THE NATURE OF HYDRATED COPPER (II) IONS m LAYER SIUCATES Thesis for the Degree of Ph. D. M\CH|GAN STATE UNNERSITY DAVID MICHAEL CLEMENTZ ' 1973 LIBRARY Michigan State University This is to certify that the w. thesis entitled THE NATURE OF HYDRATED COPPER (II) IONS IN LAYER SILICATES presented by David Michael Clementz has been accepted towards fulfillment of the requirements for Ph. D. degree in Soil Science //// //w/ Maj01( profe/ or / Date/9w!“ 16‘. m7) :7 a 0-7639 ABSTRACT THE NATURE OF HYDRATED copper (II) IONS IN LAYER SILICATES By David Michael Clements The stereocheni stry of hydrated Cu (11) ions on the interlamellar surfaces of nicrocrystalline layer silicates has been investigated by observing the anisotropic components of the g factor in the esr spectra of oriented film samples at room temperature. When a aonolayer of water occupies the interlaaellar regions the ion has axial synnetry and the symmetry axis is perpendicular to the silicate layers. The Cu (II) ion.nost likely is coordinated to four water molecules in the £1 plane and to two silicate oxygens along the 5 axis. Under conditions where two layers of water occupy the interlamellar regions. the ion is in an axially elongated tetragonal field of six water molecules and the symmetry axis is inclined with respect to the silicate layers at an angle near #50. If several layers of water nolecules occupy the interlanellar regions, the Cu(H20)§+ ion tumbles rapidly and gives rise to a single, isotrOpic esr signal analogous to that normally observed fer the ion at temperatures above 50° K. A series of Cu (II) reduced charge nontnorillonites (RCM) of varYing charge reduction was then prepared by exchange of the parent David Michael Clements Li(I)-Na(I) mineral with CuC12 in 95% ethanol solution. The Cu (11) exchange capacity, as determined by Na(I) exchange in lll (v/v) ethanol- water, is a linear function of the traction of Li(I) initially present on the exchange sites, E. Selective Cu (II)-saturatlon on internal and external sites was achieved at maximum charge reduction (3 - 1.0). Water adsorption isotherms and (001) basal spacings are interpreted in terms of an increasing tendency toward interlayer collapse with increasing charge reduction. Because of the higher hydration energy of the Cu (II) ion, however, the fraction of non-expandable interlayers at given E value is lower than those present in the corresponding Li(I)-Na(I) RCM. Electron spin resonance spectra of oriented samples show that under air—dried conditions (£3. “0% relative hunidity) the predominant Cu (II) species present, whether on internal or external sites, is the planar eu(H20)fi” ion. The eynnetry axis of the ion is oriented perpendicular to the a-b plane of the silicate sheets. In the presence of a full partial pressure of water, the Cu (II) ions on the external sites and those which are in expandable interlayers become totally hydrated (“1050):") and tumble rapidly. The Cu(H20)fi+ ions in non-expandable layers retain their restricted orientation on the silicate surface. Some general conclusions have been drawn regarding the nature of charge distribution in the mineral. THE NATURE_OF HYDRATED COPPER (II) IONS IN LAYER SILICATES By David Michael Clementz A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Crap and Soil Sciences 1973 ACKNOWLEDGMENTS The author wishes to express appreciation to his major professor Dr. Max M. Mortland for sharing his unremitting enthusiasm fer research. Dr. Mortland's imagination and high standards have allowed the author to deve10p as a scientist while pursuing broad personal interests. Gratitude is also expressed to Dr. Thomas J. Pinnavaia fer his relentless barrage of questionsiduring this study which encouraged the author to expand his capabilities and philosOphy toward research. In addition. appreciation is expressed to the HSU Chemistry Department for the use of their equipment and to Dr. Roger V. Lloyd for obtaining the Q—band spectra. Personal thanks are extended to Dr; Bernard D. Knezek who was primarily responsible for my coming to MSU and to his family for their hospitality during our stay. Appreciation is also expressed to Dr. T. A. Vogel and Dr. B. G. Ellis for serving on the committee. Horde cannot express the special thanks due to the author's wife Carol. for her patience. understanding. and steadfast devotion during the course of his graduate studies