spamomts smmEsor. ' THE ENVERGWERT 0F EXCHANGE ‘ j , ,CATIGNS m mamas. - _ ; Disserfiatéon fog the Degree Qf'Phgv'D {fiWHEGANS'EAYE UKEVERSSTY . - ' » mm 33m mam ’ ‘ £9.74 ~ U; University This is to certify that the thesis entitled Spectroscopic Studies of the Environment of Exchange Cations in Smectites presented by Murray B. McBride has been accepted towards fulfillment of the requirements for __ElL.L__degree in ill—5.1.0 C ence amomc av _ an 8 81—80149 3 BMMMWM glgmm amoERs ABSTRACT SPECTROSCOPIC STUDIES OF THE ENVIRONMENT 0F EXCHANGE CATIONS IN SMECTITES By Murray Brian McBride The behavior of interlayer cations in smectites has been studied by electron spin resonance (esr) and infrared spectroscopy. 2+ and Mn2+ ions tumble rapidly as hexaaquo complexes in the The Cu fully hydrated interlayers, but move into the hexagonal cavities of the silicate surface upon thermal dehydration. Limited layer charge reduc- tion by these small divalent cations can occur as the dehydrated cations migrate further into vacant octahedral sites of dioctahedral minerals. While charge reduction is irreversible, cations move out of hexagonal holes as the interlayers are expanded during resolvation of the clay. No evidence for specific adsorption of these cations on the silicate structure is observed. Studies of line-broadening of the esr signal of interlayer 2+ 2 2+ 3 2 -Mn + and Mn Mn + indicate that Mn —Fe + dipolar interactions are important. With increased hydration of the interlayers, the average 3+ 2+ 3+ distance between Mn2+ and structural Fe increases, and the Mn -Fe interaction decreases. The interlamellar water in fully hydrated smectites has an apparent viscosity only 30% greater than water in solution. The Mn(H20)§+ complexes of the interlayers become much less ,.§j’i Murray Brian McBride u’ffl' V (fl mobile in air-dry smectites, and for clays heated to 200°C. the Mn2+ demonstrates an esr spectrum typical of crystalline matrices. Studies of the esr of structural Fe3+ impurities in the octa- hedral layers of smectites have demonstrated the perturbation of structural Fe3+ by electrostatic interaction with exchange cations. This phenomenon allows the positions of cations in interlayers to be determined. Strongly solvating cations tend to tumble freely in fully expanded interlayers away from the silicate surface. while cations with low solvation energies remain near or within hexagonal cavities of the structure. A dielectric medium, such as water, in the interlayer reduces the electrostatic attraction between silicate and exchange cation, and the perturbation of structural Fe3+ is thereby decreased. SPECTROSCOPIC STUDIES OF THE ENVIRONMENT OF EXCHANGE CATIONS IN SMECTITES By Murray Brian McBride A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Crop and Soil Sciences 1974 ACKNOWLEDGMENTS The author wishes to thank Dr. Max M. Mortland for his enthusiastic and expert guidance in research. The constant conmuni- cation of ideas in the laboratory created an atmosphere of scientific inquiry that represented the most important aspect of the author's educational experience at MSU. Appreciation is also expressed to Dr. Thomas J. Pinnavaia. whose advice from a Chemist's point of view proved to be invaluable and often quite necessary. His inquisitive attitude toward clay mineralogy has produced many important questions and at least a few answers. Gratitude is expressed to Dr. Bernard D. Knezek and Dr. Boyd G. Ellis for serving on the committee, and to Dr. David M. Clementz, who first inspired the author's interest in electron spin resonance. Finally. the author's fiancée, Janice, deserves special thanks because of her continued encouragement and patience. ii TABLE OF CONTENTS Page LIST OF TABLES ........................ iv LIST OF FIGURES ......... . ..... . ....... v INTRODUCTION ................... . ..... 1 PART I: Cu(II) INTERACTIONS WITH MONTMORILLONITE: EVIDENCE FROM PHYSICAL METHODS ........... 3 PART II: ELECTRON SPIN RELAXATION AND THE MOBILITY OF MANGANESE(II) EXCHANGE IONS IN SMECTITES . . . . . . 29 PART III: EXCHANGE ION POSITIONS IN SMECTITE: EFFECTS ON ELECTRON SPIN RESONANCE OF STRUCTURAL IRON . . . 48 PART IV: PERTURBATION OF STRUCTURAL Fe3+ IN SMECTITES BY EXCHANGE IONS .................. 56 SUMMARY AND CONCLUSIONS ............. . ..... 73 LITERATURE CITED ....................... 75 LIST OF TABLES Table Page PART I 1. X-ray OOl spacings of Cu (II)-montmorillonites after 24-hour heat treatment and equilibration in several solvents for two weeks ........... 9 2. CEC values for Cu (II) montmorillonite heated to various temperatures for 24 hours ......... 9 3. Ability of interlamellar cations to occupy octa- hedral sites of montmorillonite upon 215°C heat treatment for 24 hours ..... . .......... 18 PART II 1. Mn2+-saturated layer silicates ............ 32 2. Basal spacings (A) of Mn2+-saturated layer silicates . 34 3. Esr line widths of oriented Mn2+-smectites ...... 42 iv LIST OF FIGURES Figure PART I l. Cu(II)-montmorillonite, infrared 0H deformation region . a. air-dry b. after 24 hour 200°C heat treatment c. after 24 hour salvation of b in l:l water-95% ethanol d. after NaOH titration of b Cu(II)-montmorillonite, infrared 0H deformation region . a. air-dry b. after heating 24 hours to 100°C c. after heating 24 hours to 156°C d. after heating 24 hours to 200°C e. after heating 24 hours to 270°C Ca(II)-montmorillonite, infrared 0H deformation region . a. air-dry b. after 24 hour 200°C heat treatment, spectrum taken immediately after heating c. same as b, but spectrum taken after 15 minutes exposure to air (50% relative humidity) d. same as b, but spectrum taken after 24 hours exposure to air ESR spectra of Cu(II)-montmorillonite films heated to various temperatures for 24 hours ........... a. air-dry b. soaked in 95% ethanol several hours The vertical lines represent the resonant position (9 = 2.0028) of a standard strong pitch sample. The symbols II and indicate the orientation of the ab plane of the si icate layers with respect to the magnetic field, Ho. Page 12 14 16 20 Figure Page 5. ESR spectra of Cu(II)-montmorillonite and Cu(II)- hectorite heated to 215°C for 24 hours ......... 22 a. not exposed to moisture after heating b. exposed to a free water surface for 24 hours PART II 1. Room tempe ature esr spectra for a) MnCl in methanol (5.0 x lo‘ M), and powder samples of hectorite; b) fully hydrated; c) air-dried; and d) dehydrated at 200°C for 24 hours. The vertical lines represent the resonance position of a standard pitch sample (9 = 2.0028) ........................ 37 2. Deaendence of the average m1 = 15/2 line widths of Mn T on interionic distance. Open points are for MnC12 in methanol solution, solid oints are for nontronite (N), Upton (U) and Cham ers (C) montmorillo- nites, and hectorite (H) under fully hydrated conditions ....................... 39 3. Esr spectra for an oriented film sample of air-dry Upton montmorillonite with the magnetic field direction II and l_to the plane of the silicate sheets . . . ...................... 40 4. Roam temperatgre esr spectra of powder samples of 5% Mn +-doped M -hectorite a) fully hydrated, b) air- dried, and c thermally dehydrated at 200°C ...... 45 PART III l. Effects of thermal dehydration and resolvation of Na+, Li+, and Ca + smectites on the esr signal of structural Fe3+ ..................... 51 l. hydrated mineral under ambient conditions 2. mineral dehydrated at 205°C for 24 hours 3 mineral resolvated in ethanol after 205°C heat treatment 0 O + The arrows indicate the weak Fe3 resonance 2. The Fe(III) esr signal of K+, Na+, Li+, and Ca2+ smectites after equilibration at various relative humidities (RH). (The upper, middle, and lower spectra are for 93%, 45%, and 0 +RH, respectively.) The arrows indicate the weak Fe resonances ...... 54 vi Figure PART IV 3+ 2+ . . 1. Structural Fe esr spectra of Ca -montmorillon1te . . . A. air-dry B. heated at 210°C for 24 hours C. resolvated in 95% ethanol after 210°C thermal treatment 2. Relationship of high-field Fe3+ signal intensity to the basal spacing of montmorillonite. (Dehydration of clays exchanged with monovalent ca ions was achieved by 110°C heat treatment, while the Ca + exchange form was dehydrated at 210°C.) ................ O dehydrated clays I air-dry clays ‘ clays solvated in 95% ethanol 3+ Structural Fe esr spectrum of freshly-prepared H30+-montmorillonite ................. . A. air-dry B. heated at 225°C for 30 minutes C. resolvated in 95% ethanol after 225°C thermal treatment Relationship of the size of interlayer cations to the position of the structural OH deformation band. (A11 clays are dehydrated by degassing and heating to 110°C ). vii 'Page 60 62 67 69 INTRODUCTION Recent interest in heavy metals as toxic pollutants of the environment has led to considerable research regarding the interaction of these metals with soils. Transition metals such as copper and manganese are stable in the soil environment as cations (e.g., Cu2+- Mn2+), and as such are able to occupy exchange sites of the clay minerals in the soil. Therefore, the nature of these adsorbed cations must be studied in order to determine the degree of toxicity and avail- ability of clay-bound heavy metals. Such studies are difficult by chemical methods, but spectroscopic techniques allow the clay to be examined undisturbed. Electron spin resonance (esr) spectroscopy may 2+ or Cu2+ on clay surfaces at very be used to detect exchangeable Mn low concentrations because of the paramagnetic properties of these ions. Similar investigations of natural soils are possible, but organic matter strongly chelates many transition metal cations. The present study uses only pure natural smectites; thus, the competing effect of metal chelation by organic matter is not present. Cation positions in clay interlayers are known to be related to the hydration energy of the cation and the hydration conditions of the environment. Although most methods do not allow clay samples saturated with solvent to be studied directly (e.g., infrared spectro- scopy). esr does not have this limitation. Thus, fully hydrated clays may be analyzed in addition to dry clays to determine the nature and position of interlayer transition metal cations as well as the properties of interlamellar water. Paramagnetic exchange ions in the smectite interlayers may reflect their environment through changes in the esr spectrum. Similarly, paramagnetic iron of the clay structure may be affected by conditions at the silicate surface. In effect, these ions act as chemical "sensors" or "probes" to give information about their immediate surroundings. PART I Cu(II) INTERACTIONS WITH MONTMORILLONITE: EVIDENCE FROM PHYSICAL METHODS Introduction Considerable research has been devoted to the study of adsorption and exchange properties of copper ions in soils and pure clay minerals. Although several workers have indicated that specific adsorption of Cu(II) ions on clay takes place (i.e., DeMumbrum and Jackson, 1956a and b), others have concluded that copper retention is similar to that of other divalent ions, suggesting error in interpreta- tion due to the precipitation of Cu(OH)2 (Bingham gt_al., 1964). More recent work in infrared (Calvet and Prost, 1971) and electron spin resonance (ESR) spectroscopy (Clementz et_gl,, 1973) has revealed the strength of physical methods in determining the nature and position of interlamellar cations. For example, exchangeable copper on smectite is strongly hydrated as the Cu(HZO);+ ion in suspension, but upon air- drying, loses ligand water to form the interlamellar Cu(H20)Z+ species. Its hydrated nature is important in considering the feasibility of specific adsorption mechanisms that have been proposed. In this study, Cu(II)-montmorillonites were heated in order, to determine the changes associated with dehydration of the copper ions on the clay surface. Although Li(I) ion movement into the vacant octahedral positions of dioctahedral clays upon heating is a well-known phenomenon, divalent cation migration into the clay structure has not been closely studied. Farmer and Russell (1967) reported that 400°C heat treatment of Mg(II)-montmorillonite reduced the cation exchange capacity (CEC) by one third. Calvet and Prost (1971) concluded from infrared studies that Ni(II) and Be(II) ions are able to occupy octa- hedral sites as well, while Ca++ and K+, because of their size, cannot penetrate the silicate layer. It was expected from these previous studies that Cu(II), because of its small ionic radius, could enter the octahedral positions. Electron spin resonance spectroscopy has been used to assess the hydration state and stereochemical orientation of copper ions on montmorillonite surfaces (Clementz gt_§l,, 1973; Clementz gt_gl,, in press) and could, therefore, be applied to these heated clays. Also, the changes in the l'OH deformation" region of the infrared spectrum of heated montmorillonites allow interpretations to be made regarding the position of the interlamellar cations. Calvet and Prost (1971) described the effect of Li(I) movement into octahedral sites on the 600-950 cm'1 infrared region. Farmer and Russell (1964) have shown that perturbation of these bands can result simply from the dehydration of the interlayer cations, even when no ions penetrate to the octahedral positions. The actual extent of migration of cations into octahedral positions (i.e., the degree of CEC reduction) has been difficult to measure because layer charge reduction produces non-expanding clays which contain some cations in the "hexagonal holes" formed by oxygen atoms on the surface of the 2:1 structure. Brindley and Ertem (1971) found that a few solvents, including ethanol, would expand these reduced- charge clays. Thus, in this study, ethanol was used to expand heated montmorillonite and solvate ions positioned in the hexagonal holes of the lattice and make them available for exchange, thus permitting an accurate measurement of the actual loss in layer charge upon heating. Methods and Materials The Cu(II)-saturated montmorillonite was prepared by separa- tion of the < 2p clay fraction of an Upton, Wyoming bentonite having the chemical formula: ++ , M 0.32 [A‘3.06 Feo.32 ”90.663 (A'o.io 5‘7.9o) 020(0H)4 (Ross and Mortland, 1966). It was washed in CuCl2 solution five times and the suspension was then dialyzed with distilled water until the suspension supernatant gave a negative AgNO3 test for chloride. The CEC of the clay was determined to be 89.3 meq. Cu(II) per 100 grams by conductometric titration with NaOH, and 86.9 meq. Cu(II) per 100 grams by repeated exchange with lMCaC12 solution at pH 4 and analysis by atomic absorption. This establishes the validity of the conductometric method as well as the divalent nature of the exchangeable copper ion. Heat treatments were done, in an oven controlled to :_1°C, on self-supporting films dried from the clay suspension. These films, placed on glass slides, were X-rayed on a Norelco diffractometer using filtered Cu-radiation. Infrared spectra of the films were obtained on the Beckman IR-7 spectrophotometer. The CEC values of the montmorillonite were determined by suspending the samples in 1:1 H20-95% ethanol and titrating this suspen- sion conductometrically with 0.01 N NaOH to determine the amount of exchangeable copper. Heat treatment for CEC analysis involved placing the freeze-dried clay samples in an oven at various temperatures for twenty-four hours. Electron spin resonance spectroscopy of the oriented clay films was done in quartz tubes using a Varian E4 spectrometer. Orienta- tion studies on these films were accomplished by the method of Clementz g5;gfl,(1973). Magnetic susceptibility measurements were done on about 1.0 gram samples of freeze-dried clay by the Gouy method. Results and Discussion X-ray diffraction and CEC data.--While an air-dry Cu(II)- montmorillonite film has a 12.4A basal spacing, representing a monolayer of Cu(H20)Z2 ions (Clementz gt_al,, 1973), the collapsed spacings observed for clays heated to 156°C and above indicate loss of ligand water (Table 1). This does not imply that the ion is totally dehydrated; thermogravimetric studies of the Cu(II)-montmorillonite (unpublished data) indicate that 1-2 water molecules per Cu(II) ion are present at 200°C. Such a monohydrate (or dihydrate) form is likely very similar to that proposed by Farmer and Russell (1964) for Mg(II)-vermiculite, with both the Cu(II) ion and its associated water molecule(s) positioned in hexagonal holes of the silicate layer. Ethanol expanded the basal spacings of all montmorillonite samples to 17.0-17.3 A regardless of the temperature of heat treatment (Table l). Brindley and Ertem (1971) had similar observations for reduced charge Li(I)-montmorillonites that were unable to swell in water. However, these Cu(II)-clays were able to slowly expand in water after heating to 150-200°C, but failed to re-expand after the 270°C heat treatment. Although both 200°C heated Cu(II) and Li(I)- montmorillonites swell to 17 A in ethanol, subsequent washing of these clays in water results in collapse of the Li(I)-clay to 9.9 A while the Cu(II)-clay expands further to 21.5 A. The apparent difference between the two heated clays is that the Li(I) form loses enough charge to become non-expandable in water, while the Cu(II) form must retain enough charge to remain expandable. The greater the hydration energy of the exchange cation, the lower is the critical layer charge still permitting swelling in water (Brindley and Ertem, 1971). An interesting observation from Table 1 is that 1:1 water-95% ethanol swells the clay more as the temperature of heat treatment is increased. From Table 2, no change in CEC is observed until 156°C, while CEC reduction is essentially complete at 200°C. Thus, the loss of layer charge explains the increasing expand- ability of the clays in 1:1 water-95% ethanol as temperature of heating increases. Brindley and Ertem (1971) found similar evidence for increased swelling of clays in ethanol and other solvents as the charge density was lowered. The 1:1 water-95% ethanol solvent expands the layers further than 95% ethanol alone (Table l). The higher dielectric constant of water gives it greater capabilities than ethanol in swelling clays to high basal spacings; however, ethanol is much more effective than water in allowing montmorillonite layers, totally collapsed by heat, to re-expand at all. The 270°C heated Cu(II)-clay gives evidence for this, remaining collapsed in water despite having considerable layer charge (57.0 meq. per 100 grams), yet swelling in both 95% ethanol and 1:1 water-95% ethanol. As Brindley and Ertem have concluded, water and ethanol appear to have completely different mechanisms of layer swelling. Table 1. X-ray 001 spacings of Cu(II)-montmorillonites after 24-hour heat treatment and equilibration in several solvents for two weeks. Heat treatment (C°) Solvent used None 50° 100° 156° 200° 270° air-dry 12.4 12.0 11.7 9.7 9.7 9.3 H20 19.4 19.4 19.6 (9.8) (9.9) 9.7 14.2* 15.2* (9.6) 1:1 21.5 22.0 22.1 23.2* 26.7* 26.7* water-95% ethanol (23.9*) (26.0*) (9.7) 95% ethanol 17.3 17.3 17.0 17.0 17.0 17.0* * very interstratified Parentheses indicate basal spacings observed immediately