroscopy, Raman
Spectroscopy, X-ray Photoelectron Spectroscopy, Auger

Electron Spectroscopy and Secondary Ion Mass Spectrometry.20

 

18

One of the most extensively studied coupling agents is
gamma-aminopropyltriethoxysilane (APS). Although its
behavior is similar to other coupling agents, it is in many
ways unique. Hydrolysis, for example, is much faster than is
typical of triethoxysilanes. This is because the amino
functionality acts as a catalyst for the hydrolysis reaction.
Another anomaly exhibited by this alkoxysilane is its ability
to remain in monomeric form when acidified prior to
hydrolysis at low concentrations (5% by weight).21 When the
pH is not adjusted, a 0.15% concentration is necessary in
order to get a large number of monomeric silanetriols. In
neutral solutions, hydrolysis is slowed and micelles form.
The natural pH of this silane in solution is 10.0. The
stability of the silane in solution is also unique. No
precipitation is seen even in fairly concentrated solutions
after aging. The reason for this stability has been the
center of much discussion in the literature. All of the
researchers agree that the explanation must involve an
interaction centered around the aminofunctional group, but
there are several different ways in which the interaction has
been described. Plueddemann22 first suggested that the
silane may cyclize and form a five membered ring through
association between the silicon and the nitrogen atoms as
follows:

—0\ c-c

-O-Sl I
—o/ ‘N-C

19

He has also proposed the existence of an internal zwitterion

23 24

structure under acidic conditions. Boerio et al. have

studied the structure of APS, not in solution, but deposited
on polished iron mirrors. They believe that the five

.membered ring may exist for thick films several hundred

angstroms thick by virtue of an infrared band at 1575cm-1.

1

Thin films, however, gave a 1510cm- band which was assigned

to a symmetric NH + deformation of a cyclic internal

3
zwitterion structure. Further studies were performed,

centered around the analysis of spectra obtained under both
dry and moist dehydrating conditions. This led them to
propose a new structure24 consisting of a hydrolyzed oligomer

with amino groups hydrogen bonded to silanol groups:

OH

0

/CH2— ca \| /o‘--oII ---NH2\
cu -_ _. CH
2 l 0 Si 2

th--.H ... 0

2

0H
Also at that time, Chiang25 proposed a similar but monomeric

structure for the silane in solution:

—0 CH CH
\\ 2 2
/Si\/ \/CH2
—0 OH-n-NH,

This structure was similar to the one proposed by Moses26 et

al. for APS bonded to the surfaces of SnO2 electrodes. The

Chiang structure was suggested on the basis of changes in the

NH2 deformation stretch in the IR under various heat

treatments. Chiang25 proposes that the dimeric siloxane is

20

condensed to the surface of the silica, but rather than
associated with the silanols, the amino group is associated

with other surface hydroxyls as follows:

Plueddemann27

later agreed that the monomeric structure for
APS in solution was consistent with the chemical reactivity
of the silanols, but that a structure of that nature would

exist as a seven membered ring:

 

The latest structure for APS deposited on a surface was

28

proposed by Boerio et al. and supported by experimental

29 et al. The structure is that

evidence collected by Naviroj
of an amine bicarbonate salt which appears to be formed when
APS is deposited onto a substrate at natural pH and dried in

air. This hypothesis was supported by infrared spectra taken

21

of the samples dried in nitrogen, carbon dioxide and air.
Also in support of this, heating of the surface resulted in
detectable loss of C02.

In summary, it appears that in acidic solutions, the
unusual stability of APS arises from the fact that it takes
on a six or seven membered cyclic structure which involves
the internal hydrogen bonding of the amino group with one of
the silanols. This structure is also responsible for the
fact that APS exists chiefly in monomeric form at low
concentrations. On the surface of a substrate, APS condenses
as a siloxane dimer. Depending upon the drying conditions
and substrate, its structure may be that of an amine
bicarbonate salt, a cyclic structure with the amine

associated with the APS silanols, or one with the amine

associated with the surface hydroxyl groups.

Chapter 2

EXPERIMENTAL METHODS

A. Imogolite Synthesis

Imogolite was synthesized according to the method
described by Farmer and Fraser.8 Aluminum tri-sec butoxide
(Aldrich) was hydrolyzed in a mixture of 0.10M perchloric
acid and deionized water sufficient to give an initial
aluminum concentration of 33mM. The amount of perchloric
acid used was equal to half the number of moles of aluminum
being hydrolyzed. The hydrolysis required from 2 to 6 hours
to complete. At this point the solution changed from cloudy
to clear in appearance. Deionized water was then added to
bring the overall aluminum concentration to 10-30mM.
Tetraethylorthosilicate (Aldrich) was added to the hydrolyzed
aluminum solution in an amount equal to half the number of
moles of aluminum. The mixture was then adjusted to pH 5.0
with 1M sodium hydroxide and immediately acidified to pH 4.0
using 1M acetic acid. The concentration of acetic acid did
not exceed the concentration of silicon. The mixture was
heated for 2 to 3 days at a temperature of 90-100°C. The
final solution was water-clear after heating. Occasionally,
a cloudy appearance was observed due to the formation of
insoluble products such as gibbsite. These insoluble
products were not easily separated from the imogolite. An

aliquot removed from the clear reaction product gave a gel

22

23

upon addition of 1M ammonium hydroxide, which was indicative
of the presence of imogolite. Soluble by-products of the
reaction were easily removed by dialyzing the solution for
one week in cellulose dialysis tubing (Spectrapor-Spectrum
Medical Industries) molecular weight cutoff 12,000-14,000.
This treatment yielded suitably pure imogolite for use in

subsequent reactions.

B. APS-imogolite Synthesis
A 2% solution of gamma-aminopropyltriethoxysilane

(Petrarch Systems) in deionized water was hydrolyzed for half
an hour in the presence of enough 1M acetic acid to achieve a
pH of 3.5 to 3.7. The solution was-water clear. The silane
solution was added to a solution of imogolite and the mixture
was allowed to stir overnight. The amount of silane added
was equal to the aluminum concentration used in the synthesis
of the imogolite. Since some soluble by-products were always
formed in the imogolite synthesis, the silane was in excess

of the amount needed for monolayer coverage.