and for the typing of this manuscript. ii TABLE OF CONTENTS LIST OF TABLE 0 O O C O O O O O O O O O O O O 0 O O O O 0 LIST OF FIGURES O O O O O O O 0 O O O 0 O O 0 O 0 O O O 0 IMRmUCTIOR O O O O O O O O 0 O O 0 O O O O O O O O 0 O 0 PART I a Stereochemistry of Hydrated Capper (II) Ions in Layer Silicates. An Electron Spin Resonance St‘dye e e e e e e e e e o e 0 Introduction . . . . . . . . . EXperimental Methods . . . . . Results and Discussion . . . . PART II 3 PrOperties of Reduced Charge Montmorillonite: Hydrated COpper (II) Ions as a Spectroscopic Pmbe 0 O O 0 O O 0 0 O O O O O 0 O O O O 0 0 Introduction . . . . . . . . . . . . Experimental Methods . . . . . . . . Results and Discussion . . . . . . . SUMMARYANDCONCLUSIONS ................. BIBLIOGRAPHY O O O O O O O I O O O 0 O O O O 0 O O O O O 0 iii PAGE iv mkw \a) 17 17 18 21 39 ’42 LIST OF TABLES TABLE PAGE PART I l. Esr’Data for Hydrated Cu (II) Ions on the Interlanellar Surfaces of Various Layer Silicates O O O O O O O 0 O O O O O 0 O O O O 0 O 0 e O 0 11 PART II 1. Cation Exchange Capacity and Ear Data for Hydrated Cu (II) Ions on the Surfaces of Reduced Charge Montmorillonites . . . . . . . . . . . . . 29 2. Cation Exchange Data Obtained b Conductometric Titration of Selectively Cu (II ~8aturated mn.‘ . . . O O O O O C C O O C C O O C O O O O O O O O O 38 iv LIST OF FIGURES FIGURE PAGE PART 1. 2. 3. PART 2. I Esr spectra (first derivative curves) for Cu (II) hectorite. Aoand B) randomly oriented powder samples at 300 and 770 K, respectively. C) oriented film sample at 300 K with the a,b-planes of the silicate layers positioned parallel to H. D) a.b-planes positionedperpendiculartofl............... 9 Schematic representation of the stereochemistry of hydrated Cu (II) under conditions where (A) one layer and (B) two layers of water occupy the interlamellar r661 one O O O O O O O O O O C O O O O O O C O C O O O O O 10 Ear spectra (looo Gauss scans at 300° K) of Cu (II) in oriented film samples of layer silicates. In each case the tOp spectrum is for the a.b-p1anes positioned parallel to H, and the bottom spectrum is for the a.b-p1anes positioned perpendicular to H. A) Cu (II) montmorillonite (Upton) when a monolayer of water occupies the inter- lamellar regions. B) Cu (II) vermiculite (Llano) with two layers of water in the interlamellar region; ”x" denotes an unidentified resonance line which was also observed in the Na(I) exchange form. C) Interstratified Cu (II) montmorillonite (Chambers) with varying layers of water in the interlamellar regions. H increases from left to right. and the vertical line indicates the resonance position of a standard pitch sample with g - 2.00280 O 0 O O O O O O C O O O O O 0 O O O 0 O O O 0 O O O 1“ II Solid line. plot of milliequivalents of exchangeable ions on Cu (II)-saturated RCM, as determined by Na(I) exchange in 1:1 (v/v) H O-ethanol. versus E, the fraction of exchangeable L (I) in the preheated clay. Dashed line. analogous plot of exchangeable Na(I)19gd Li(I) in ROM as determined by Bfll‘flley and. man s e e o e e e e o e e 23 Water adsorption isotherms: A, Cu (II)-saturated RCM having varying amounts of charge reduction; B and C. Cu (II)-exchanged forms of ROM and their unexchanged V LIST OF FIGURES -- Continued PART II PAGE 3. 5. Na(I), Li(I) counterparts with g; values of 0.6 and 1.0, respCtinl-yctotoo00.000000000000000 25 Ear spectra (first derivative representations) for Cu (II)-nontnorillonite (Ran. g - o). A, 4000 Gauss scan of an air-dry. randomly oriented powder sample; B, 1000 Gauss scan of an air-dry. oriented film sample with the W of the silicate layers positioned parallel to H; C, the spectrum of the same sample when the a,b~plane§ of the silicate layers are positioned perpendicular to H. The solid vertical line indicates the resonance position of a standard strong pitch sample with g - 2.0028. All spectra obtained at 300 K O O O O O O O O O O I O O O O O O O O O O O O O O O 28 Ear spectra of ROM at E - 1.0. A. baseline of air- dry, unexchanged Li(I)-Nam powder: a. air-dry Cu (II)- saturated powder; a and D. an air-dry. Cu (II)- saturated pressed disk with H parallel and perpendi- cular to the silicate layers, respectively; E, Cu (11)- saturated powder exposed to a free-water surface for 24 hours. Field setting is identical for all Banploseoeeeeeeeeeeeeroeeeeeeeeeooe32 Esr spectra of selectively Cu (In-saturated