upon addition of solvent. Table 2. CEC values for Cu(II) montmorillonite heated to various tempera- tures for 24 hours. Heat treatment (C°) Solvent of T'trat'°" None 50° 100° 155° 200° 270° water 83.7 80.7 84.8 61.9 38.9 17.0 1:1 water- 95% ethanol 89.3 82.5 89.8 68.2 59.6 57.0 ID The CEC data (Table 2) show a correlation of layer charge reduction to the collapsed basal spacings (air-dry) of Table l. Titra- tion in 1:1 water-95% ethanol is considered to give actual layer charge measurements, while titration of the clay in water results in CEC data which depend on the quantity of layers which can be re-expanded after heating. As Table 1 indicates, increased temperatures of heating caused more and more clay layers to be non-expanding in water, until the 270°C treatment resulted in a CEC corresponding to only external surface Cu (II). The value of 17.0 meq. per 100 grams is very near the expected CEC of external sites of smectite. As with Li(I)-montmorillonites, the loss of layer charge is interpreted as being a result of Cu(II) migra- tion into vacant sites of the dictahedral clay. Unlike the Li(I)-clays, charge loss is only about 1/3 of the total CEC, even at 270°C. A second heat treatment of a reduced-charge Cu(II)-montmorillonite (58.0 meq. Cu(II) per 100 grams) was done at 200°C after resaturation of remaining exchange sites with Cu(II) using three washes in 1:1 95% ethanol-1.0M CuCl2 solution. The resulting CEC from NaOH titration of the clay sus- pended in 1:1 water-95% ethanol was 58.3 meq. Cu(II) per 100 grams, indicating that the initial reduction in charge is a maximum. Greater Coulombic repulsion forces for the divalent copper ion than for the monovalent lithium ion in the silicate lattice may account for the reduced entry of Cu(II) into octahedral sites. Movement into these sites requires considerable dehydration of the Cu(II) ions so that they may first enter the hexagonal cavities of the lattice, as shown by the 9.3-9.7 A basal spacings of the reduced charge clays. The almost totally dehydrated Cu(II) ion positioned in a hexagonal hole of the ll silicate is the transition state for migration into the octahedral vacancy of the dioctahedral mineral (Calvet and Prost, 1971), at which time complete dehydration must occur. For cations with no direct access to vacant octahedral sites, the "transition state" is a metastable position as long as the clay is prevented from rehydrating. Infrared Spectroscopy Significant perturbations of the infrared 0H deformation region for Cu(II)-montmorillonite after heating to 200°C are apparent (Figure 1. a,b). The 848 cm.1 band, assigned to OH associated with AlMg pairs (Calvet and Prost, 1971), is lost upon heating and seems to 1 be shifted to a frequency higher than the 855 cm' band observed by Calvet and Prost upon heating a Li(I)-montmorillonite. It appears as a shoulder on the low frequency side of the 889 cm"1 , assigned to OH associated with AlFe pairs (Farmer and Russell, 1967). However, re- expanding the heated clay in 1:1 water-95% ethanol or titrating the clay with NaOH in 1:1 water-95% ethanol give identical spectra (Figure l. c, d). A shoulder around 850-860 cm"1 appears, while the shoulder on the 889 cm'1 band is lost, indicating a significant shift of the AlMgOH band to lower frequency. Both of the above treatments swell the layers, allowing resolvation of only the Cu(II) ions that are present in hexa- gonal hole positions, thus causing the ions to move out of these sites. The titration with NaOH after swelling the layers produces Cu(OH)2, assuring the removal by precipitation of all Cu(II) ions from the hexa- gonal sites. Thus, the effect of removal of Cu(II) ions from the lattice hexagonal holes is to reduce the perturbation of the 0H associated with 12 um» in in in an WAVE NIGER ten") Figure l. Cu(II)-montmorillonite, infrared 0H deformation region: a. air-dry b. after 24 hour 200°C heat treatment c. after 24 hour solvation of b in 1:1 water-95% ethanol d. after NaOH titration of b l3 AlMg pairs. This is expected, since Cu(II) ions embedded in hexagonal hole positions would be situated directly above structural OH groups, and could change the direction of the dipole moment of the OH group as well as perturb its deformation vibration. Movement of Cu(II) into vacant octahedral sites must account for the remaining shift of the AlMgOH group from 849 to 850-860 cm" after the other Cu(II) ions have been removed from hexagonal cavities, since the spectrum of the unheated Cu(II)-montmorillonite cannot be regained by the solvation and titration techniques mentioned above. This "octahedral shift" is very similar to that observed for Li(I) montmorillonite after heating (Calvet and Prost, 1971). The larger "hexagonal hole shift" was not observed for the Li(I)- clay, probably because a divalent ion in the hexagonal hole perturbs the 0H deformation much more than a monovalent ion in the same position. The AlFeOH band near 889 cm"1 and the MgMgOH band at 803 cm"1 (Calvet and Prost, 1971) are not affected by heating the Cu(II)- montmorillonite to 200°C. However, the AlAlOH band at 920 cm'] is shifted to near 930 cm'] upon heating, and shifts back to 925 cm-1 when the hexagonal hole Cu(II) ions are resolvated (Figure l). The explana- tion for this observation is very similar to that for the AlMgOH band shift, but the effect on the A1A10H groups by Cu(II) moving into vacan- cies associated with AlMgOH groups must be less direct. From Figure 2, it is apparent that Cu(II) ion movement into hexagonal hole and octahedral positions of the silicate lattice does not occur until well above 100°C. This observation is confirmed by the CEC and X-ray data (Tables 1 and 2). The infrared spectrum of the 0H deformation region, after being altered at 156°C, changes little from l4 ABSORBANCE 4 1000 900 860 760 600 um NUMBER 1cm") Figure 2. Cu(II)-montmorillonite, infrared OH deformation region: air-dry after eating 24 hours to 100°C after heating 24 hours to 156°C after heating 24 hours to 200°C after heating 24 hours to 270°C {DD-DUO) 15 156°C to 270°C, indicating that most of the Cu(II) migration has occurred at 156°C. The CEC data show that 2/3 of the maximum layer charge reduction took place at 156°C, while maximum reduction was reached at 200°C. No change in the OH deformation region has been observed for Ca(II)-montmorillonites heated to 400°C (Calvet and Prost, 1971). However, from the above discussion it would be expected that Ca(II) ions, although too large to enter vacant octahedral sites, could occupy hexa- gonal holes and perturb the 0H deformation vibrations in a manner similar to Cu(II) ions. This was observed (Figure 3), since heating Ca(II)-montmorillonite to 200°C caused the 847 cm"1 band to broaden and shift to 854 cm". After a few minutes of exposure to air, the band returned to its original position, indicating that the Ca(II) ions were moving out of hexagonal cavities and rehydrating, thus removing the perturbation on the structural OH groups. The regeneration of the original 0H deformation spectrum was nearly complete after 24 hours (Figure 3, c), indicating almost complete rehydration. In contrast, the reduced charge Cu(II)-montmorillonite does not rehydrate readily and Cu(II) ions remain in hexagonal holes, as indicated by the lack of change in the 0H deformation region of a Cu(II)-montmorillonite exposed several days to air (40% relative humidity). This inability to rehy- drate in the atmosphere is most likely related to the loss of layer charge. Montmorillonite samples exchanged with various other cations were heated to 215°C for 24 hours, and the spectra were examined for irreversible changes in the 0H deformation region after rehydration in ABSORBANQL in in ’fifi' an mwtmmum urn Figure 3. Ca(II)-montmorillonite, infrared 0H deformation region: a. b. C. d. air-dry after 24 hour 200°C heat treatment, spectrum taken immediately after heating same as b, but spectrum taken after 15 minutes exposure to air (40% relative humidity) same as b, but spectrum taken after 24 hours exposure to air 17 air. All clays showed the marked shifts due to movement of dehydrated cations into hexagonal holes, but only the Cd(II) and Ca(II) clays 1 band as rehydration in air took showed regeneration of the 847 cm' place. This indicates that Cd(II) cations (as well as Ca(II) as dis- cussed before , and Na(I) and K(I) as determined by Calvet and Prost) cannot migrate into octahedral sites; the ionic radii of these ions are too large to permit penetration (Table 3). However, as Calvet and Prost (1971) have stated, movement into vacant octahedral sites is a general phenomenon of all small cations, as Table 3 indicates. Although the enthalpy of hydration of the cations must influence the temperature at which dehydration begins, and thus affects the temperature at which penetration of the structure begins, it is not a factor in determining whether or not migration to octahedral sites will occur. For example, Ca(II) and Cd(II), although having the lowest hydration enthalpies of all the divalent cations listed in Table 3, are not able to penetrate to the octahedral layer. While multivalent ions have the ability to penetrate the structure, charge seems to affect the extent of penetra- tion as shown by the limited layer charge reduction by Cu(II) as com- pared to Li(I). ESR Spectroscopy The Cu(II)-montmorillonites, after heat treatment to various temperatures, were investigated in the air-dry state by electron spin resonance spectroscopy, using oriented films parallel and perpendicular to Ho, the magnetic field. The orientation technique allows determina- tion of the