C. APS-alumina Synthesis
Gamma-aminopropyltriethoxysilane was hydrolyzed as
described above. A 0.3% suspension of gamma-alumina, surface

area 280m2/g, (Catapal B, Vista) was adjusted to pH 4.2 with

24

1M acetic acid. The two solutions were combined and allowed
to stir overnight. The amount of silane added was equal to
the amount of hydroxyls on the surface of the gamma alumina.
The surface hydroxyl group concentration was assumed to be

16.6 OH/lOOAz.

D. Sodium Lauryl Sulfate-imogolite Synthesis
A 0.20% solution of sodium lauryl sulfate (Fisher) in
deionized water (below the critical micelle concentration of

8.34 x 10’3

mole/l) was prepared and the pH was adjusted to
4.3 by the addition of 1M acetic acid (a few drops). This
was added to a solution of imogolite prepared as outlined
above. The number of moles of sodium lauryl sulfate added
equaled the number of moles of aluminum used in the original
imogolite synthesis. Since imogolite was not formed in 100%
yield, the amount of sodium lauryl sulfate was in excess of
that needed to form a monolayer. The solution was stirred
overnight and then dialyzed for one week to remove excess

sodium lauryl sulfate. The imogolite was then present as a

flocculated mass.

E. Fourier Transform Infrared Spectroscopy

The FTIR instrument used to obtain vibrational spectra
was an IBM model IR 408 with an IBM AT workstation equipped
with IR44 software. All IR specimens were KBr pellets

containing 2 weight percent of the air-dried samples. The

25

KBr (Mallinkrodt) was spectroscopic grade. The 13mm pellets
were made by pressing 0.049 of powder under 20,000psi for ten
seconds.
F. MAS 2951 NMR

The instrument used to obtain solid state NMR spectra
was a Bruker WH-180 equipped with Nicolet NTCFT-1180 computer
software and Doty solid state probe spinning at the magic
angle of 55°44'. Air-dried samples were packed into sapphire
rotors and spun at a rate of 3.5-4.2kHz. Chemical shifts
were obtained by setting a talc peak equal to the literature
value of -78.1ppm with respect to TMS. Pulse width (P2) is
set equal to 6.0 usec and relaxation delay (D5) is set equal

to 2.0 sec for each sample.

G. Time Dependent Dialysis Experiment

Aqueous suspensions of silylated imogolite, silylated
gamma—alumina and hydrolyzed gamma-aminopropyltriethoxysilane
were prepared as described above. Each mixture contained the
same amount of silane. Thirteen dialysis membranes were made
for each of the three systems studied, each membrane
containing 25mls of solution. The membranes were placed in
S-liter buckets of deionized water. A membrane was removed
every two hours for the first eighteen hours, and at longer
intervals thereafter. Each time a sample was removed, the

deionized water was replaced. The samples were removed from

26

the membranes and air-dried. Samples of the initial
solutions (25ml) were also air-dried. Infrared spectra were
obtained for each of the samples collected, except for the
pure silane. The weights of each of the air-dried silane

samples collected were determined by difference.

H. Elemental Analysis

Carbon and hydrogen analysis was done by Galbraith
Laboratories in Nashville, Tennessee. Silicon and aluminum
analysis was done at the inorganic chemistry laboratory of
the Department of Toxicology at Michigan State University.
The analysis was done on a Jarrell-Ash 955 Atom-Comp
instrument. J.T. Baker instra-analyzed grade standards were
used. NBS plastic clay 98a served as a clay reference.
Samples were prepared by mixing 0.0259 of sample with 0.1509
lithium borate (Aldrich, Gold Label) in a carbon crucible and
heating to 1000°C for 12 minutes. The resulting melt was
immediately transferred to a mixture of 20mls 6% hydrochloric
acid and 20mls 6% nitric acid and stirred for fifteen
minutes. This solution was diluted to 100mls and submitted

for analysis.

Chapter 3

RESULTS AND DISCUSSION

A. Imogolite Synthesis

Prior to this investigation, imogolite was regularly
synthesized by a method similar to the one reported in the
experimental section of this thesis, but the success rate was
extremely low.. Some somewhat subtle, but important changes
have since been made to render the current synthesis method
quite reliable. The most important of these changes was a
reduction of the concentration of Al in the final solution
from 60mM to 30mM. Because the synthesis of imogolite is the
result of a nucleation process, the overall concentration of
reactants in solution play an important role in determining
what products will be formed. Higher reactant concentrations
lead to the formation of insoluble products in addition to
the desired product imogolite, as evidenced by the cloudy
appearance of the reaction mixture after refluxing. As may
be expected, the concentrations of other ions in solution
also have a marked effect upon the imogolite nucleation
process. Perchlorate ion, from the perchloric acid used to
hydrolyze the aluminum tri-sec-butoxide will inhibit
imogolite formation if present in concentrations greater than
that of silicon. The presence of dissolved salts and

3

phosphate ion 0 have also been found to decrease the yield of

imogolite, as determined by the height of the gel formed when

27

28

NH OH is added to a volume of imogolite solution in a test

4
tube and centrifuged. Acetate ion, on the other hand, in
quantities no greater than that of the silicon concentration
seems to be necessary for the formation of imogolite under
laboratory conditions. The final determining factor for
successful imogolite synthesis is pH. Allowing the pH to
increase beyond 5.0 will promote the formation of insoluble
products. Successful imogolite synthesis results in a drop
in pH during the reflux step. Even under ideal conditions,
some soluble products are formed along with the imogolite.
These soluble products can easily be removed through dialysis
of the contents of the entire reaction vessel against
deionized water for approximately one week. These products
have a much lower molecular weight than the imogolite and
pass easily through the pores in the tubing. After dialysis,
what remains is pure imogolite in solution. The imogolite
remains stable over long periods, but after nine to twelve

months precipitation can be seen to occur even in dilute

samples.

B. Characterization of Imogolite
Once the synthetic procedure has been carried out, the
first step is to determine whether a portion of the product

gels upon addition of 1M NH4OH. Gelation is indicative of

 

29

the presence of imogolite. The next step is to dialyze the

29

solution. Further characterization includes MAS Si NMR,

electron microscopy and FTIR.

FTIR is the fastest and easiest method of
characterization. Representative spectra of both dialyzed
and non-dialyzed samples are shown in figure 3. Both spectra

have prominent water peaks at around 3500cm-1 due to the OH

I

stretch and 1630cm- from the HOH bending vibration.