RCM (E - 1.0) pressed disks: A and B, predominantly external Cu (ID-saturation under air-dry conditions with H parallel and perpendicular to the silicate layers, respectively; c. the same sample after exposure to a free-water surface (essentially orientation independent); D and E. internally Cu (ID-saturated under air-dry conditions with H parallel and perpendi- cular to the silicate layers, respectively; F and G. the previous sample after exposure to a free-water surface with H parallel and perpendicular to the silicate layers. respectively; H and I. the previous sample after heating to 200 C for one hour, sealed. and allowed to return to room temperature, with H parallel and perpendicular to the silicate layers, “metivelyo O 0 0 O O O O O O O O O O O 0 O O O O 0 O 0 O 35 INTRODUCTION The nature of hydrated copper (II) ions in layer silicates is of interest from several viewpoints. First. copper (II) is an essential element required for plant nutrition. Yet it can become a pollutant at high concentrations. This paradox automatically draws our attention since Cu (II) can adhere to the exchange sites on layer silicates present in soils and sediments. Secondly, Cu (II) has recently been shown to have unique ability to coordinate aromatic molecules when present on layer silicate surfaces. Coupled to these interests are the electronic configuration and nuclear properties of Cu (II) itself which allow it to be studied via electron spin resonance (esr) spectroscopy. Bar has been recognized as a powerful tool in examination of Cu (II) ions in various environments. However. very few esr studies of Cu (II) ions in layer silicates have been made. To the author's knowledge, no studies to date have shown in what hydrated form Cu (II) exists on exchange sites of layer silicates or what accounts for its unique coordinating abilities on these surfaces. Clearly. any attempt to make such observations must come from studying the 1 2 Cu (II) ion 22. gig; and esr is particularly suited for this task. Once the nature of the ion is understood in known submicroscoPic environments. it can be introduced into unknown systems and therein reflect its environment. Therefore. this study was made in two parts. The first involved investigating the stereochemistry of hydrated capper (II) ions in a representative variety of layer silicates which are well characterized by the latest chemical and physical methods. Once that data was obtained, the Cu (II) ions were introduced onto the exchange sites of artificially altered minerals and used to describe some of their preperties. PART I Stereochemistry of Hydrated Capper (II) Ions on the Interlamellar Surfaces of Layer Silicates. An Electron Spin Resonance Study Introduction Recent investigations have shown that Cu (II) ions can form complexes with various aromatic molecules when present at the inter- lamellar cation exchange sites of certain layer silicate minerals, known as montnorillonites, or better. smectites.l-5 The silicate layers undoubtedly play an important role in stabilizing the com- plexes since Cu (II) in other environments. including homogeneous solution as well as the solid state. is not known to form arene complexes. In view of this observation, information concerning the nature of Cu (II) ions on layer silicate surfaces is of interest. The objective of the present work was to investigate the stereochemistry of exchangeable hydrated Cu (II) ions in the inter- lamellar regions of various layer silicates by means of ear spectro- scopy. Although the minerals are microcrystalline. they are potentially well suited for such studies because highly ordered films can be prepared in which the crystallites are oriented with their planes of hydrated metal ions parallel to each other. Thus it should be possible to deduce the orientations of the anisotrOpic components of g with respect to the anionic silicate surface. Although esr l; spectroscopy has been used to study hydrated Cu (II) and other paramagnetic ions in amorphous resins,6 isotrOpic zeolites. 