13601
J. Phys. Chem. 1994,98, 13601-13606
Interactions of Binary Solvents with Charged Expandable Clays. 2. Experiment Joseph R. Feldkamp* Applied Research Associates Inc., P.O.Box 40128, Tyndall AFB, Florida 32403-0128
Thomas B. Stauffer Armstrong Laboratory, Environmental Research, Division, I39 Barnes Drive, Suite 2, Tyndall AFB, Florida 32403-0070 Received: December 9, 1993; In Final Form: September 13, 1994@
FT-IR spectroscopy is used to directly measure the composition of the sorbed phase on a charged, expandable organo-montmorillonite placed into contact with binary solvent solutions. Experimental data are compared with thermodynamic computations of the sorbed phase composition. The theoretical treatment, developed in part 1, examines the extent of partitioning due to electrical charging of the clay-water interface which is in addition to any partitioning that might result from purely chemical differences between the interlayer environment of layered aluminosilicate clay minerals and an external solution. Provided the extent of partitioning is expected to be large, as it is for the acetone-toluene system, the agreement between theory and experiment is very good. On the other hand, as the predicted extent of partitioning becomes smaller, the agreement between theory and experiment degrades. In addition to composition measurements, the magnitude of the interfacial electric field is characterized by examination of the carbonyl stretching frequency of acetone, which is common to all three solvent combinations studied. As the component of smaller dielectric constant is displaced, the carbonyl stretching frequency is observed to rise. This is expected since an increased dielectric constant will reduce the electric field and thereby favor the nonpolar form of acetone with its higher stretching frequency. The implications of our results to electrical double-layer theory, which treats the dielectric constant as spatially fixed, are briefly discussed.
Introduction Whether the naturally occurring exchange cations of an expandable clay have been replaced or not, the chemical environment of the interlayer solution is expected to be different from that of the typical dilute aqueous solution filling the pore space of most soils. When exchangeable cations have been replaced with organic cations, then the chemical environment of the interlayer solution has usually been altered sufficiently that partitioning effects will be observed when placed into contact with a binary solvent (or some solute-solvent system). In other words, the composition of the sorbed phase is expected to differ from that in the external or bulk solution. In addition to providing a different chemical environment, the interlayer solution of many clays is exposed to a rather intense electric field that will polarize the sorbed phase to an extent depending upon its dielectric constant. The polarization of the sorbed phase will in turn cause the constituent chemical potentials to be different from that in a field-free region of space. Thus, just as an altered interlayer chemical environment promotes partitioning due to changes in chemical potentials, so too, an electrified interface leads to altered chemical potentials that will contribute to partitioning. Partitioning is necessary in order to restore equality of chemical potentials throughout the system. The extent of this effect has been examined theoretically in part 1 of this series. Under constant confining pressure, the sorbed phase composition of a two-component solution is altered due to electrical charging of the clay -solution interface according to the following integral:
* Corresponding @
author. Abstract published in Advance ACS Abstracts, November 15, 1994.