Substantial differences appear in the lower wavenumber region
of the two spectra. The dialyzed sample contains the two

peaks characteristic of the Si-O-Al vibrations in imogolite,

1 1

one at 995cm- and the other at 940cm- . These peaks are

consistent with the presence of isolated orthosilicate units

in imogolite. That same region in the non-dialyzed sample

however, shows a peak at 943cm"1

1 and 1090cm-1. The small peak is due to imogolite,

but the absorbances at wavenumbers greater than 1000cm"l are

and two large peaks at

1143cm'

due to Si-O-Si vibrations. It is easy to identify these
peaks with impurities because there are no Si-O-Si bonds in
the imogolite structure. There is only one additional peak
that differs widely between the two spectra. This one is
sharp and falls at 630cm-1. This peak is attributed to the
C00. vibration of the acetic acid that is added during
synthesis. From this analysis it becomes clear that dialysis

is essential in order to obtain imogolite in absence of

soluble impurities. The FTIR spectrum of imogolite seems

30

 

TRANSMITTANCE

 

'" 'l' ... (”I "la-"I "' I“"’I”°-l-_]——'l" "T" "I I ' I" I
4000 3000 2000 1000

WAVEMERS 400 . 0

b)

 

0. 7023 I --..

0.650

0.500

TRANSH I T T ANCE

 

 

Figure 3 IR spectra of a) Dialyzed imogolite.
b) Non-dialyzed imogolite.

31

almost too simple to satisfactorily identify imogolite. But

in the region from 900-1000cm-1

, the ability to detect two
separate peaks is unique to imogolite. For example, in
allophane, another aluminosilicate found with imogolite in
nature, there is only one broad peak observable in that
region. In fact, the degree of definition achieved between
the two peaks is an indirect measure of how well-formed the
tubes are in the sample. Another peak, not shown in these
spectra, can be found at 348cm-1. This medium intensity band
is convenient for differentiating between allophane and
imogolite because it is absent in allophane. It is present
in other ordered aluminosilicates such as montmorillonite,
however.

Another useful means of imogolite characterization is
MAS 2gsi NMR. This is somewhat less convenient both because
large amounts of sample and large blocks of time on the
instrument are needed as a result of the low silicon content
of imogolite. The spectrum is a singlet at -79 ppm with

respect to TMS. The singlet is due to the isolated

orthosilicate units which correspond to a 00 site.

C. APS-imogolite Synthesis

Retaining imogolite in its non-agglomerated state in
solution during this reaction was desired. As has been
mentioned previously in this thesis, imogolite begins to

flocculate above pH 7, so acidic pH synthesis conditions were

32

necessary. APS exhibits a pH of 10.0 in aqueous solution so
acidification was necessary during the prehydrolysis step in
the coupling reaction. The acidic conditions proved to be
beneficial in two additional ways. Hydrolysis of the ethoxy
groups is accelerated in acidic environments and the
aminofunctional group is protonated. Since the silanol
groups have an isoelectric point at pH 2-3, it is likely that
a negative charge is associated with the silanol. The amino
functionality is expected to be positively charged in the pH
range 3-6.27 Therefore, at pH 4 the positive external charge
on the imogolite tubes will assist in preferentially
orienting the silane on its surface by repelling the
positively charged amino group and attracting the negatively
charged silanols. Acetic acid was chosen to adjust the pH
because of its use in imogolite synthesis. The tubes can
tolerate exposure to it without any adverse effects.

The quantity of coupling agent needed in the reaction
proved to be difficult to determine. As was indicated
previously, only indirect methods of determining imogolite
yield are possible. A useful indirect method involves the
measurement of the height of a gel formed by raising the pH
of a standard volume of solution. This method fails to allow
subsequent calculation of the amount of coupling agent needed
in the reaction. The procedure adopted to circumvent this
problem was to use the concentration of aluminum from the

imogolite synthesis for the calculation of the amount of

33

coupling agent needed in the silylation reaction. Because
the yield of imogolite is never 100%, this method results in
the use of excess coupling agent. There are 1.5 hydroxyls
available for each aluminum on the external surface of
imogolite and three silanols for every mole of APS. Thus for
an aluminum to APS ratio made equal to one, there is a
two-fold excess of coupling agent. This ratio should provide
sufficient APS for monolayer coverage where every surface
hydroxyl is condensed with a coupling agent silanol.

Once the reaction has been completed, a new problem
arises. How can unreacted silane, present as a result of the
addition of excess coupling agent, be separated from the
silylated imogolite? Filtration is not possible because the
tubes are too small. Dialysis was the obvious choice because
of its utility in the purification of imogolite from soluble
synthetic byproducts. After dialysis of the reaction mixture
for several days, no trace of coupling agent could be found
in the FTIR! This revelation lead to the subsequent
quantitative study of the hydrolysis reaction of the
silylated imogolite and in attempts at stabilizing the

product.

D. Characterization of APS-imogolite

silylated imogolite has been characterized by MAS 29Si

NMR, elemental analysis and FTIR.

34

The FTIR spectra of APS polymerized by air-drying on a
glass slide is shown in figure 4(a). Table 1 lists some
typical IR stretches and the frequencies at which the
vibrations occur. The peak at 3370cm-1 is due to the N-H

stretch, the one at 2932cm-1

1

is the CH2 stretch. The peak at
2, at 1484cm-1 due to NH3+, 13:35am"1 a

and 1028cm"1 are due to Si-O-Si stretches,

is due to NH
1

1580cm-

CH2 bend, 1131cm'
and 930cm"1 is attributed to Si-OH. The weak band at

2150cm”l has been assigned to an NH3+ combination band
vibration.

When an IR is taken of a sample consisting of APS
reacted with imogolite as described in the experimental
section of this thesis, several things can be noted. Figure
6(a) is a spectrum of a two to one ratio of APS to aluminum
in imogolite calculated in the usual way. There are two

peaks present just above 1000cm-1. One peak is at 1127cm-

and the other occurs at 1033cm-1. Identification of these

1

two peaks indicates the presence of a network of highly
polymerized silane covering the imogolite tubes. A monolayer
of individual silanols and dimers would have been represented
by a single peak near 1016cm-1.25 The polymerization of the
silane on the surface is controlled in part by the
conformation of the APS on the surface. Polymerization of
the silane on the surface is prohibited in the same way as it

is prevented in solution, that is, through hydrogen bonding

between the NH2 functionality on the silane and a hydrogen,

.35

 

 

 

 

 

 

 

 

 

 

a)
1.:22
:.:0
1
:.00-
w d
g -
4
E5 -
I% 0.90-
2
( _
I!
0.
c1
0'79°_ r 1 I I l I I I r I I I
4000 3000 2000 :000
uAvsuuuaens 400.0
b)
0.670 ‘—-
fi
d
0.60-
.I
..I
o.so-
w d
U -I
‘5
I: 0.40"
g a
Z a
E5 0.30-4
4
0.:ee-'
I 1 f I l I I I T I I
4000 3000 2000 1000
uavenuueens 400.0
Figure 4 IR spectra of a) Air dried APS. b) Air dried

imogolite.