degree of anisotropy of the Cu(II) resonance (Clementz 18 Table 3. Ability of interlamellar cations to occupy octahedral sites of montmorillonite upon 215°C heat treatment for 24 hours. Cation Ionic radius (K)+ Enthalgy]o;m2 grationi ozigfizgag1oqa32r 8e++¥ 0.35 -596 + Mg++ 0.55 -459 + Li+ 0.68 -124 + Rh+++ 0.68 Unknown + N1++ 0.69 -503 + Cu++ 0.72 -502 + 0o++ 0.72 -477 + Zn++ 0.74 -489 + co++ 0.97 -432 - Na+* 0.97 -97 - Ca++ 0.99 -377 - K+* 1.33 -77 - * From Calvet and Prost (1971) T From Handbook of Chemistry and Physics * From Cotton and Wilkinson, Advanced Inorganic Chemistry, 3rd edition 19 §t_gl,, 1973). Decreasing Cu(II) signal intensity is apparent with increasing temperature of heat treatment (Figure 4 a). However, regen— eration of signal occurred upon re-swelling the films in 95% ethanol (Figure 4 b), indicating that Cu(II) ions in the hexagonal holes of the unexpanded clay either form bonds which cause the unpaired d-orbital electrons of Cu(II) to become paired, or the ions in the hexagonal environment experience relaxation effects which broaden the ESR signal to the point of being unobservable. The latter explanation is more probable for two reasons: (a) a covalent Cu(II) bond should not simply be broken by resolvation of the Cu(II) ion by ethanol, unless it is very weak; (b) enhanced interaction of the Cu(II) ion with lattice vibration and paramagnetic Fe(III) ions of the structure can result when the ion is no longer hydrated and rapidly tumbling, but is posi- tioned in the hexagonal holes of the lattice. This interaction causes a very short spin-lattice relaxation time which broadens the ESR signal greatly (Adrian, 1968). The 156°C heat treatment reduces signal intensity greatly, with little more reduction in intensity from 156° to 270°C, verifying the conclusion from infrared data that layer col- lapse and Cu(II) dehydration and movement into hexagonal holes first occurs (and is virtually complete) with the 156°C treatment. Part of the weak Cu(II) signal of the clays heated to 156°C-270°C and then equilibrated in air is isotropic with an electronic 9 factor of about 2.16 (Figure 4 a), and probably results from rehydration of external surface Cu(II) ions in the air, allowing the Cu(HZO);+ species to form and tumble rapidly at the surface (Clementz gt_al,, in press). The more narrow resonance near 9 = 2.06 is an anisotropic signal, showing little --.o’\- .i l omsmn‘ Modulation - B Gain - 1250 Modulation . 8 Coin - 1600 ' Modulation - 8 Gain ' 2000 n Modulation- 8 Figure 4. 20 b Gain - 630 Modulation . 12.5 Goin - 2000 “000101100 ' 8 t A 200' 4;" ‘ Gain . 2000 Modulation . 8 ESR spectra of Cu(II)-montmorillonite films heated to various temperatures for 24 hours: a. air-dry b. soaked in 95% ethanol several hours The vertical lines represent the resonant position (9 = 2.0028) of a standard strong pitch sample. The symbols II and l_indicate the orientation of the ab plane of the silicate layers with respect to the magnetic field, Ho. 21 intensity when the clay film is oriented perpendicular to the magnetic field, ”0' Thus, 2.06 is the value for gL(Clementz et_al,, 1973), and the anisotropic signal represents Cu(II) in a restricted interlamellar environment. The guvalue for the anisotropic signal could not be accurately determined because of low signal intensity, but is known to be near 2.34 (Clementz §t_al,, 1973). As the anisotropic spectra of the clays heated to 50°C and 100°C reveal, (Figure 4 a), Cu(II) is largely in a restricted environment of tetragonal symmetry after heating to these temperatures, the Cu(H20)Z+ species predominating in the inter- layer regions. Resolvation of Cu(II) ions by swelling the layers of heated clays in ethanol for several hours regenerates much of the ESR signal, as stated above. This signal, although quite isotropic for clays heated to low temperatures, shows more anisotropic character for those heated at higher temperatures. The isotropic signal at g = 2.15 indicates that the Cu(II) ions are solvated and tumbling rapidly between layers that are known from X-ray studies to be about 7.0A apart. The anisotropy at higher temperatures indicates that some layers did not totally expand upon addition of ethanol, thus preventing complete resolvation of all Cu(II) ions. Some layers of the clays heated at high temperature must not have re-expanded at all, since signal intensity after ethanol treat- ment was reduced as temperature of heating increased. If Cu(II)-montmorillonite is not exposed to air after 24 hours of heating at high temperature (215°C), almost no signal can be observed (Figure 5 a), indicating that almost completely dehydrated Cu(II) ions give almost no ESR signal (a weak anisotropic signal is present), nor 22 0 I ' Goin=5000 " Gom=6,300 ,. . Modulation=25 Modulation 8 0 Montmorillonite r4000m$* Hectorite Gain = 4,000 GMn=1200 ” MMUIOIIW‘B MO0MON00=8 Figure 5. ESR spectra of Cu(II)-montmorillonite and Cu(II)-hectorite heated to 215°C for 24 hours: a. not exposed to moisture after heating b. exposed to a free water surface for 24 hours 23 do Cu(II) ions that enter octahedral sites of the structure. For both types of Cu(II), the loss of resonance can be explained by a too effi- cient spin-lattice relaxation, as described before. However, spin- pairing through a partially covalent Cu-O bond is a possible explanation for signal loss of Cu(II) which has migrated to vacant octahedral sites of the silicate lattice. Irrespective of the mechanism of signal loss, it can be stated that none of the ESR signals observed can have any appreciable intensity resulting from structural Cu(II). Thus, the isotropic resonance observed for air-dry films after heating to 156°C- 270°C (Figure 4 a) must have resulted from rehydration of external sur- face Cu(II) as proposed above, since the X-ray diffraction data indicate that the clay heated to 270°C cannot re-expand in water to allow inter- layer cation rehydration. The weak anisotropic signal of the same films must have been due to restricted Cu(II) of the internal surfaces. In Figure 5, a similar anisotropic signal is observed for hectorite not exposed to air after heating to 215°C, again indicating that partially or totally dehydrated Cu(II) in hexagonal holes of the internal surface produce the signal. Penetration of Cu(II) into the octahedral layer is impossible in hectorite since it is trioctahedral. This resonance, hardly detectable for Cu(II)-montmorillonite if not exposed to the air after heating to the same temperature (Figure 5), may be reduced in intensity by the presence of Fe(III) in montmorillonite, hectorite containing very little Fe(III). Equilibration of both the heated hector- ite and montmorillonite with a free water surface for 24 hours resulted in isotropic signals becoming fairly strong (Figure 5 b), indicating hydration of internal as well as external surface Cu(II) ions. 24 Magnetic Susceptibility Studies As mentioned above, loss of ESR signal intensity may be a result of electron pairing (bonding) or relaxation processes. The former reduces the number of unpaired electron spins in a system while the latter does not. The magnetic susceptibility of a sample is a measurement of these unpaired spins and does not depend on spin-lattice relaxation processes. The magnetic moment of the electron interacts with an external magnetic field to change the apparent weight of the sample. Such susceptibility measurements were made on three freeze- dried montmorillonite samples: Cu(II)-saturated, Cu(II)-saturated and heated 24 hours at 200°C, and Ca(II)-saturated. For the Ca(II)- montmorillonite, the susceptibility was totally attributed to Fe(III), the only paramagnetic ion of appreciable quantity in the clay. The resulting effective magnetic moment, ueff’ was 6.44 B.M. (Bohr magnetons), close to the value of 5.9 observed in many high-spin Fe(III) compounds (Figgis and Lewis, 1964). The theoretical magnetic moment for the high- spin Fe(III) ion (S = 5/2) is 5.92 B.M., and can be altered little by crystalline electric fields of a lattice, since its total orbital angular momentum is zero (L = 0) and there can be no spin-orbit coup- ling (J. Smart, 1966). Thus, Fe(III) in montmorillonite is in the high-spin state and its spin-only magnetic moment is not much changed by the silicate lattice. Paramagnetic impurities in the clay may account for the somewhat high value for Fe(III). Still calculating susceptibilities on the basis of one mole of Fe(III) in montmorillonite, Cu(II)-clay had "eff = 6.27 B.M., and 25 the same clay heated to 200°C for 24 hours had ueff = 6.61 B.M. The Cu(II)-montmorillonite would be expected to have a susceptibility higher than the Ca(II)-clay, since Cu(II) in most compounds has ”eff = 1.9 B.M. The low value observed for the unheated Cu(II)-clay indicates that spin interaction between Cu(II) and Fe(III) may be occurring, a process of antiferromagnetism which is often observed in lattice structures (Figgis and Lewis, 1964). Superexchange, or spin exchange through intervening nonmagnetic atoms is the explanation for this phenomenon, and oxygen atoms are very effective in providing spin transfer, which would be necessary for superexchange to operate in silicates. Spin exchange between Cu(II) and Fe(III) may occur over several Angstrom units through structural oxygen atoms coordinated to these transition metals. The fact that heated Cu(II)-montmorillonite has a magnetic susceptibility similar to the unheated Cu(II)-clay substantiates the conclusion that the loss of ESR signal of this clay upon heating must not be a result of Cu(II) spin pairing either by bonding to the silicate