The symbols have the following meaning and units: xPq mole fraction of the more polar component after charging; xpi,mole fraction of the more polar component before charging; yp, activity coefficient based upon pure solvent as reference; R, universal gas constant (8.3147 J/(mol deg)); A,Margules parameter (J); T, absolute temperature (K); K, dielectric constant of interlayer solution phase; V, total molar volume (m3/mol); UO, surface charge density of clay (C/m2); €0, permittivity of vacuum (8.854 x lo-'* C2/(N m2)); model-dependent dimensionless parameter (2 < < 6). Assumptions inherent to the derivation of eq 1 include the following: (a) Counterion charges are considered to be either uniformly smeared throughout the interlayer space or smeared uniformly over a plane located midway between adjacent aluminosilicate sheets. = 6 applies when the former charge distribution is assumed while = 2 applies in the case of the latter charge distribution. Additionally, is assumed to remain unchanged over the range of integration. (b) The variables yp, K, and V were assumed to be dependent only upon composition. (c) The interlayer solution composition is uniform throughout the interlayer space. Hence, yp, K, and V would also be uniform given the assumption in (b). As already pointed out, the mole fraction x: would be identical to that in the external solution phase if the interlayer and external spaces were chemically identical. If the two regions are not chemically identical, then some degree of partitioning prior to charging is expected. As suggested in part 1, xp' might then be suitably related to the mole fraction external to the interlayer space by the simple partition coefficient E
x,
x
x
x
0022-3654/94/2098-13601$04.50/0 0 1994 American Chemical Society
x
13602 J. Phys. Chem., Vol. 98, No. 51, 1994
Feldkamp and Stauffer
Gasketts n
where +io is now the mole fraction of the more polar component in the external or bulk solution phase. A vast literature has developed which considers partitioning due to chemical effects (e.g., refs 1 and 2), principally chemical effects resulting from the prior adsorption of naturally occurring organics. The present series of papers examines the extent of partitioning due to interface electrification. To the best of our knowledge, this phenomenon has not been previously studied. A theoretical background was presented in part 1 while part 2 focuses on experimental verification. Experimental verification is obtained by exposing an organoclay complex to the vapor phase above an appropriate binary solvent solution and subsequently measuring the composition of the sorbed phase using Fourier transform infrared spectroscopy (FT-IR). The measured sorbed phase composition is compared with predicted values obtained by assuming that partitioning occurs only as a result of the interfacial electric field. In other words, the chemical environment of the interlayer solution is assumed to be reasonably close to that of the binary solvent solution.
Experimental Section Organo-Clay Complex. Measurements of the sorbed phase composition were carried out on specimens of cetyltrimethylammonium montmorillonite (CTMA-M), prepared from a sodium montmorillonite that originated from Crook County, WY. The organo-clay complex was prepared by dispersing 15 g of the sodium clay in 0.5 L of distilled water and subsequently washing six times with 16 mL (96 mL total) of aqueous CTMA chloride solution (0.75 equivk). The resulting suspension was washed three times with distilled water and then three times with denatured ethanol. Finally, a portion of the clay-ethanol suspension was washed twice with toluene. It was noticed that the CTMA-clay had a considerably larger gel volume in toluene than in denatured ethanol. Residual ethanol was undoubtedly present. Mounting of Clay Films. Thin films of CTMA-clay were deposited on both ZnSe windows from a standard, demountable liquid IR cell (Perkin-Elmer 126-XXX series). Film deposition was accomplished by layering a very small volume of the clay dispersion onto the ZnSe windows and then allowing the toluene to evaporate. Films faced toward the inside of the liquid cell and had densities of ~ 0 . mg/cm2. 6 After reassembling the demountable cell, the cell assembly was placed under vacuum (0.01 Torr) for at least 5 h to remove all traces of water, ethanol, or toluene from the CTMA-clay films. The IR spectrum of the evacuated film was obtained prior to any exposure to solvent vapor in order to provide a reference. Upon completion of a set of experiments, the clay films were removed from the ZnSe windows by gently rubbing with a cotton swab, soaked with acetone. Clay Film-Vapor Experiments. Deposited clay films were exposed to the vapor phase above a binary solvent solution by recirculating air through a bubbler submerged in the desired solvent and were connected in series with the liquid IR cell. A flow rate of nearly 0.4 mL/min was maintained by a simple bellows pump for at time period ranging from 45 to 60 min. A diagram indicating the layout of the system is shown in Figure 1. When exposure to the appropriate vapor was complete, the liquid IR cell was disconnected from the setup, and the two ports were immediately stoppered with Teflon plugs. The 4560 min exposure time was judged adequate since IR examination revealed no spectral change beyond this time. No special
/
I
\
n
Vapor In
t
To Pump
Vapor Out
ZnSe Windows Figure 1. Apparatus used to expose clay films to binary solvent vapors.