36

Table 1. IR active vibrations and their frequencies

Functional_§reun 6 cm
N-H stretCh. O O O O O O O O O O O O O O O O 3370 to 3390

-1

CH2 stretch. . . . . . . . . . . . . . . . . 2928 to 2932

H003'. . . . . . . . . . . . . . . . . . . . 1630 and 1332

NH2 bend O O O O O O O O O O O O O O O O O O 1550 to 1600

NH3+ O O O O O O O O O O O O O C O O O O O O 1470 to 1492

Si-CH2 bend. . . . . . . . . . . . . . . . . 1410 to 1412
Si-O-Si stretch. . . . . . . . . . . . . . . 1000 to 1130
Si-O-Al. . . . . . . . . . . . . . . . . . . 990 and 940
81-03. . . . . . . . . . . . . . . . . . . . 930

C00. 0 O O O O O O O O O O O O O O O O O O O 63 o

37

either on the surface or on one of the silanols. The

1 1

presence of strong IR bands at 1480cm- and 1550cm- to

1

1575cm- (NH3+) signify the presence of such bonding,

although the exact nature of these bands is still the subject
of current debate in the literature. These assignments are
based upon studies of thin films of APS on polished iron

24 in dry atmosphere and of APS deposited on high

25

mirrors
surface area silica gel. The silane on the silica gel is
actually stabilized by hydrogen bonding to the anionic
surface hydroxyls characteristic of that surface. The silane
on the iron mirrors is believed to contain intramolecular

hydrogen bonds between NH and a silanol hydrogen. This

2
hydrogen bonding is extremely sensitive to exposure to

atmospheric moisture. It is believed that the slightly
acidic effect of water vapor in the presence of carbon
dioxide causes disruption of this hydrogen bonding. This is

indicated by the IR spectra of samples aged under atmospheric

conditions. The 1480cm-1 peak disappears and the 1550cmm1

1 1

peak increases to 1585cm- (which approaches the 1585cm-

peak observed for the free NH2 vibration in polymerized APS.)
Deposition of silane under basic conditions (pH 12) results

in a subsequent decrease in the intensity of the 1480cm-1
band after prolonged atmospheric exposure. In addition,
those films deposited at lower pH (9.1) or exposed to the
atmosphere for long periods of time showed a shift of the

1

Si-O-Si stretch to higher wavenumbers, from 1105cm- to

38

1

1130cm— , indicating that in the absence of hydrogen bonding

the APS films were more highly polymerized. Both of these
results were attributed to the weakening of the hydrogen

bonding between the NH and the silanol protons. Elsewhere,

2
aminosilanes have been shown to form an amine bicarbonate
20
2.
would also explain the higher degree of polymerization

salt when exposed to moisture and CO Salt formation

because the NH is no longer associated with the silanol.

2
Because the APS-imogolite system is adjusted to a pH of
4 through the addition of acetic acid, it is expected that
the protonated aminofunctionality will have little affinity
for the protonated imogolite surface hydroxyls. Under these
conditions, it is very likely that the amine remains fully
protonated after condensation of the silanols to the
imogolite surface. Silane deposition onto the iron mirrors
and the high surface area silica, to which the results
obtained here are being compared, is performed at pH 9-12.
The surface coverage obtained in these experiments is also
much lower than the surface coverage obtained in this study.
Both pH and degree of surface coverage severely effect the
conformation adopted by the silane on the surface. It is not
surprising, then, that there are both similarities and
differences in the infrared spectra obtained here and in

1 to 1400cm-1 region in

these other studies. In the 1600cm-
particular, the spectral bands obtained in all three cases

are similar in number and relative intensity. The difference

39

arises in the frequency at which the bands occur. A shift in

1

the 1580cm- band to lower wavenumbers is associated with the

presence of hydrogen bonding between the amine and surface
hydroxyl or silanol hydrogens. Weakening hydrogen bonding of
this type in the iron mirrors as described above, was

accompanied by a weakening or disappearance of the band at

1480cm-1. In the APS-imogolite system, the band at 1580cm-

1 and isvzcm'l. The 14806111"1

1 and 1414cm-1! The

1

1

is shifted between 1564cm- band,

however is shifted between 1410cm-

1

Si-O-Si region contains stretches at 1127cm- and 1033cm-

suggesting a large degree of polymerization on the surface.
Combination of the above data provides evidence for the
presence of an amine salt on the surface of imogolite under

air-dried conditions. Acetate is the probable anion for the

protonated amine. The presence of a sharp band at 650cm"1 is

analogous to the band detected in the non-dialyzed imogolite

sample at 630cm'1. The sharp band present in the IR of the

original polymerized APS spectra is also present in the

1 1

APS-imogolite sample at 680cm_ , so the 650cm- band is not

characteristic of pure APS. The large degree of

polymerization is due to the lack of protection of the

+
3

being associated with the acetate anion. The decrease in the

silanols by amine hydrogen bonding as a result of the NH

frequency of the NH + band may be due to the presence of

3
fully protonated amine species.

40

These observations collectively support the assumption
that there is a direct interaction between imogolite and APS.
But because these IR spectra are taken of samples dried from
the reaction vessel, it is desirable to show that there is an
interaction between the APS and imogolite, and to dispel the
possibility that the spectra obtained may be the same as if
the residue from the reaction were just a mixture of APS and
imogolite powders. This was done through the analysis of
spectra obtained by digitally subtracting the imogolite
spectrum from each of four spectra. The four spectra .
represent the patterns obtained from reaction mixtures of
four different mole ratios of APS to A1: 0.5 to 1.0, 1.0 to
1.0, figure 5, and 2.0 to 1.0, 2.5 to 1.0, figure 6,
calculated in the usual way. The resultant spectra after the
subtraction process are shown in figures 7 and 8.

Shifts in the placement of the bands can be seen in the

1 1

unprocessed spectra in the 1200cm- to l400cm- range by

noting that a band, initially a shoulder on the Si-O-Si peak,
becomes isolated and then becomes a shoulder on the 1410cm"1
peak as the concentration of APS increases. At low
concentrations of APS a higher value is obtained in the

1570cm-1 range. The region of interest in the subtracted

spectra is around 1140cm-1. This peak is the Si-O-Si stretch
arising from the self condensation of hydrolyzed APS units.
Recall that there are no Si-O-Si bonds present in the

imogolite structure. In each of the four resultant spectra

41

0)

 

0.637

THANSMITTANCE

 

 

 

 

4000 3000 2000 1000

b)

 

0.6243 :
man-

u
‘

ojmr:

0.500"

-

 

 

-
d
‘

0.450“

 

THANSMITTANCE
1 111

 

0.400"

 

 

1111

0.3394

 

 

Figure 5 IR spectra of APS-imogolite samples containing the
following APS to A1 mole ratios: a) 0.5 to 1.0 b) 1.0 to
1.0.