or by Cu-Cu interaction. The susceptibility for the heated Cu(II)- montmorillonite appears to be higher than that for the unheated Cu(II)- clay because of loss of hydration water upon heating. General Discussion Analysis of all the data presented indicates that Cu(II) ions remain hydrated on the clay interlamellar surfaces until enough thermal energy is provided to remove ligand water and permit migration into hexagonal holes of the lattice. Further movement through these sites 26 into empty octahedral positions of the montmorillonite occurs simultaneously but is limited, probably by the divalent charge on the ions. Ions considerably larger than Cu(II) cannot enter octahedral sites because of their size, and thus cannot reduce the layer charge. The bond between Cu(II) and structural 0H of clay proposed by DeMumbrum and Jackson (1956b) is not possible at ambient temperatures because the interlamellar CU(H20)Z+ species of air-dry montmorillonite has a structure which would permit closest approach of Cu(II) and structural CH of more than 4A. This distance is longer than any con- ceivable bond-length. Heating to about 150°C removes ligand water, permitting the dehydrated Cu(II) ions to approach structural 0H by mov- ing into hexagonal cavities. Still no evidence for bond formation is found, since resolvation of the Cu(II) in hexagonal holes occurs upon layer expansion. The infrared evidence given by DeMumbrum and Jackson to support Cu-O bonding should be reconsidered for the following reasons: 1) mixing and compression of clay samples into KBr pellets allows ionic exchange with K(I) from the KBr to form K(I)-saturated clays (Mortland, unpublished data). Thus, the spectra reported would not be of Cu(II)-clay, but of K(I)-clay; 2) the bands at 6.4 and 7.0u (1562 and 1428 cm'1), attributed by DeMumbrum and Jackson (1956b) to Cu-oxygen bonds, are at a frequency too high for such vibrations, which usually are found in the far infrared region (i.e., 300-600 cm“). An interesting observation is that these two bands coincide with the two most intense absorption bands for acetate anion, suggesting that if the clays were prepared with copper acetate, not all of the excess salt had been removed when the clay was saturated. This further suggests that 27 excess Cu(II) found above the exchange capacity might be due to Cu acetate; 3) the width of the 0H stretch bands of their spectra indicate that most of the peak intensity is due to adsorbed and coordinated water (which varies with the nature of the exchangeable cation), so that measurement of the structural 0H peak intensity under the conditions described is not a valid quantitative method. I Hodgson gt_al, (1964) have proposed endothermic chemisorption to explain "specific adsorption" of Co(II) on montmorillonite surfaces. However, the heat of reaction they calculate (about 15 kcal./mole) for Co(II) adsorption is not nearly sufficient to overcome the hydration energy of -477 kcal./mole for Co(II) and this exchange ion, like Cu(II), remains hydrated to high temperatures in the interlayer and on external surfaces. Thus, interaction of Co(II) with the lattice OH groups at the experimental temperatures that Hodgson gt_al, used is not likely for the same reasons given in the discussion on the feasibility of specific adsorption of Cu(II). The present study agrees with the find- ings of Bingham gt_al, (1964), that Cu(II) behaves much like other divalent ions on clays. A thermodynamic investigation of Cu(II)-Ca(II) exchange on montmorillonite (El-sayed gt_al,, 1970) supports this view, indicating that Ca is somewhat preferred to Cu on the exchange sites even though the entropy term points to a more orderly structure of Cu ions on the surface. The latter observation implies structured hydra- tion of Cu(II) ions on interlamellar surfaces as has been demonstrated by ESR (Clementz gt_al,, 1973). This ordering would be expected to be greater for Cu(II) than Ca(II) for two reasons: (a) the Cu(II)-H20 bond is partially covalent with d orbital-ligand orbital interaction; (b) the 28 coulombic ion-dipole interaction between cation and ligand is stronger for Cu(II) than Ca(II) because of the smaller copper ion radius. The penetration of the octahedral layer by Cu(II) and other ions (Table 3) is not likely to be of importance in soils, but may be possible in the geologic column where the environment (temperature and pressure) might promote such interaction. PART II ELECTRON SPIN RELAXATION AND THE MOBILITY OF MANGANESE(II) EXCHANGE IONS IN SMECTITES Introduction The significant chemical and physical properties of smectites often depend on the nature of the interlayer exchange ions. To better elucidate the structure and mobility of the exchange ions, several workers have recently applied electron resonance spectroscopy to certain Cu2+ 2+-montmorillonites, - and Mn2+-saturated forms. Fully hydrated Cu for example, have been shown to possess tetragonal Cu(HZO)62+ ions which tumble rapidly in expanded interlayers containing several molecular layers of water (Clementz gt_al,, 1973). Upon drying the mineral in air, the exchange ions lose two axial water ligands which are held weakly because of Jahn-Teller distortion of the d9 electronic configuration of the metal ion. The resulting planar Cu(H20)42+ ions are confined to 2.8A-thick interlayer regions with the symmetry axis of the complex ion oriented at 90° to the silicate lamellae. Furuhata and Kuwata (1969) have reported that the widths of the hyperfine (hf) lines of hydrated Mn2+ are broader on the exchange sites of montmorillonite than in bulk solution. The increase in line width was attributed to relaxation effects of the more restricted surface-adsorbed ions. Also, Mn2+-montmorillonite has been reported to exhibit broader hf lines when larger molecules (i.e., pyridine) replace water on ligand positions, a result again interpreted in terms of reduced mobility of the Mn2+-solvent complex because of the size or bonding nature of the ligand molecules (Pafomov, et a1., 1971; 3D 31 Taracevich and 0vcharenko, 1972). However, in addition to mobility effects, other factors such as the site symmetry of the paramagnetic ion and dipolar interactions can also contribute to the observed esr line widths. The present study investigates the esr spectra of Mn2+ 3 smectites of differing charge density and structural Fe + content under hydrated and anhydrous conditions. The relaxation mechanism control- ling the hf widths of the mineral-bound ion has been defined, and a quantitative estimate of interlayer mobility has been obtained for fully hydrated hectorite. Materials and Methods The smectites used in this study are given in Table 1 along with their cation exchange capacities and reported unit cell formulas. The Mn2+-saturated exchange forms were prepared by washing the native mineral (<2 u fraction) with aqueous 1.0 M_Mn012. Excess salt was removed by dialysis with distilled water, and the mineral was recovered 2+ 2 from the slurry by freeze-drying methods. Average Mn -Mn + distances within an interlayer were estimated from the CEC values and the theo- retical surface area of 800 mz/g (Grim, 1968c). X-ray basal spacing were determined with a Norelco diffracto- meter and Ni-filtered Cu-radiation. The magnetic susceptibilities of 2 Mn +-hectorite, -montmorillonite, and -nontronite were measured by the Gouy method. The susceptibilities were corrected for paramagnetic contributions due to structural Fe3+ by subtracting the susceptibilities 2+ exchange forms of the minerals. Only 3+ obtained for the Na+ or Ca Na+-hectorite, which has a low Fe content, showed no paramagnetism. 32 me Humvee; .~m< VAIOavaNCAOComPWVflNoooFgm> + .eeweeowceoeeomeooee meeoeoeecc .o:e_ cocooeeccee eeoem r 34 +mm.e_ .emm.ep Aoee_nv oeepsoe5eo> 5N.m_ «m.m_ Ameoeseeov eo_eo_FeeoEp=oz ¢.mF m.ep muwcocucoz m.m_ m.e_ Aeooeav ooe:o__e262oeoz o.NN o.m~ ooeeooooz chmcwz chwcwz vacate»: »__=o ago te< .mmmeVme cmxop empmczummu+ ch mo A><-o|t3%§>, |13/2><—->|:1/>, and |-l/><——>|+lléwhich are not resolved at X band frequencies. The non-degeneracy of the AmS = 1 transitions leads to inhomogeneous line broadening. The line widths are the sum of two + AH contributions, AHi = AH where AHI is the width arising from Ii Di ion-solvent collisional relaxation processes (Rubinstein et a1., 1971; Luckhurst and Pedulli, 1971) and AH is the width due to dipolar inter- D actions between neighboring Mn2+ ions (Hinckley and Morgan, 1966). The AHD term is concentration dependent, because the dipolar interactions 3 2+ - Mn2+ distance. are proportional to r‘ , where r is the average Mn In dilute solution (<0.01 M, r > 55 A) the lines are narrow and deter- mined exclusively by AHI. Increasing the concentration causes the six hf components to broaden markedly until at concentrations of 2.3 M_or greater (r < 9.0 A) the hf structure is lost and the spectrum appears as a single broad line (Hinckley and Morgan, 1966). A typical spectrum for MnCl2 in dilute solution is shown in Figure l a. Fully hydrated Mn(II)-hectorite exhibits a similar “solution- like" spectrum, except that the hf lines are broader (Figure l b). Reducing the amount of interlayer water from several to two molecular layers by allowing the mineral to dry in air at m50% relative humidity causes the lines to broaden markedly (Figure l c). Thenmal dehydration at 200° for 24 hours leads to still further line broadening and almost complete loss of hf structure (Figure 1 d). Similar increases in line width with decreasing hydration are observed for the Mn2+-saturated montmorillonites and nontronite. 