precautions were taken to control temperature which was approximately 22 f 2 "C. Binary solvent combinations under study included acetone/toluene, acetone/chloroform, and acetonitrile/acetone. Experiments were performed with at least 20 different liquid phase compositions. A second liquid IR cell was used to obtain the IR molar extinction coefficient ratios for the three solvent combinations. For example, known masses of toluene and acetone were dissolved in pentane, injected into the liquid cell, and then subjected to IR examination. Absorbances were measured at selected wavenumbers for acetone ( ~ 1 7 2 2cm-') and toluene (m727 cm-') and then ratioed (absorbance acet0ne:absorbance toluene). Dividing this ratio by the molar ratio in the liquid cell then yielded the desired result, the molar extinction coefficient ratio. For acetone and toluene, the ratio of molar extinction coefficients at the selected wavenumbers was 0.67. For acetone and chloroform (in pentane) at 1722 and 762 cm-', respectively, the ratio of molar extinction coefficients was determined to be 0.274 (acetone:chloroform). Finally, for acetone and acetonitrile (in toluene) at 1722 and 2253 cm-', respectively, the ratio of molar extinction coefficients was determined to be 14.5 (acet0ne:acetonitrile). Needless to say, the extinction coefficient for acetonitrile is relatively weak. The ratio of molar extinction coefficients measured in either pentane or toluene was assumed to hold for the clay interlayer solution as well. FT-IRSpectra. Infrared spectra of clay films were obtained using a Nicolet Model 740 FT-IR spectrometer equipped with a liquid nitrogen cooled MCT (mercury cadmium telluride) detector. Instrument parameters were controlled via a Nicolet Model 680 workstation to yield a spectral resolution of 2 cm-'. Averaged spectra were obtained from 128 scans on all samples. Spectral subtraction was implemented, using the untreated, evacuated clay film as reference, and followed by base line correction, if necessary, to isolate the desired peaks from the absorbance spectrum. Measured absorbances, together with the molar extinction coefficient ratios, were then used to compute the composition (i.e., mole fraction) of the sorbed phase on the clay film.
Results and Discussion Sample IR spectra of the four sorbed solvents involved in experiments, subsequent to spectral subtraction, are shown in Figures 2-5. The liquid phase mole fractions prevailing at the time the various spectra were collected are indicated in the respective figure legends. If the base line adjacent to a particular absorbance peak was reasonably flat, then that base line was used as a reference to compute the peak absorbance. Otherwise, the average of absorbances measured on either side of a particular peak served as a base line or reference absorbance
J. Phys. Chem., Vol. 98, No. 51, 1994 13603
Interactions of Binary Solvents with Clays. 2 1 0
'1
09 08
C7
a
06
X
-
0 5
Q
a 4
53
Figure 2. IR spectrum of sorbed acetone from the acetone-toluene system. The liquid phase mole fraction of acetone equals 0.62. Absorbance measurements were made at ~ ~ 1 7 cm-l. 10
02
3'
i.02 wm
'
3
"""
4
5
'
'
'
"
lo-'#
'
'
' 2
'
" " "
3
4
' ' ' 5 -5 ~
10c
Figure 6. Mole fraction of acetone in the sorbed phase minus that in the extemal solution phase for the acetone-toluene system, plotted against the extemal mole fraction of acetone.
I
ys -
'
m
a
liquid phase is plotted against the base 10 logarithm of the extemal liquid phase mole fraction for the more polar component (i.e., xp' - xp' vs log(xp')). Data presented in such a way directly show the extent of partitioning and are readily compared with theory. All theoretical results have been obtained by setting = 2 (see eq 1) and by modeling activity coefficients in accordance with the Margules e q ~ a t i o n : ~
x
Figure 3. IR spectrum of sorbed toluene from acetone-toluene system. The liquid phase mole fraction of acetone = 0.62. Absorbance measurements were made at 3 7 3 0 cm-'. m
7 0 0
00
2975 2250 Zi2S WAVENUMBER
2b00
Figure 4. IR spectrum of sorbed acetonitrilefrom acetone-acetonitrile system. The liquid phase mole fraction of acetone equals 0.587. Absorbance measurements were made at ~ ~ 2 2 5cm-'. 0
'1
0
In Figure 6, experimental and theoretical data appear together for the acetone-toluene system. The parameter A, obtained from vapor pressure data? was set equal to 2000 J/mol. Two theoretical curves are shown in order to illustrate the sensitivity of predicted results to variation in surface charge density. Surface charge density was set equal to either 0.12 C/m2 (the value appropriate for our montmorillonite clay) or 0.06 C/mz. Obviously, theory fits the experimental data extraordinarilywell, and reducing the surface charge density by half leads to a substantial reduction in the partitioning effect. Thus, not only is partitioning due to electrification of the clay-water interface a very important phenomenon, the model developed in part 1 appears to be entirely appropriate. Note, however, that the partitioning effect we are reporting herein favors the more polar component, which is unlike the partitioning ,commonly caused by chemical effects in aqueous systems, as reported by Karickhoff' and others. For very general situations, chemical and electrical effects must be considered together in order to have a complete understanding