42

0)

 

0.611

1

 

 

tmmxnmce
P
L
O
I

 

 

 

 

 

 

4000 3000 2000 £000
IAVENUNDERS 400.0

 

 

 

TRANSMITTANCE

 

 

 

 

 

 

 

Figure 6 IR spectra of APS-imogolite samples containing the
following APS to Al mole ratios: a) 2.0 to 1.0 b) 2.5 to
10°.

43

 

 

 

 

 

TBANSMITTANCE
P
N
S
l

 

 

 

 

 

4000 3000 2000 1000
NAVENUNBERS 400.0

 

 

 

TRANSMITTANCE
9
0|
3

 

 

0.5500

 

 

4000 3000 2000 1000
WAVENUMBERS 400.0

Figure 7 IR spectra resulting from the subtraction of a pure
imogolite spectrum from the spectra in figure 5. APS to Al
mole ratios a) 0.5 to 1.0 b) 1.0 to 1.0.

44

 

 

 

TRANSMITTANCE

 

 

 

 

 

 

 

TRANSMITTANCE
1

 

 

 

4000 3000 2000 1000
WAVENUHDERS - 400.0

Figure 8 IR spectra resulting from the subtraction of a pure
imogolite spectrum from the spectra in figure 6. APS to Al
mole ratios a) 2.0 to 1.0 b) 2.5 to 1.0.

45

this Si-O-Si band is shifted to lower wavenumber with
increasing concentration of APS. Table 2 gives a list of
concentration ratios versus the wavenumber observed for the

Si—O-Si stretch. The value for pure polymerized APS is also

1 1

included. The 12cm- shift from 1131cm- in the pure

1

polysilane to 1143cm— in the 0.5:l.0 mole ratio APS to Al

sample is expected to be significant on a FTIR with a

resolution of 2.00cm-1. A measure of the consistence

achieved on the instrument for the CH stretch at 2929cm-1 to

2

Table 2. Si-O-Si stretching frequency vs. APS concentration

M0LE RATIO FREQUENCY (cm-1) FREQUENCY (cm-1)
AR§LAL Si:9:§i_§treteh QHZ_§tretch

pure APS 1131.4 2932

2.5 to 1.0 1137.2 2929

2.0 to 1.0 1137.0 2932

1.0 to 1.0 1141.0 2932

0.5 to 1.0 1143.0 2929

1 is included in the table. The shift in the Si-O-Si

29326m'
peak to lower wavenumber with increasing concentration of APS
requires explanation. It is believed that the shift in the
Si-O-Si stretch is being caused by the increasing amount of
Si-O-Al character nearby as a result of the partial
condensation of the polymerized APS to the Al-OH sites on the

surface of imogolite. At high concentrations of APS, the

bulk Si-O-Si signal will show essentially polysilane

46

character because all of the imogolite surface sites will be
occupied by silane and the remaining silane will be forced to
polymerize with the APS already on the surface. The APS far
from the surface of the imogolite will be essentially
identical to the pure polysilane. The Si-O-Si band due to
APS close to and bonded to the imogolite surface, however,
will be strongly affected by the presence of the nearby
Si-O-Al bonds and, therefore, at lower concentrations of
silane a larger shift in the band resulting from the Si-O-Si
vibration would be expected. Unfortunately, the IR pattern
of imogolite consists largely of Si-O-Al bands, so simply
looking for the presence of peaks in the Si-O-Al portion of
the spectrum is not a viable test for imogolite-coupling
agent interaction. Even with spectral subtraction
capabilities it is difficult to remove the substrate signals
completely because of concentration differences between the
two spectra being subtracted. The direction of the shift is
also consistent with this model. The frequency (v) of a
vibration in the IR vtx Ik70, where k is the force constant
and u is the reduced mass. When Si-O-Al begins to enter into
the Si-O-Si vibration the reduced mass is lowered, increasing
the frequency of the vibration. Since frequency is directly
proportional to wavenumber, the wavenumber of the observed

vibration should also increase. As the concentration of APS

47

decreases, the Si-O-Si peak increases in wavenumber, as
predicted by an increase in aluminum character due to nearby
Si-O-Al bonds.

Further evidence for interaction has been obtained

through MAS 29

29

Si NMR spectroscopy. Figure 9(a) shows the

Si NMR spectrum of pure imogolite. It consists of a single
peak located at -79.3ppm with respect to TMS corresponding to
a Q0 site. The next spectra, figure 9(b) is that of air-
dried APS. This consists of three peaks, Q1 at -50.9ppm, Q2
at -60.3ppm and Q3 at -68.6ppm with respect to TMS. Typical
structural representations of each of these sites are shown
in figure 10. The last spectra, figure 11, is that of a mole
ratio 0.5:1.0 APS to Al mixture of APS and imogolite. Three
peaks are observed here. The peak at -79.7ppm corresponds to
imogolite. The two peaks at -69.7ppm and -61.0ppm are due to
the silane. The difference between the individual APS and
imogolite spectra and that of the APS-imogolite system lies
in the absence of the -50.9ppm peak in the mixture. The
presence of imogolite seems to cause the Q1 sites, those
corresponding to end groups in the polymer chain, to
disappear. The continuous nature of the tubular imogolite
surface may allow for a more continuous polymer network to
form in the presence of imogolite. The conformation of the
APS on the surface of imogolite may also promote
crosslinking, leaving fewer oxygen sites uncondensed,

transforming Q1 sites to 92 and Q3 sites.

48
1:)

 

WWII: IIIIIIIIII I IIIIIIIIIIIIIIIII III

Lulu“
500 400 300 200 0 -IM 4200 '1!) -400 '500

 

b)

 

 

 

 

 

WWWMWII IWIMIIIWUIIIIIWI

11m1111..,..

 

Figure 9 2981 MAS NMR of a) Air-dried dialyzed imogolite and
b) air-dried APS, with chemical shifts reported with respect
to TMS.