37 0 1 1 IOOGAUSS H 5 b 66111411102 C II Figure 1. Room temperature esr spectra for a) MnClZ in methanol (5. 0 x 10 5M), and powder samples of hectorite; b) fully hydrated; c) air-dried; and d) dehydrated at 200°C for 24 hours. The vertical lines represent the resonance position of a standard pitch sample (9 = 2.0028). 38 Since the average interlayer exchange ion distance for each mineral is in the range 10-14 A, the widths of the Mn2+ signals should be determined mainly by the AHD term. This is verified by the compari- son in Figure 2 of the average width of the mI = i5/2 lines for MnCl2 in methanol solutions and the fully hydrated minerals. Further evidence for the importance of dipolar broadening is provided by the spectrum of Mn2+-vermiculite. The interlayer exchange ion distance in this latter mineral (6.9 A) is substantially smaller than 9.0 A, and, as expected, only a single, broad line with a width of 710 G is observed. The broadening is similar to that observed for the solid MnCl2 salt (830 G) and consistent with dipole-dipole coupling between magnetic ions 3 to 8 A apart (Abragam and Bleaney, 1970). In addition to dipolar coupling between Mn2+ ions within an interlayer, analogous interactions between ions in adjacent interlayers may occur. Also, the Mn2+ 3+ i ions may be relaxed by coupling to Fe n the silicate structure. These dipolar interactions along the crystal- lographic g_direction should differ from those in the a_b_plane and should be manifested as differences in line widths and g values when the magnetic field direction is oriented II and 1_to the silicate sheets. Figure 3 illustrates the spectra obtained for an oriented film sample of air-dried Upton montmorillonite. The average width of the mI = :5/2 lines is 15 G larger for the II than for the 1_orientation. The 9 values also differ slightly for the two orientations, 2.005 gs, 2.000 for the II and l_orientation, respectively. Figure 2. 39 120-. ‘ C N 1 1 Inq P c I I l I 0 “q 35 u a ‘ -3 3 c1 '2' 3 HI I 3 “1 .1 40‘ 1 + mi 7 V 1 V f 20 40 00 .0 100 avenue: Mo-Mo nuisnloulc olsnucul) Dependence of the average mI = :5/2 line widths of Mn2+ on interionic distance. Open points are for MnCl2 in methanol solution, solid points are for nontronite (N), Upton (U) and Chambers (C) montmorillonites, and hectorite (H) under fully hydrated conditions. 40 it Figure 3. Esr spectra for an oriented film sample of air-dry Upton montmorillonite with the magnetic field direction I] and l_to the plane of the silicate sheets. 41. Table 3 summarizes the widths for oriented samples of each mineral under air-dried and fully hydrated conditions. With the excep- tion of hectorite, the widths for the air-dried samples differ by 93, 15 G for the two orientations, whereas the widths are more nearly equal for the fully hydrated samples. The dependence of the magnetic aniso- tropy on hydration state is not unexpected. In the air-dried minerals, the motion of the Mn(H20)52+ ions is confined to the a_b_plane, but in the fully hydrated state the ions may tumble more nearly randomly. Near random tumbling would tend to average the magnetic anisotropy. Both dipolar interactions between Mn2+ and structural Fe3+ 2 and differences in the average Mn2+ - Mn + distances in the a_b_plane and the 9 direction appear to contribute to the observed anistropy. Among the f0ur minerals, hectorite has the lowest Fe3+ content (<0.14%). Also, in the air-dried state it exhibits the minimum disparity between Mn2+ distances within an interlayer and across interlayers. Conse- quently, it shows little or no anisotropy in the air-dried, as well as the fully hydrated, state. Air-dried nontronite and the montmorillon- 3+ ites, on the other hand, contain greater amounts of Fe (gj,, Table l) and exhibit a greater disparity in exchange ion distances. Thus, these minerals exhibit anisotropic line broadening in the air-dried state. Even in the fully hydrated state, however, the tumbling of Mn(H20)62+ in nontronite and Chambers montmorillonite does not appear sufficiently random to completely average the anisotropy. 2+ _ Fe3+ The importance of Mn dipolar interactions in the 2+ case of Mn -nontronite is indicated by line widths which are larger than those for Chambers montmorillonite, despite longer exchange ion 42 .cmxmpcmucw cm segue: meow :z cmmzumn mucmymwu mmmgm>m mgp m? mesh + +~ .mcm_a.m.m uwcqmcmoppmpmxgo use op.fl ucm __ vmacmwco :owuomgwu upmwm uppocmme ms» saw: mucmcoaeoo oucmcomoc N\mn u He one to; mmsmm cw mozpm> wmmcm>m mzp mew mguuwz on» r me me om mm m.mp wpwgouum: Acouaav mm mm opp mm_ o.NF mpwcoppegospcoz mo_ mFF wep mop m._p mpwcogpcoz AmcmnEmcov um mo_ opp mmp n.o_ muwcoppwgospcoz A __ A __ L .8535 as ~mcwzwz umumcnaz a—Fsm chmcwz umwcoucw< 1+~cz cmxupcmch .rmmuwuumEm- c: vmucmwgo mo msuuwz weep Emu .m «Fame +N 43 distances in the former mineral. Their importance is further underscored in the dehydrated mineral where the ion occupies a hexagonal position or a vacant octahedral site very near Fe3+ in the silicate structure. Under these conditions, the lines are so broadened that they are not detect- able. There is little doubt that much of the line broadening observed on passing from fully hydrated to air-dried and thermally dehydrated 3 nontronite is due to increasing dipolar interactions with Fe + as the 2+ Mn ions move nearer the silicate structure. Similar effects are pro- bably less important but still operative for the montmorillonites. In hectorite, where little Fe3+ is present, the increase in line widths with decreasing hydration state can only be interpreted in terms of reduced mobility of the interlayer. However, even in this latter case, it is difficult to assess quantitatively the interlayer mobility, because the line widths are still determined by an interionic dipolar relaxation mechanism involving neighboring Mn2+ exchange ions. In absence of dipolar interactions, spin relaxation of Mn(H20)62+ in solution results from molecular collisions between the solvated ion and solvent molecules which cause random distortions of the complex and induce a zero field splitting (Rubinstein gt_gl., 1971; Luckhurst and Pedulli, 1971). Under appropriate conditions, it is pos- sible to obtain a quantitative comparison of the correlation time T for the ion on the exchange surfaces and in bulk solution from the relative esr line widths in the two environments. When wot<< > |+llg>> transition is directly proportional to T and the inner product (0:0) 44 of the zero field splitting tensor (Burlamacchi, 1971; Burlamacchi g5;gfl,, 1970). Therefore, if the reasonable assumption is made that (0:0) is the same in bulk solution and on the exchange surfaces of the mineral, then the relative correlation times should be directly propor- tional to the ratio of line widths. Dipolar interactions between Mn(H20)62+ ions in hectorite were eliminated by doping a Mg2+ exchange form of the mineral with 5% Mn2+. As illustrated in Fugure 4 a, the doped sample exhibits six, almost fully resolved hf lines under fully hydrated conditions. The width of the fourth highest field component at room temperature, which is a reliable estimate of the width of the l+l/2>< > |-l/2> transition (Garrett and Morgan, 1966), is 28.7 G. In comparison, the width of Mn(H20)62+ in dilute aqueous solution at room temperature is 22 G. Therefore, the value of t, which can be taken physically to be the precollision lifetime of the ion (Rubinstein §t_al., 1971), is only £3, 30% longer in the interlayer than in bulk solution where it has been estimated to be 3.2 x 10']2 sec. (Rubinstein gt_al., 1971). Thus, the interlayer of the fully hydrated mineral is indeed very much solution like. In contrast, T for Mn(H20)62+ has been estimated to be $2: 2.2 times larger in three- dimensional synthetic zeolites than in bulk solutions (Tikhomirova gt_al., 1973). Intuitively, drying the mineral down to two molecular layers of water should cause the mobility of the interlayer to decrease. This is confirmed by an increase in the line widths for the doped mineral as illustrated in Figure 4 b. However, the lines are too broad and 45 H—D Figure 4. Room temperature esr spectra of powder samples of 5% Mn2+- doped MgZ+-hectorite a) fully hydrated, b) air-dried, and c) thermally dehydrated at 200°C. 46 overlapping (average width 48 G) to obtain a simple quantitative estimate of the I-1/2>< 1973). >|+1I2>>transition (Burlamacchi et al., Figure 4 c shows the spectrum of the doped mineral under ther- mally dehydrated conditions where the interlayers are collapsed and the Mn2+ ions are coordinated to silicate oxygens in hexagonal positions. The spectrum consists of six main lines which represent the allowed AmI = 0 transitions and five pairs of weaker doublets which are due to forbidden transitions with AmI = :1. This type of spectrum is character- istic of Mn2+ in certain crystalline matrices and in frozen glasses in absence of dipolar coupling (Allen and Nebert, 1964). Thus, as expected under anhydrous conditions, there is no solution character to the inter- layer Mn2+ ions. Conclusions Because of the short exchange ion distances and the presence of structural Fe3+ in most smectite minerals, the esr line widths of 2 interlayer Mn + ions under hydrated and anhydrous conditions is controlled by anisotropic dipolar coupling between paramagnetic centers. In the 3 2+ - Mn2+ interactions case of hectorite, which has a low Fe + content, Mn can be eliminated by doping Mn2+ into a diamagnetic Mg2+ exchange form of the mineral. When the mineral is fully hydrated with the Mn(H20)62+ ions in ca, 12.5A interlayers containing several molecular layers of water, the interlayers are very much