7 and layer silicates,8 the special utility of the technique when applied to oriented samples of the latter types of compounds has been only recently recognized.8° All of the layer silicates investigated in the present study are related in that the silicate layers consist of two silica sheets that enclose an octahedral layer which is occupied by non—exchangeable cations such as A1(III). Fe(III), Mg(II) and Li(I). The negative charge on the infinite two-dimensional silicate framework originates from positive charge deficiencies in the octahedral layer or by the replacement of Si(IV) by a trivalent ion (35.. Al(III)) in the tetrahedral silica sheets. Minerals of both types were investigated in order to assess the effect of the site of positive charge defici- ency in the silicate framework on the stereochemistry of the hydrated Cu (II) ions in the interlamellar regions. garmental Methods The following naturally occurring layer silicates of known unit cell composition were used: Hectorite (Hector. California); Montmorillonite (from Upton, wyoning'. Chambers. Arizona’. and Otay. California); Saponite (Scotland) . and Vermiculite (from Llano. Texas', and Libby. Montana). The Cu (II) exchange forms were prepared by slurrying for several hours 2;, 2.0 g of the <.2,afraction of the mineral in 500 ml of an aqueous or methanol solution of 1.0! CuClz. centrifuging. and discarding the supernatant liquid. The procedure was repeated three times. and then the excess chloride was removed 5 by washing with water or methanol until a negative chloride ion test with AgNO was obtained. Each sample was given a final wash with 3 methanol, dried in air. and then allowed to stand several days at 100% relative humidity to displace any adsorbed methanol with water. The sodium exchange forms were prepared in an analagous fashion. except that the exchange reactions were carried out in aqueous solution, and the products were isolated from aqueous suspension by freeze-drying. The ear spectra of randomly oriented powder samples of the sodium exchange forms were recorded to identify the resonances due to non- exchangeable paramagnetic ions in the silicate layers. Only the vermiculite from Libby, Montana: exhibited a very broad (A H - 320 Gauss) resonance with a g value near 2.0. This resonance is believed to be due to iron (III) ions which occupy octahedral positions in the silicate layers as similar spectra have been observed previously 9 fer hydromicas in which Fe(III) ions occupy octahedral environments. Although the sodium exchange form of each of the other silicates gave resonance signals near g - n.0, which.may arise from Fe(III) in tetrahedral sites,10 none showed resonances near g - 2.0 which were sufficiently intense to obscure Cu (II) signals in X-band spectra. In the case of the Libby vermiculite, the Cu (II) signals could be resolved from the iron signal in the Q-band spectrum at 77° x. Some dipolar interactions probably occur between iron and copper. because the width of the copper resonances increased with increasing iron concentration in the silicate layer. In general. highly ordered self-supporting films of the Cu (11) exchange forms of the layer silicates were prepared by evaporating at room temperature an aqueous suspension of the mineral on a flat 6 polyethylene or teflon surface and then peeling the films away. Since the crystallites are oriented with their silicate layers parallel to the film surface, narrow strips of film (22. 3 x 10 am) placed vertically in a 4 mm quartz glass esr tube or on a teflon holder could be positioned in the cavity of an ear spectrometer with the silicate layers at a known angle to the external magnetic field. Films of the montmorillonite sample from Otay, Califbrnia,exhibited poor mechanical strength, suggesting that only partial orientation of the crystallites occurs upon evaporation of the suspension. In this case, however, an ear spectrum for a partially oriented sample was obtained by evaporating the suspension on a thin teflon strip and placing the entire strip in an ear tube. Ear X-band spectra were obtained with a Varian E-h spectrometer: the Q-band spectrum for'Cu.