49

A B C

R’ R’ x R’ R’ R’ R’

a \ / I. l I. I, I,
EtO-Si-OEt /Si; x-si-o-sa- Is‘a [S‘i-O-Si-O-

(5 o o o 0 0'0 0 o o 6

Figure 10 Structural models illustrating thelnoleculer
bonding at the silane/silica interface. a) Q site

b) Q site c) Q site.

50

 

 

AlLllLlLLLlllLllllllLllllLlLL4LJlLLLllLlLlLlLlllll

200 1 00 0 - 1 00 -200

Figure 11 2981 MAS NMR of air dried APS-imogolite with mole
ratio 0.5APS to 1.031.

51

The combined results from the IR and MAS 29Si NMR
studies support the conclusion that APS is bonded to the
surface of imogolite. The fact that the APS can be
completely removed by dialysis of the aqueous solution of APS
and imogolite suggests that the resulting bond is
hydrolytically unstable.

Attempts at forming a more stable bond by altering
synthetic procedures proved to be unsuccessful. The mixture
was air-dried, to allow maximum APS-imogolite contact, then
resuspended in deionized water and dialyzed. The APS
completely disappeared. The mixture was air dried, heated to
120°C for twenty four hours to drive out water adsorbed on
the surface, resuspended in deionized water and dialyzed.
This treatment appeared to prolong the process of APS
removal, but the APS was still reversibly bonded. The
APS-imogolite solution was heated at various temperatures
overnight with no change in the resulting IR spectra.

This additional data coupled with several reports from

31 of acid catalyzed hydrolysis,

the literature by Borisov
suggest strongly that hydrolysis is the cause of the
instability of the APS modified imogolite. Although Si-O-Al
hydrolysis was responsible for the degradation of the
APS-imogolite system, it can be used to study the hydrolysis

rate.

52

E. Time Dependent Dialysis Experiment

Once it had been determined that the Si-O-Al bond
between imogolite and APS was hydrolytically unstable, it
became apparent that this system was ideally suited for a
semi-quantitative study of the siloxy alumane hydrolysis
rate. An experiment was subsequently designed to compare the
rate of hydrolysis of the alumasiloxane bond of the
APS-imogolite system to that of the APS-gamma alumina system.
The bonding in both systems occurs between APS and surface
aluminum hydroxyls. The major difference between alumina and
imogolite are the surface area, (280 vs. 900 mz/g),
respectively and the number of hydroxyls per A2 (16 vs. 18
OH/IOOAZ), respectively. There is also the fact that the
imogolite surface is curved while the gamma-alumina surface
is planar. In addition, the crystallinity of gamma-alumina
is less than that of imogolite.

The experiment entails dialysis of three different
systems for a period of 50 hours: APS-imogolite, APS-alumina
and APS in aqueous solution. IR spectra of the imogolite and
alumina substrates are shown on figure 12. Details can be
found in the experimental section of this thesis. Samples of
APS-imogolite and APS-alumina were taken at various intervals
during the dialysis, dried and analyzed by Fourier Transform
Infrared Spectroscopy. Since the APS passes readily through
the dialysis tubing, the rate of diffusion of APS from

solution was monitored by weighing the amount of APS

513

 

 

 

 

ABSORBANCE
O
b
O
l

 

 

 

I1 I I I T l I I l l I I l l
4000 3000 2000 1000
uavenuuaens ‘ ‘00.0

b)

 

0.726

'2

 

ABSORBANCE
5’
A
O

is

O
:3
lllllllllllllllllllll
___=;

 

0.159

 

 

I
4000 3000 2000 1000
uaveuuuaens 400.0

Figure 12 IR Spectra of a) Gamma-alumina b) air-dried
imogolite.

54

recovered as a solid at each sampling. The individual
weights were converted to percent APS values by setting equal
to 100% the number of grams of solid obtained by air drying
the first sample.

For the APS-alumina and APS-imogolite, the conversion
from absorbance to percent APS was more complex. Absolute
solution absorbances were unacceptable measures of
concentration because of the irreproducibility of the KBr
pellets. Solution state spectra were not useful because of
difficulties in keeping the gamma-alumina particles suspended
and because of the low concentrations of APS being used.
Imperfect mixing of components and different pellet
thicknesses were the major factors attributing to the poor
reproducibility of the pellets. To overcome this problem,
ratios of the absorbances of spectral bands due specifically
to the substrate and to APS were used to determine the
concentration of APS in each of the samples. IR spectra of
the initial APS-alumina and APS-imogolite samples are shown
in figure 13. In the APS-imogolite system the APS peak at

1 and the imogolite peak at 954cm-1 were

approximately 1570cm-
chosen, in an attempt to avoid regions in the spectra which
overlap. In the APS-alumina system, the same APS absorbance

1 1 were used.

at 1570cm- and the alumina absorbance at 1070cm-
IR spectra of APS-alumina and APS-imogolite after twelve
hours of dialysis are shown in figure 14. IR spectra of the

final sample of APS-alumina and APS imogolite are shown in

55

 

 

 

 

 

 

ABSORBANCE
O
8
11111

 

 

 

 

 

 

 

ABSORBANCE
1

 

 

 

 

I T I l I I I I I I F I I I
4000 3000 2000 1000
HAVEMMEHS 400 . 0

Figure 13 IR spectra of initial Time Dependent Dialysis
Bxperiment samples, 1.0 to 1.0 mole ratio APS to A1,
a) APS-alumina b) APS-imogolite.

S6

 

 

ABBOFBANCE
O
8

l L 1

 

 

 

I I I I r fl I 1 fi r —I I I I
4000 3000 2000 1000
HAW 400 . 0

 

 

1111

 

0.150!

 

 

IAVENUMERS 400 . 0

Figure 14 IR spectra of Time Dependent Dialysis Experiment
samples after twelve hours, a) APS-alumina b) APS-imogolite.

S7

figure 15. After the absorbance ratios were determined, they
were converted to %APS by assigning the value calculated for
the initial conditions to the absorbance ratio obtained for
the initial samples collected, and zero to the absorbance
ratio obtained when a spectra of the substrate alone is
taken.

A plot of the percent APS versus time was made to
compare the rate of loss of APS for the three systems. The
curves for the three systems studied, are superimposed on the
plot shown in figure 16. The pure APS drops to nearly zero
within four hours of dialysis at which point a more gradual
rate of disappearance begins. It takes approximately
fourteen hours before the mixed systems begin to show a
reduction in the rate of change in APS concentration. This
is due to the fact that the APS is initially associated with
the substrates, which are unable to diffuse through the
dialysis tubing. Another important observation is that the
concentration of APS at the point of the change in slope of
the APS-imogolite and APS-alumina samples are markedly
different. The APS-alumina curve levels off close to zero
percent APS. The APS-imogolite curve, on the other hand,
levels off near 25% APS.