of partitioning phenomena. Of the three solvent systems under evaluation, the acetonetoluene system is expected to display the greatest partitioning response resulting from electrical effects since toluene has the lowest dielectric constant of all, and replacing toluene with a high dielectric constant liquid such as acetone is a highly favored process. In view of such logic, the experiment was repeated, leaving acetone, but replacing toluene with chloroform which has a somewhat higher dielectric constant. K = 2.568 for toluene while that for chloroform is 4.806. In this case, though replacement of chloroform with acetone is still favored, such replacement is less favored than for toluene. Our expectation is fulfilled as indicated by Figure 7. Two sets of theoretical computations were made, one considering the sorbed phase solution to be ideal and the other nonideal with A= -2000 J/mol (obtained using vapor pressure data found in ref 4). a0 = 0.12 C/mz in either case. As Figure
I \ o
1 00
WAVENUMBER
Figure 5. IR spectrum of sorbed chloroform from acetone-chloroform system. The liquid phase mole fraction of acetone equals 0.81. Absorbance measurements were made at ~ 7 5 cm-I. 5 when computing peak absorbances. Altogether, a total of about 65 spectra were obtained. In all cases, the gap between the pair of ZnSe windows was sufficiently small that IR absorption by background vapor was completely negligible. The format used to graphically present sorbed phase composition data is the same for all systems studied. The difference between the sorbed phase mole fraction and that in the extemal
13604 J. Phys. Chem., Vol. 98, No. 51, 1994
Feldkamp and Stauffer 0 33
:[/
ACETONE/ C H LCRO KO R V
0 25
07
.-
a
06
-
05
X
c 4
0 ,o
-a
x
X
015
c
n
Ti
X
01C
33
0 05
32
51 \ 2
3
L
5
6
7
130
XP
0 oc IC-2
2
3
0
5
XO
10-1
2
3
4 5
10
- ACETCW'R LE
Figure 7. Mole fraction of acetone in the sorbed phase minus that in the external solution phase for the acetone-chloroform system, plotted against the external mole fraction of acetone.
Figure 8. Mole fraction of acetonitrile in the sorbed phase minus that in the external solution phase for the acetonitrile-acetone system, plotted against the external mole fraction of acetonitrile.
7 shows, theory and experiment still agree rather well, and both
As stated in the Introduction, the interlayer solution residing between aluminosilicate sheets is exposed to a very intense electrical field. As the composition within this sorbed phase changes, the dielectric constant must change, leading to a variation in field strength. Conveniently, it turns out that acetone is a very sensitive indicator of the strength of this electric field. This is so by virtue of the fact that a substantial contribution to the ground state molecular structure of acetone comes from its polar form:
show that a partitioning effect is now less pronounced than with the toluene/acetone system. Also, the results of Figure 7 demonstrate quite clearly the importance of accounting for nonideality since the two theoretical curves are significantly different. Though the theoretical prediction obtained under the assumption of ideality appears to do a better job matching experiment, the use of a real solution model is probably more appropriate for comparison with experiment. Thus, Figure 7 would suggest a small but significant discrepancy between theory and experiment. The source of the discrepancy is believed to be due to a chemically based partitioning that would tend to show itself in a more dramatic fashion if the partitioning effects due to interfacial electrification were less important. The hypothesis of the preceding paragraph was tested by requiring that both components of the binary solvent have a high dielectric constant. According to eq 1, such a scenario would force the integrand to remain large over the entire range of integration. As a result, the upper limit of integration would not need to be as large in order to satisfy the equation. Also note that when the two dielectric constants are close together, the same result is obtained. Thus, with solvents chosen to have large K values, even though they may be widely separated, a partitioning response due to electrificationat the interface should be greatly suppressed. The solvent system chosen to produce such behavior was acetonitrile (K = 37.5) and acetone (K = 20.7). The experimental and theoretical results for the acetonitrile-acetone system are shown in Figure 8. Theoretical predictions were made for both the ideal and nonideal solution case (A= 2000 J/mol for the nonideal solution case). Regardless of ideality or not, theory and experiment show substantial disagreement. The source of the disagreement is possibly due to chemical partitioning, now made more apparent because of the suppression of electrical partitioning. For example, the smaller size of acetonitrile molecules may permit a closer association with the organic counterions. Nevertheless, partitioning due to electrical effects is still appreciable, and the shape of the experimental curve still reflects that of the theoretical curve. That is, both show a maximum, unlike the behavior exhibited by the other two-solvent systems. Admittedly, the discrepancy might not be entirely due to chemical effects, but rather the result of a reduced molar extinction coefficient for adsorbed acetonitrile.