(II) was recorded on a Varian Model vu503 Q—band spectrometer. A Phillips X-ray diffractoaeter with copper radiation and a nickel filter was used to determine the (001) spacings of the capper (II) exchange forms of the layer silicates. Results and Discussion Hectorite is an example of a layer silicate in which the negative charge on the silicate layers originates exclusively from a positive charge deficiency in the octahedral positions. Uhen allowed to equilibrate in air under ambient conditions. the Cu (II) exchange fern exhibits a 001 spacing of 12.h3 and a water to copper ratio of about 831. Since the silicate lattice g_dimension is 9.68. the thickness of the interlamellar region (2.82) indicates that the Cu (II) ions are hydrated by a monolayer of water molecules. Ear spectra of the Cu (II) ions under these conditions are illustrated in Figure 1. Spectra A and B for randomly oriented powder samples at room temperature and at 77° K, respectively, consist of clearly defined g‘L and g" components as expected for Cu (II) with axial symmetry and (gu -gl )prn hA. Although hyperfine splitting due to 63 Cu and 65 Cu (1-3/2) is well resolved for the parallel component, none was observed for the perpendicular component. When the spectrum of an oriented film sample is recorded with the silicate layers parallel to the magnetic field direction (g;., spectrum 0) only 5i. is observed. 0n the other hand, when the film is oriented with the silicate layers perpendicular to H, only the A,, components of g" are visible. Thus the syametry axis of the hydrated ion is positioned perpendicular to the silicate layers. The ear results, together with the fact that a monolayer of water occupies the interlamellar region, are consistent only with an environ- ment in which copper (II) is coordinated to water molecules in the xy;plane and to surface oxygens of the silicate lattice along the 5 axis. Most likely, four water molecules are bound to copper as shown schematically in Figure 2A. The remaining water molecules must occupy outer spheres of coordination. If most of this latter water is removed by heating the silicate under vacuum at 110°. the esr spectral features of an oriented film sample remain unchanged. Thus the amount of outer sphere water. which has been recently described by Far-er and Russell11 as forming a dielectric link between the exchangeable cation and the silicate surface, does not alter the basic stereochemistry of the ion. The ear spectral parameters for siredried Cu (II) Hectorite are presented in Table 1 (Part A) along with those for related Cu (II) Figure 1. Ear spectra (first derivative curves) for Cu (II) hestorgte. A and B) randomly oriented powder samples at 300 and 77 K, respectively, C) oriented film sample at 300 K with the a,b-planes of the silicate layers positioned parallel to H. 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In each case an anisotropic spectrum with g1_ and g" components was observed at room temperature but the hyperfine splitting of the parallel component was better resolved at 77° x. l 7 Among the three montmorillonite samples shown in the table. 77 to 95% of the net negative charge on the silicate layers originates from cationic charge deficiencies in the octahedral sheet of the silicate framework, but the number of Cu (II) ions per unit cell and the nature of the ions occupying octahedral position.differ from those in hectorite. Since the surface area of one face of the unit cell in all of these silicates is approximately 50 R 2, the average distance between Cu (II) ions is _c_a_. 16 R in hectorite and 9;. 9.0-12. 5 R in the montmorillonites. In saponite, and the two vermiculites. where all of the negative charge on the silicate layers is due to positive charge deficiencies in the tetrahedral sheets, the average distance between Cu (II) ions are estimated to be 10 and 7 2. respectively. Despite these differences in Cu (II) - Cu (II) distances, posi- tion of positive charge deficiency in the silicate layers, and the nature of the metal ions in the octahedral and tetrahedral sheets of the silicate framework, oriented film samples of each layer silicate showed the same esr spectral changes as those described for Cu (II) hectorite when the film is positioned parallel and perpendicular to the applied magnetic field. Moreover, in each case the magnitude of g“ is greater than g ‘L (5;, Table 1). This result is consistent with Cu (II) ion in an axially elongated tetragonal crystal field 12 and the unpaired electron occupying a d 2 2 orbital. Axial x -y 13 elongation is also indicated by the magnitudes of the interlamellar thicknesses. In the case of Cu (II) hectorite, where the interlamellar thickness is 2.8 R, if the radius of a silicate oxygen atom is taken to be 1.4 X , 13 then the capper (II) silicate oxygen bond length is 22, 2.8 R . The expected distance of the Cu-OH bonds is 95, 2.0 2.13 2 An example of the ear spectra obtained for an oriented film of Cu (II) montmorillonite is provided in