Elemental analysis of the weight percent carbon in the
APS-imogolite sample taken after fourteen hours of dialysis
gave a mole ratio of APS to imogolite of 0.28, assuming the

water content of the air dried imogolite to be 22%. Ideal

5E3

 

 

Aesonaauce
P
‘
O
iJ,1

 

 

 

 

ABSORBANCE

 

 

 

rI I I I I—I I I I—I.I I r
4000 3000 2000 1000

HAVENUHBEHS 400.0

Figure 15 IR spectra of Time Dependent Dialysis Experiment
samples after fifty hours, a) APS-alumina b) APS-imogolite.

75‘AFES

59

 

 

 

100
A‘APS
O APS-Imogolite
80 I! APS-Alumina
a
60- (’85!
a
0 o
40‘ A 2 ° 0 ° 0
o
o
n
20— °
A
A A A x ‘ ‘ * ‘
0 I ‘I** I ' I I I I‘ I
, 0 10 20 3O 4O 50

Figure 16

Tkne (hours)

Plot of percent APS versus dialysis time.

 

60

60

monolayer coverage results in each available silanol
condensing to a surface aluminum hydroxide. This leads to a
one to one mole ratio of APS to imogolite. When the fact
that APS tends to condense to surfaces in the form of a dimer
is taken into account, there are only 2 hydroxyls per mole
APS. This leads to a more realistic monolayer of 0.67 mole
APS per mole imogolite. The true mole ratio for monolayer
coverage is probably lower still, due to the presence of
uncondensed silanol hydroxyls and inaccessible surface
aluminum hydroxyls which have been rendered unreactive by the
close proximity of an alkyl ammonium chain. As a result, the
group of samples dialyzed for fourteen hours or longer, can
be expected to have near monolayer to below monolayer silane
surface coverage. This information allows for the
explanation of the data when they are plotted on a semilog
plot.

0n the plot of the logarithm of the percent APS versus
time, figure 17, the APS-imogolite data can be treated as two
different sets of linear points, each with a different slope.
The slope of the first six data points is greater than that
of the slope of a line drawn through the last seven data
points. Both slopes are much smaller than the aqueous APS
slope. When only the first six data points in the
APS-alumina data set are considered, a line of very nearly
the same slope as was obtained for the APS-imogolite data can

be drawn. The subsequent data points for longer times,

61

 

 

 

 

2.2 -
A APS
0' APS-Imogolite
1.8- g. ‘ I! APS—Alumina
o 8
o
a A Q 0 ° 0 0
Q_ 1.4""I fl 0 o
<
N II
V A
on 1.0-
.9. A A ,l
a
A
0.6- A ”I
x
at
0.2 1 I . l . —I . I . I
0 10 20 :50 4O 50 60
Time (hours)
Figure 17 Plot of the log(%APS) versus dialysis time.

62

however aren’t nearly as well behaved, but seem to approach
the behavior of the aqueous APS system. The data points for
the aqueous APS seem to lose their linearity at low percent
silane also. This suggests that the nonlinearity of the
APS-alumina data may be a result of the error involved in
measuring such low levels of APS. The average error in

the value determined for percent APS was calculated by
averaging the values obtained from the spectra of several
different KBr pellets of the same sample. The average error
was found to be 2%, with the individual errors ranging from
less than half a percent to 5%. As the concentration of APS
in the sample decreases, the error arising from the method of
measurement becomes more and more significant. The linearity
of sections of the data in a logarithmic plot at long
dialysis times suggest that the hydrolysis rate is pseudo
first order. The hydrolysis rate depends upon both the
concentration of APS and the concentration of water, but
since water is present in large excess, the rate becomes
independent of its concentration. This means the slopes of
the lines drawn through the second seven data points will be
directly proportional to the rate constant for the
hydrolysis. The slopes were calculated by the least squares
method using the first six data points for the APS-alumina
and the APS-imogolite system. The values -0.033 and -0.029
were obtained, respectively. The slope was also obtained in

the same manner for the last seven points for the

63

APS-imogolite system. The value obtained for this slope was
-0.006. From these slopes, the initial rate appears to be
equivalent for the APS-imogolite and the APS-alumina system.
The rate at long dialysis times decreases dramatically for
the APS-imogolite system.

Recall the fact that both of these systems were
synthesized in the presence of APS in excess of that needed
to form a monolayer. A plausible explanation of this data is
that there is a bimodal distribution of silane coverage. The
first type of APS removed is due to the solubilization and
removal of excess silane polymerized on the surface of the
silylated substrate due to the change in the conformation of
the silane in the presence of the substrate. This APS does
not have the opportunity to interact directly with the
substrate. The second rate is actually a measure of the rate
of hydrolysis of the Si-O-Al bond due to the remaining APS
close to the surface of the substrate. At short times, the
rate of disappearance of APS is expected to be approximately
independent of the nature of the substrate. At longer times
the rate is expected to depend highly on the substrate. The
stability of the Si-O-Al bond toward hydrolysis determines
the resulting slope. A steep slope indicates the bond is
readily hydrolyzable and conversely a small value for a slope
indicates a low propensity for hydrolysis of the
alumosiloxane bond. This analysis for the APS-alumina system

leads to the conclusion that APS is much more stable with

64

respect to hydrolysis when bonded to imogolite than when
bonded to alumina. Although it is not possible to draw a
meaningful line through the latter points plotted for the
APS-alumina system, it is clear that the rate of
disappearance of APS is much faster in the APS-alumina system
than in the APS-imogolite system. Further studies must be
done to determine the reason for the significant difference

in APS hydrolysis rates for imogolite and alumina surfaces.

F. Conclusions

The success rate of imogolite synthesis has been
improved dramatically through the adjustment of several of
the synthetic variables. Dialysis of the reaction mixture
facilitates separation of imogolite from other reaction
products formed. An imogolite-APS complex was formed, as
determined by FTIR and MAS 29Si NMR studies, but found to be
hydrolytically unstable. The hydrolysis rate was measured by
a time dependent dialysis experiment, where the amount of
silane present at various dialysis times was determined by
FTIR. The rate of APS hydrolysis was measured using this
method for the substrates imogolite and gamma alumina. It
was found that the APS-imogolite system was more stable

toward hydrolysis than the APS-alumina system.