solution-like with the mean lifetime between ion-solvent collisions only slightly longer than found for bulk solutions. Under air-dried conditions, where the Mn(H20)62+ ions are 47 sandwiched between silicate sheets in interlayers two molecules of water thick, the interlayers are still solution-like but considerably less mobile than bulk solutions. Thermal dehydration transforms the solution- like esr spectrum into one characteristic of the solid state as the Mn2+ ions move into hexagonal arrays of oxygen atoms in the silicate structure. PART III EXCHANGE ION POSITIONS IN SMECTITE: EFFECTS ON ELECTRON SPIN RESONANCE OF STRUCTURAL IRON The positions of exchange ions in smectite minerals depend in part on the hydration energies of the cations and hydration condi- tions. Under ambient conditions, they may be solvated by one or two molecular layers of water as in Cu(H20)42+ and Cu(H20)62+, (Clementz §t_al., 1973), whereas in the wet silicate they are generally present in greatly expanded interlayers as fully hydrated cations. Thermal dehydration of the cations allows the silicate layers to collapse as the ions move into hexagonal cavities formed by oxygen atoms on the interlayer surfaces. In these hexagonal sites, the dehydrated cations are adjacent to structural OH groups (McBride and Mortland, 1974). Since the dioctahedral smectites possess vacant octahedral positions, small cations such as Li+ can migrate irreversibly at sufficiently elevated temperatures through the hexagonal cavities into the empty octahedral sites (Calvet and Prost, 1971). The present work demonstrates that the position of the interlayer cation can be readily determined from the nature of the electron spin resonance (esr) signals of Fe3+ ions present in the aluminosilicate layers of the mineral. Isolated structural Fe3+ ions in distorted tetrahedral or octahedral sites of silicate minerals (Matyash gt_al,, 1969; Kemp, 1971; Novozhilov gt_al,, 1970) and glasses (Castner gt_al,, 1960) commonly exhibit a broad signal with an isotropic 9 value near 4.3. Hydrated + Na+, Li and Ca2+ exchange forms of the smectite in this study (Upton, Wyoming montmorillonite, MO.G4(A13.06 Feo.32 ”90.66) (810.10 517.90) 49 50 020(OH)4 exhibits two Fe3+ signals near 9 = 4.9 as shown in Figure 1.1. 3+. 2+ in two-thirds of the octa- 3+ . 1n Since the distribution of A1 Fe3+ and Mg hedral positions is random and the A13+/Mg2+ ratio is 4.64, Fe octahedral positions formed by sharing edges with octahedra containing A13+ should be almost five times more likely than Fe3+ ions which share edges With M92+- The ”92+ ions are the source of net negative charge in the silicate structure, and this charge imbalance must cause the . Fe3+ environment adjacent to Mg2+ to differ from those adjacent to A13+. Thus, the stronger Fe3+ signal is attributed to Fe3+-Al3+ pairs with orthorhombic symmetry (Angel and Hall, 1972), whereas the weaker signal most likely arises from Fe3+-Mg2+ pairs. The Fe3+ signals for the Na+, Li+ and Ca++ exchange forms after dehydration at 205° for 24 hours are illustrated in Figure 1.2. Subsequent to thermal dehydration, the samples were allowed to equili- brate in 95% ethanol to allow re-expansion of the interlayers, and the esr spectra shown in Figure 1.3 were obtained. [Even in smectites where irreversible charge reduction takes place by thermal migration of cations into the octahedral position, ethanol is known to effectively expand the collapsed layer silicates (Brindley and Ertem, 1971). The esr results indicate that for the Na+ and Ca2+ smectite, thermal dehydra- tion eliminates the weaker Fe3+ signal (indicated by arrows in spectra), while resolvation and expansion of the interlayers regenerates this signal. However, the weak esr signal of the dehydrated Li+ smectite does not return upon solvation. Thermal dehydration of the Na+ and Ca2+ exchange cations allows movement of these ions into hexagonal cavi- ties and the ionic charge to approach the source of negative charge on 51 co mmuwoomEm + no N .mucmcommc +mwu xemz mcu mumuwccw mzogcm wee “coaummcp one; omom cmucm Focmgpm cw umpm>Fomwc chmcwz .m meson eN not QOmON ea eeoeeeszoe _etocez .N mcowpwucoo ucmwnsm cone: Focuses umpmcczz .F . we Pocaauzcnm mo chmwm two as“ no new .+w4 .+mz eo cowum>~0mwc u“ cowumcuxcmu Fascmcu co muoommm .F mczmwu I! 52 2+ 3+ 2+ oxygens associated with structural Mg Therefore, the Fe -Mg pairs will no longer experience charge imbalance and will become more like Fe3+-Al3+ configurations which possess no net charge. Dehydration 3+ 2+ pairs to shift into 3+ 3+ 2+ may then cause the weak Fe3+ resonance of Fe -Mg the invariant strong resonance as the environments of Fe in Fe 3+ -Mg and Fe -Al3+ pairs become more alike. Resolvation of the mineral in ethanol simply reverses the effects of dehydration as evidenced by the 3+ 2+ reappearance of the weak Fe resonance. The Ca and Na+ ions move out of their hexagonal cavities due to their energy of solvation, and this ion migration reestablishes the non-equivalence between Fe3+-A13+ and 3+ 2 Fe -Mg + pairs. In the case of Li+ smectite, the exchange ions are small enough to migrate irreversibly into vacant octahedral sites of the sili- cate at elevated temperature and solvation in ethanol does not restore 3+ 2+ the weak Fe3+ signal. The resulting Li+-Fe -Mg configurations have no imbalance. Apparently, both silicate charge reduction of smectite 2+ and Na+ions into hexagonal 3+ by Li+ ions and movement of dehydrated Ca cavities have a very similar effect on the environment of Fe associated with charge sites. This result could only occur if both exchange ions in hexagonal cavities and ions that have migrated into octahedral posi- tions are positioned as closely as possible to the oxygens associated 2+ with structural Mg The electrostatic attraction between positive exchange ions and the structural negative charge associated with Mg2+ insures this positioning. The tendency for the exchange ions to occupy hexagonal cavities should increase as the electrostatic attraction between the cation and 53 silicate surface competes more favorably with the hydration energy of 2+ montmorillonite, the ion. As shown in Figure 2 for K+, Na+, Li+ and Ca there is a decrease in the intensity of the weak Fe3+ signal as the relative humidity decreases. Also, at a given relative humidity the signal increases with the hydration energy of the exchange ion (-77, -97, -124, and -377 kcal/mole, respectively, for K+, Na+, Li+ and Ca2+). Thus, at 0% humidity, K+ resides exclusively in hexagonal positions, whereas Ca2+ remains hydrated in the interlayer. Even at 93% relative humidity, some K+ and Na+ appear to be partially dehydrated and in hexa- gonal sites since the weak Fe3+ resonances are less intense than those for the Li+ and Ca2+ exchange forms of the mineral at the same humidity. These results are in qualitative agreement with the distribution of exchange cations calculated from hydration energies and the charge densities of layer silicates (Shainberg and Kemper, 1966). The weak resonance of structural Fe3+, because of its apparent sensitivity to cationic charge in the hexagonal cavities, may also be useful to determine the degree of "keying" of organic cations into the silicate structure. For example, a smectite exchanged with tetramethyl- ammonium ions and dehydrated by heating to 110°C retained most of its weak Fe3+ resonance, whereas a similarly dehydrated methylammonium smectite lost much of the weak signal intensity. The latter result supports the earlier suggestion that the (CH3)NH3+ ions key into the hexagonal cavities adjacent to structural MgZ+ (Cast and Mortland, 1971). Steric factors, however, prevent keying of the (CH3)4N+ ion. 3+ Further esr studies of structural Fe in other organic and inorganic cation exchange forms of smectite should supply additional 54 .mmocwcommc +mmm xmmz ms» mpmowucw mzoccm one coo one mcgomam cmzop new .mfiuvwe .Lmaaz mzkv pm :o_pmgnwpw:cm Lopes mmpwpomEm +N any A.»_o>coooemee .1“ No new .Nme .Nmm .Azm + v mmwpwuwsac m>vumch msomcm> mu ucm .+w4 . mz . x we chmwm me AHHvau ogp + o’— .N menace 55 useful information on the position and orientation of the ions on the interlayer surfaces. PART IV PERTURBATION OF STRUCTURAL Fe3+ IN SMECTITES BY EXCHANGE IONS Introduction Exchange cations in the interlamellar regions of layer silicates are known to vary their positions relative to the silicate surface depending upon the cationic species and the hydration state of the mineral. For example, an air-dry K+-montmorillonite is largely collapsed (10 A basal spacing) with potassium ions embedded in the hexagonal cavities of the surface structural oxygen atoms (Grim, 1968a). In contrast, montmorillonite exchanged with a strongly solvating cation 0 2+ has an air-dry basal spacing of 14.5-15.0 A, indicating a such as Mg double layer of interlamellar water formed by Mg(H20)62+ ions (Walker, 1955). Thus, the Mg2+ ions are in the center of the interlayer. By heating strongly hydrated clays to near 200°C, most of the ligand water is removed and the cations then enter hexagonal cavities of the struc- ture, allowing total collapse of the montmorillonite to a basal spacing of about 9.7 A (McBride and Mortland, 1974). In these positions, the cations perturb structural hydroxyls, and it has been possible to corre- late changes in the structural CH stretch and deformation bands in the infrared region with the state of dehydration of the mineral (Russell and Farmer, 1964; McBride and Mortland, 1974). Recently, the electron spin resonance (esr) of Upton, Wyoming montmorillonite (and several other layer silicates) near 9 = 4.9 has 3+ been assigned to structural Fe of orthorhombic sites in the octahedral layer (McBride et al., 1974; Angel and Hall, 1972). The 9 = 4.9 signal 57 58 appears to be composed of two resonances: a low-field strong resonance that is invariant, and a slightly higher field overlapping resonance that is eliminated in montmorillonites by dehydration (McBride et al., 1974). The high-field signal is considered to be produced by F83+ adjacent to octahedral MgZ+. Since isomorphous substitution of Al3+ by Mg2+ produces most of the layer charge in montmorillonite, the high- field Fe3+ is adjacent to unbalanced negative charge and, therefore, resonates at a different position from the Fe3+ next to octahedral Al3+. However, dehydration of the clay allows exchange cations to enter hexa- 2+ and balance the negative layer charge. 