Figure 3A. We turn now to the deduction of the stereochemistry of hydrated Cu (II) ions when two layers of water molecules occupy the interlamel- 1ar region. Earlier x-ray diffraction studies of layer silicates 1“ indicate that when two layers of water are present, divalent metal ions should be octahedrally coordinated to six water molecules. However, in the case of'Cu (II) the octahedron should be distorted, as required by the Jahn - Teller theorem. Vermiculite is especially well suited for obtaining two layers of water'molecules in the interlamellar regions as the high surface charge density permits a maximum of only two layers even when the silicate is fully hydrated. Samples of the other layer silicates with all inter- lamellar regions uniformly occupied by two layers of water molecules are more difficult to achieve as they can be swelled beyond two layers of water molecules and thus tend to give interstratified systems. The presence of’two layers of’water molecules in the interlamellar regions of airedried oriented film of Llano Cu (II) vermiculite was verified by observing X-ray reflections of several rational orders that corresponded to a 001 spacing of lu.22. The ear spectrum of the film at room temperature consisted of g“ and 51. components when the silicate layers were positioned‘bg§g_p§£;llel and perpendicular to the Figure 3. Ear spectra (1000 Genes scans at 3000K) of Cu (II) in oriented film samples of layer silicates. In each case the top spectrum is for the a,b-planes positioned parallel to H, and the bottom spectrum is for the a,b-planes positioned perpendicular to H. A) Cu (II) montmorillonite (Upton) when a monolayer of water occupies the interlamellar regions. B) Cu (II) vermiculite (Llano) with two layers of water in the interlamellar region: "x" denotes an unidentified resonance line which was also observed in the Na(I) exchange fern. C) Interstratified Cu (II) montmorillonite (Chambers) with varying layers of water in the interlamellar regions. H increases from left to right, and the vertical line indicates the rgsonance position of a standard pitch sample with g - 2.002 . 15 external magnetic field as shown in Figure 33. The lack of any appreci- able change in the relative intensities of g" and g1_ upon altering the position of film in the magnetic field indicates that the symmetry axis of the tetragonal ion is inclined with respect to the silicate surface at an angle near #50. Also, the Cu-OH bonds along the 2 symmetry axis are longer than those in the xy plane, as g" > El. . A schematic representation of the stereochemistry of the ion is shown in Figure ZB. AnisotrOpy in the g factor of Cu(hzo)62+ is rarely observed at room temperature. One previous example was reported by Fujiwara and his co-workers in a study of cupric sulfate solutions confined in the molecular space of polyvinylalcohol gels.15 But isotrOpic thermal motions are normally sufficiently rapid above 50°x to give a single esr line.16 One suggested motion involves the rapid exchange of the ion between three equivalent Jahn - Teller distorted states which correspond to axial elongation along the three possible sets of HZO-Cu-HZO axes.17 However in the absence of rapid tumbling this motion will not lead to averaging of g" and 5d when the ion is sorbed on a surface in the manner illustrated in Figure 23. Ear spectral parameters fer two Cu (II) vermiculites with two layers of water in the interlamellar regions are presented in Table 1 (part B). The g values are somewhat larger than those found for the planar aquo complex, and the calculated average values of g are in good agreement with the observed values of g av for Cu (II) in 18 aqueous solution 6 and forCu(H20)62+ at the exchange sites of resins. The exceptionally large line width observed for the Libby Cu (II) vermiculite is undoubtedly due to magnetic interactions between Cu (II) 16 and Fe (III) which is present in large amounts in the silicate frame- work (g;,, experimental sections). Attempts to observe the stereochemi stry of Cu (II) ion hydrated by more than two water layers were complicated by interstratification of the silicate as indicated earlier. An example of the esr spectra obtained fer an interstratified system is given in Figure 30. The high field line in the figure is due to the 5i. component of Cu (II) ions hydrated by a monolayer, whereas the lower field line, which is