Chapter 4

FUTURE STUDIES

A. Imogolite Synthesis

Much progress has already been made to improve the
success rate of the imogolite synthesis. More experiments
need to be done, however, to try to understand the process of
tube formation itself. The key to this is the understanding
of the step which involves the subsequent raising and
lowering of the solution pH. Is the pH adjustment critical
or is the addition of acetic acid to the mixture the true
purpose of that step? This could be tested by adding
equivalent amounts of acetic acid and sodium hydroxide to
otherwise identical reaction mixtures. To one, the acid and
base would be added together, not allowing the pH of the
solution to change. To the other, the acid and base would be
added separately as is done at present, allowing the pH of
the solution to adjust. Another aspect of the synthesis that
requires further study is the yield with respect to both
reflux time and overall concentration. The yield could be
monitored by the gel test, FTIR spectroscopy and electron
microscopy. The present method of synthesis involves low
concentrations and a long reflux time, which make imogolite

less attractive for potential industrial applications.

65

66

B. APS-imogolite Synthesis

Although silylated imogolite is too unstable in aqueous
solution for use in a mixed organic-aqueous system such as
phase transfer catalysis, it should be stable in purely
organic systems where the amount of water present is quite
low. The current methods being developed for applying silane
coupling agents17 involves depositing them from non-aqueous
solvent, and either adding stoichiometric amounts of water or
taking advantage of water already adsorbed on the surface to
achieve hydrolysis of the coupling agent. This method has
been shown to allow more control of the amount of coupling
agent bound to the surface. A resulting enhancement of
stability, surface properties and reproducibility has been
observed. It is possible, and even desirable to modify
surfaces with silane from purely organic solution.
Successful surface modification of imogolite may be

accomplished in this way.

C. SLS-imogolite Synthesis and Catalysis

A recent preliminary reaction between the surfactant
sodium lauryl sulfate (SLS) and imogolite has produced
promising results. The reaction was carried out as described
in the experimental section of this thesis. A suitable
interaction was anticipated at low pH between the positively
charged imogolite and the anionic surfactant SLS. The result

of the combination was flocculation of the imogolite tubes.

67

Sodium lauryl sulfate had successfully transformed the
imogolite from hydrophilic to hydrophobic in nature. When
the mixture was placed into a dialysis membrane for one week,
the concentration initially decreased according to the IR,
but eventually remained constant. Dialysis successfully
removed the excess SLS and the rest remained firmly
associated with the imogolite tubes. I

The ability of the particles to remain flocculated under
agitation such as that applied during sonification must be
determined and the surface coverage of the imogolite
analyzed. The solubility of the flocculated mixture should
be determined by attempting to extract it with organic
solvents. The synthetic conditions should be varied to see
how they affect the product. Other surfactants should be
used to try to build the most desirable characteristics into
the complex. The phase transfer catalytic properties of the

resulting complexes should then be studied.

D. Analytical Techniques

The extensive use of MAS NMR and FTIR has been made
throughout this thesis. Many methods for elucidating the
nature of surface phenomenon, in particular, have permeated
the recent literature. As a result of this, it will be
necessary to understand and, where possible, apply these new
techniques to the systems being developed. A list of some of

these techniques includes Electron Microscopy, Diffuse

68

Reflectance IR Spectroscopy, x-ray Photoelectron Spectroscopy

and Auger Electron Spectroscopy.

LIST OF REFERENCES

10.

11.

12.

13.

14.

15.

16.

17.

18.

LIST OF REFERENCES

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Cradwick, P.D.G. et al. Nature Phys. Sci. 1972, 240,
187.

Yoshinaga, N. and Aomine, S. Soil Sci. Plant Nutr.
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Gotz, J. and Masson, C.R. J. Chem. Soc. A 1970, 2683.
Gotz, J. and Masson, C.R. J. Chem. Soc. A 1971, 686.

Farmer, V.C. et al. J. Chem. Soc. Chem. Comm. 1977,
462.

Farmer, V.C. and Fraser, A.R. Developments in
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Borisov, S.N. et a1. 0 at o s a d
figtgxgggmpggngg, page 297, New York, Plenum, 1970.

Inoue, T. and Wada, K. Clay Sci. 1971, 4, 61.
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Egashira, K. and Aomine, S. Clay Sci. 1974, 4, 231.
Henmi, T. and Wada, K. Clay Miner. 1974, 10, 231.

Farmer, V.C. and Fraser, A.R. J. Soil Sci. 1982, 33,
737.

Henmi, T. and Yoshinaga, N. Clay Mineral. 1981, 16,
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Wada, K. Clays Clay Miner. 1987, 35(5), 379.

Leyden, D.E. ed. s t s,
New York, Gordon and Breach, 1985.

Waddell, T.G. et al. from §ily1atg§_§grfgge§, Leyden,
D.E. and Collins, W.T. eds. New York, Gordon and

Breach, 1978.

69

19.

20.

21.

22.

23.

24.

25.

26.

27.

28.

29.

30.

7O

Morrall, Steven W. PhD. Dissertation, Colorado State
University, Fort Collins, Colorado, 1984.

Mittal, K. L. ed. As ts f
Eelxmerig_92ating§ New York Plenum. 1983.

Ishida, H. from Symposium 93 Adhesion Aspects of
Eglxmeric_ggafings. page 49. New York. Plenum. 1983.

Plueddemann, E.P. Proc. 24th Ann. Tech. Conf. Reinf.

Plastics/Composites Div. SPI Section 19-A, 1969.

Plueddemann, E.P. from An§1y§i§_91_§iliggng§, Smith,
A.L. ed., New York, Wiley, 1974.

Boerio, F.J. et al. J. Colloid Interface Sci. 1980,
73(2), 416.

Chiang, Chwan-Hwa et al. J. Colloid Interface Sci.
1980, 74(2), 396.

Moses, P.R. et al. Anal. Chem. 1976, 50, 576.

Plueddemann, E.P. Silane_gounling_bgenfs. Chapter 3,
New York, Plenum, 1982.

Boerio, F.J. and Williams, J.W. Proc. 36th Ann. Tech.
Conf. Reinf. Plastics/Composites Div. SPI Section 2-F,
1981.

Naviroj, S. et al. Proc. 37th Ann. Tech. Conf. Reinf.
Plastics/Composites Div. SPI Section Z-C, 1982.

Henmi, T. and Huang, P.M. from Pzgggediggs 9f the
'o c , Denver, 1985, Shultz et

al. eds. The Clay Minerals Society, Bloomington,
Indiana, 1987.

"IIIIIIIIIIIIIII