3+-Mg2+ group no longer experiences gonal holes near octahedral Mg As a result, the structural Fe unbalanced negative charge and resonates at a lower magnetic field posi- 3+ adjacent to structural A13+ (Fe3+-A13+). Thus, 3+ tion, similar to Fe the appearance or disappearance of the high-field Fe resonance can be used to determine cation position relative to the silicate surface under various conditions of hydration. The objective of this study is to evaluate the usefulness of Fe3+ esr in describing the migration of different cations as the solvent content of interlayers is varied, and to compare esr results with evidence from infrared spectroscopy. Methods An Upton, Wyoming montmorillonite was used in all experiments, having the chemical formula: ”0.64 [A]3.06 Fe0 32 Mgo 66] (A10 10 Si7.90) 020 (0H)4 (Ross and Mortland, 1966). Various exchange forms were obtained by washing the <2u fraction in large quantities of aqueous chloride solution, followed by dialysis of the clay suspension until the 59 AgNO3 test showed no evidence of chloride. The proton-exchanged clay was prepared by passing a Na+-montmorillonite suspension through a protonated resin column (Amberlite IR-120) and drying immediately at room temperature by boiling off the water under vacuum. Infrared spectra of self-supporting clay films were obtained on the Beckman IR-7 spectrophotometer. These films were mounted in a specially designed brass cell with NaCl windows to allow degassing and heating to 110°C, so that infrared spectra of dehydrated clays could be obtained. Electron spin resonance spectroscopy of clay powders was done in quartz tubes using a Varian E4 spectrometer. Spectra of dehydrated clays were obtained by heating the clay powders in quartz tubes and immediately sealing the tubes to prevent rehydration during the record- ing of spectra. Discussion of Results 3+ The esr of Fe near 9 = 4.9 shows changes which can be corre- lated to the solvation state of the montmorillonite. For example, an air-dry CaZ+-clay demonstrates a relatively intense high-field Fe3+ signal indicated by the arrow in Figure l A. The basal spacing of this clay is 14.9 A, indicating a double layer of interlamellar water mole- cules, and the high-field signal is evidence that the Ca2+ ions are not in hexagonal cavities of the silicate surface but are positioned in the center of the interlamellar region as Ca(H20)62+ species (McBride gt_al,, 1974). In contrast, the loss of the high-field Fe3+ signal upon dehydra- tion of the clay (Figure l 8) indicates that the Ca2+ ions have entered Figure 1. 60 Structural Fe3+ esr spectra of Ca2+—montmorillonite. A. B. C. air-dry heated at 210°C for 24 hours resolvated in 95% ethanol after 210°C thermal treatment 61 the hexagonal cavities of the collapsed interlayers (9.7 A basal spacing) and are compensating the negative charge associated with octa- hedral MgZ+. Resolvation of the dehydrated Ca2+-montmorillonite in 95% ethanol expands the interlayers (17.0 A basal spacing), and the high- field signal reappears as the Ca2+ ions resolvate and move out of the hexagonal holes (Figure l C). Since the high-field Fe3+ is sensitive to the distance between the source of layer charge and the compensating cation, clay samples were exchanged with several metal and alkylammonium cations of different sizes in order to vary this distance. These clays were fully dehydrated by heating before esr spectra were obtained. The spectra were analyzed by integrating the areas of the main Fe3+ resonance and the weaker high- field Fe3+ resonance and utilizing the ratio of these two areas as an indicator of the relative quantity of Fe3+ unperturbed by interlayer 2+-montmorillonite expanded in 95% ethanol cations. For example, the Ca possesses a strong high-field resonance (Figure l C) which is measured as a high ratio (Figure 2) because of little interaction between fully solvated Ca2 + and the silicate. Dehydrating the clay by heating elim- inates the high-field Fe3+ signal (Figure 1 B); the ratio is then zero (Figure 2). Therefore, signal ratios close to zero indicate strong interaction between interlayer cations and sites of negative charge in the layer silicate. The esr spectra are first derivatives of the absorption spec- tra, so that areas beneath peaks do not directly give signal intensities. However, intensities are generally determined fairly accurately as (signal width)2 X (signal height) (Levanon and Luz, 1968). Since the 62 Focmfim fig 5 ummeom 933 ‘ $33 astute I mxupu umpwcuzcmu nu Adoopm pm umpfcému mm: Egom mmcmcuxm + no on» mpwcz .ucmEpmmgp pawn ooopp an um>mwgum mm: mcowumu u:m_m>ocoe suwm uwmcmcuxm mxupu mo :o_pmgu cmov .wuwcoppwgospcoe we mcwumam memn ms“ op zuwmcmwcw Pocmwm +mmm ufiwwwlzmwz mo awcmcowpmpmm .N oc:m_u :3 9.88m .88 o 0.2 0.: 3. 0.9 0.: 0.2 0.2 0.: cm. o. H 1 q q q 4 1 u . .7sz %2 w: +30 MW. 38 «NM. \ .. V0 m :2 0 mm. .\...§ :3. s. .N. 2,: N. w + 4 . | V 0 M u: . 0.. m. +_n_ ‘ m . nJ . ON. on... +~8< +08 .m. .¢~.w 63 two Fe3+ signals partially overlap, the relative intensity of the high-field Fe3+ resonance can only be estimated by measuring the area added to the lower-field Fe3+ signal by the high-field shoulder (indi- cated by thefizrrow in Figure l). The strength of the lower-field signal is arbitrarily taken as the area under the peak and above the horizontal baseline. This area represents an internal standard of structural Fe3+ content. For signals of fairly constant width, first derivative signal areas should be proportional to relative intensities. In Figure 2, the relationship between the basal spacing of dehydrated clays and the ratio of Fe3+ signal areas is apparent. The Na+, K+, NH4+, and Cs+-montmorillonites were dehydrated by ll0°C heat treatment and the Ca2+-montmorillonite was dehydrated at 2l0°C. The very low relative intensity of the high-field Fe3+ signal in these clays indicates that the cations are embedded in the hexagonal cavities of the silicate surface. The Cs+ ions, being too large to fully enter the hexagonal holes, prevent the silicate layers from totally collapsing to 9.5-9.7 R, and cannot quite fully eliminate the high-field signal. Clays exchanged with organic cations are dehydrated by 110°C heat treat- ment. The high-field signal becomes more intense as the interlamellar organic cations hold the layers further apart, a result expected from the position of cationic charge (Figure 2). Steric hindrance prevents the positive charge of tetraalkylammonium ions from approaching closely, or entering, hexagonal cavities of the montmorillonite. Methylammonium (MA+) and propylammonium (PA+) ions may "key" into the hexagonal holes to some extent (Gast and Mortland, l97l), but the presence of high- field Fe3+ indicates that the methyl and propyl groups attached to the 64 -NH3+ group prevent the latter from fully penetrating into the structure. In contrast, dehydrated NH4+-montmorillonite shows no high-field Fe3+ (Figure 2), a result of more complete penetration of the structure. In summary, the dehydrated clays substituted with organic and inorganic cations reveal a direct relationship between the relative intensity of the high-field Fe3+ signal and the basal spacing (indicated by the line in Figure 2). This relationship is evidence of decreased perturbation of structural Fe3+ associated with structural Mg2+ as the charge of the cation is moved further from the silicate surface. The presence of solvent molecules in the interlamellar regions greatly influences the position of cations relative to the surface. As previously described, Ca2+ ions in air-dry montmorillonite are separated from the surfaces by water molecules as evidenced by the strong high-field Fe3+ signal. The air-dry Li+-clay, with a basal spacing of l2.3 X, has a monolayer of water in the interlayer. The high-field signal, although less intense than in the Ca2+-clay, is strong enough to indicate that the Li+ ions are not in hexagonal holes (Figure 2). This result is consistent with the concept of coordination of three water molecules to Li+ so that the exchange cations are near the middle of the interlayer (Grim, 1968b). In contrast, the air-dry Na+-clay has a much weaker high- field signal despite the fact that the basal spacing is also 12.3 R. The Na+ ions must be partially dehydrated and close to hexagonal cavities; a portion of the ions may actually penetrate the cavities. These observa- tions agree qualitatively with calculations of expected cation positions on clay surfaces based on the known hydration energies of Ca2+, Na+. and Li+ (Shainberg and Kemper, 1966). Clays fully solvated in 95% ethanol 65 (17.0 A basal spacing) show the expected direct relationship of the hydration energy of interlayer cations to the intensity of high-field Fe3+ (Figure 2). The signal intensity increases in the order Na+