orientation independent, is due to Cu (II) hydrated by two or more layers of water. It was possible. however, to investigate the nature of the hydrated capper (II) ions when the interlamellar regions of Cu (II) hectorite were fully expanded by soaking the silicate in water for #8 hrs. Under these conditions the 001 x-ray reflection corresponded to an interlamellar thickness of about 102 and the ear spectrum of an oriented film sample consisted of a single isotropic line with g - 2.192, independent of its position with respect to H. Thus when several layers of water are present the Cu (II) ion tumbles rapidly. averaging the g" and ad. components. PART II Properties of Reduced Charge Montnorillonites: Hydrated Cu (II) Ions as a Spectrosc0pic Probe. Introduction The thermal migration of exchangeable cations, such as lithium. into vacant octahedral positions in montmorillonite has been known for nearly two decades. An important consequence of this cation migra- tion is the reduction in the surface charge of the mineral and the concomitant increase in the mean distance between the cations remaining on the interlamellar surfaces. Variable charge reduction can be achieved by introducing along with Li (I) on the initial exchange sites an ion (g‘gh, Na(I)) which is too large to penetrate the silicate structure upon heat treatment. Above a critical concentration of Li(I) corresponding to 25, 50% of the initial C.E.C., the reduced charge minerals resist reexpansion by water. However. recent studies have shown that the reduced charged minerals can be swelled by certain sol- 19 These latter solvents vents such as ethanol. glycol and morpholine. should provide a means of replacing the exchangeable Li+ and Na+ ions with any desired cation. It has been recently demonstrated that Cu (II) ions can serve as a useful probe in detecting environmental influences of silicate surfaces on exchangeable ions by electron spin resonance (ear) spectro- scopy.20 The technique has been applied in the current study to obtain 17 18 structural information for hydrated Cu (II) ions on the internal 9.“; external exchange sites of reduced charge montmorillonites (hereafter designated as ROM). In addition. CEO and water adsorption isotherms have been determined for the Cu (ID-saturated ROM in order to assess the effect of the transition metal ion on the cation exchange and swelling properties of the mineral. merimental Methods Preparation of ROM 8 A series of ROM samples was prepared by the method of Brindley and 19b Ertem. Suspensions of < 2’s Li(I)-saturated and Na(I)-saturated Upton. wyoning. Montmorillonite (A.P.I. H-25) were mixed in various proportions and classified with regard to the fraction of lithium ions occupying the exchange sites prior to heat treatment. _F_‘. The E values for the four samples in the series were 0.0 (no lithium present in the Na(1)-nontuori11onite). 0.2, 0.6. and 1.0 (no sodium added to the Li(I)- montmorillonite). The suspensions (approximately 600 m1, 0.00h7 gm/ml) were stirred for 21} hours to allow maximum randomization of ions and then dried into large thin films (approximately 6" by 12") on poly- ethylene sheets. These clay films were peeled from the polyethylene and heated for an hours at 220°C. It was believed that this technique would promote homogeneous. distribution of ions in all layers. After cooling, the clay films were suspended in 95% ethanol and dispersed in a Waring blender. The ethanol suspensions were concen- trated, and the clay was collected by vacuum filtration. One portion of each airudried, Li(I)-Na(I) Ron sample was saved for the water adsorption study. 19 Cu (II) Exchange Forms: A. Cu (II) Saturation of Internal and External Sites. Total Cu (II) saturation was achieved for R014 at all four 3 values by stir- ring the samples for 15 hour periods in l §_Cu012/9j% ethanol solution and repeating the procedure two more times. Excess CuCl was removed 2 by washing with 95% ethanol until a 01" test with AgNO was negative. 3 The samples were allowed to dry in air to remove excess ethanol and were then equilibrated at 20°C and box relative humidity for 21+ hours. All of the Cu (II) ions can be exchanged off with 3 washes of Gaol2 in ethanol, as verified by the loss of a Cu (II) esr signal. B. Cu (II) Saturation of Internal Sites. x-ray diffraction experiments (see below) indicated that Cu (II)-saturated Rah with g values of 0.6 and 1.0 would undergo limited swelling upon exposure to liquid water. Thus, it was assumed that a large organic cation in aqueous solution would readily exchange Cu (II) on external sites but have difficulty in penetrating the interlamellar space to exchange ~ internal Cu (II) ions. One-gram samples of totally Cu (II)-saturated ROM with_§ values of 0.6 and 1.0 were quickly washed three times with 50 ml of an aqueous 0.5 E tetrabutylammonium chloride solution. The wash procedure required approximately 10 minutes to complete. Excess @uuNTCl’was removed by washing with water. 0. Cu (II) Saturation of External Sites. One-gram samples of untreated Li(I)-Na(I) ROM with g values of 0.6 and 1.0 were heated to 110°C for one hour to insure removal of interlamellar ethanol. Each sample was stirred three times in aqueous l §,CuCl and then washed 2! free of excess of with water. The samples were dried and equilibrated at 20°C and nos relative humidity. This procedure should have 20 rendered all of the external exchange sites Cu (ID-saturated, while the internal sites remained largely unexchanged. Infrared Studies: Thin films of the RCM samples were dried onto Irtran windows and scanned on a Beckman IR-7 spectrOphotometer. Lithium migration into the structure was verified by observing the changes in the 0H "wag" vibrations in the 700-900 cm"1 region as discussed by Calvet and Prost.l9a Charge Reduction Measurements 3 Charge reduction in ROM was verified by deterring the amount of exchangeable Cu (II) present in the totallybsaturated Cu (II) exchange forms. A modification of the Mortland and Mellor technique21 was used which consisted of suspending the clays in a 50:50 (v/v) mixture of 93% ethanol and water, and titrating conductrometrically with standard NaOH. The titration curves were linear with only one end- point. Hater Adsorption: The Cu (ID-saturated samples and the unexchanged RCM'S were equilibrated at 20°C for one week under seven different partial pres- sures of water vapor provided by various saturated salt and sulfuric acid solutions and the resulting weight gain with increasing partial pressure was recorded. X-ray Diffraction Studies: The (001) reflections of the samples under various conditions were measured by depositing thin films of the mineral on glass slides. A Philips X-ray diffractometer with capper radiation and a nickel filter were employed. 21 Electron Spin Resonance : Some degree of orientation of crystallites was necessary to give detailed spectral information. Where possible, the oriented film 21 technique employed earlier was used. Samples which did not make good oriented films (e.g,, E a 1.0) were pressed into disks 8c under 10.000 psi for three minutes on a Carver Press. Narrow strips were sliced from these disks for use as oriented samples. x-band spectra were recorded on a Varian E-h spectrometer. Results and Discussion The CEO of Cu (II)—saturated ROM, as determined by Na(I) exchange in 1:1 (v/v) ethanol-water, exhibits a linear dependence on E. the fraction Li(I) initially present on the exchange sites (see Figure 1). At E_- 1.0. a residual CDC of 27 meq per 100 gm is retained which corresponds to about twice the negative charge arising from tetrahedral substitution of A1(III) for Si(IV) in the mineral (unit cell formula. I“(1)0.64 E13.06F'0.32"50.oa (A10.10317.9)°20(°H)u)' Included in Figure l for comparison purposes are the data of Brindley and Ertem 19“ for exchange of Li(I)-Na(I) RCH in ammonium acetate solution. The nonlinear dependence on‘E and lower CEC values relative to those obtained fer Cu (II) RCM may be due in part to dif- ferences in exchange conditions. That is, Cu (II) in ethanol may be more effective than NH+4 in aqueous solution in replacing Li(I) and Na(I) in these reduced charge systems. The differences in the data may also arise because of differences in experimental technique. It is believed that protons are generated during the heat treatment of these l9c,22 clays. The protons could migrate to octahedral sites, satisfy 22 .n seems ens sameness an eenefloeoe no :8 3 33 one Ens oflnsowsncoxo mo uon msomo .es Mo sages...“ on». .M urea? .Hossnveé m H screen .amao possesses one 5 ES oSsowcesowo r $55 as 5 essences Ems a... confidence no £8 ggeomAHC so so used 03.33388 mo 33333333 mo 90.3 .33 snow .H enema 23 H 0.53.... 3% o; 0.0 md 50 0.0 nd v.0 md «.0 —.o r _ _ p _ _ . a _ _ E sidoz ms