-
The importance of this form is commonly supported by the wellknown blue shift of the n n* transition in the TJV absorption spectrum which occurs upon placing ketones in more polar solvents. Such solvents would tend to afford some degree of stability to the above polar form. The carbon-oxygen double bond of the nonpolar form of acetone is expected to exhibit a higher IR stretching frequency than the polar form pictured above with its single bond. As the imposed electric field is strengthened, the polar form should become increasingly favored so that the observed IR stretching frequency ought to decrease. This shift has already been observed by Parfitt and M ~ r t l a n d , ~ who examined the effect of various exchange cations on the IR spectrum of acetone. Thus, the position of the IR absorption band for acetone should provide a sensitive measure of electric field strength in the interlayer solution. Figures 9- 11 show the wavenumber of maximum IR absorption, Ym,by acetone as a function of mole fraction in the external solution on a log scale. In all cases, the figures support the contention that the electric field, as manifested by Y,, varies with composition of the sorbed phase. In Figures 9 and 10, the gradual replacement of the relatively nonpolar solvents toluene and chloroform by acetone leads to an increase in dielectric constant. Such an increase will reduce the electric field and cause Ym to move toward the value normally encountered for the pure ligand, ~ 1 7 2 cm-'. 2 The range of Y, values in Figures 9 and 10 is essentially the same, which is not surprising since the dielectric constants for chloroform and toluene are not too different. For the acetonitrile-acetone system, however, the dielectric constant remains larger across the entire composition range. Thus, the entire range of measured Y, values lies above that measured for either
Interactions of Binary Solvents with Clays. 2
J. Phys. Chem., Vol. 98, No. 51, 1994 13605
Electrical Double Layer Theory for Binary Solvents
1702
C
0-2
2
3
4
5
10-1
2
3
4
5
'IC0
Xp - A C E T O N E
Figure 9. Wavenumber of maximum IR absorption by acetone for the acetone-toluene system, plotted against the mole fraction of acetone in the external solution.
AC ET0 NE/ C H LCROFC R V
The well-known treatment of the diffuse portion of the electrical double layer in terms of the Poisson-Boltzmann equation breaks down when the one-component solvent, always considered a continuum by traditional Poisson-Boltzmann theory, is replaced with a multicomponent solvent. In this final section, we would like to briefly consider the ramifications of our two papers to electrical double layer theory when the solvent is binary. As our results have amply demonstrated, the composition of the sorbed phase occupying the interlayer space depends dramatically on the magnitude of the electric field. Thus, the composition and dielectric constant for the sorbed phase and the electric field will be interdependent. The theoretical treatment offered in part 1 of this series considered the composition, and therefore the dielectric constant, to be invariant throughout the interlayer space. For large interlayer spacings, such an assumption could not hold. Thus, a theory is needed which would allow the solvent composition and therefore the dielectric constant to vary with distance from the clay surface. The usual Poisson-Boltzmann equation originates from the differential form of Gauss' law:
0
where D is the electric displacement vector and e is the charge density due to the counterions in the vicinity of the clay surface. For a linear, isotropic binary solvent, D = rE, where E is the permittivity of the binary solvent and E is the electric field. Relating electric field to a gradient in electrostatic potential, q, yields the result 1762 c
17'2
where K is the dielectric constant given by K = € / E O and EO is the permittivity of vacuum. Now Q may be developed in the classical manner of electrical double layer theory in terms of the Boltzmann factor. The dielectric constant however cannot be handled in the traditional manner by removing it from under the divergence operation and shifting it to the right-hand side of the equation. Instead, we must now explicitly account for the spatial dependence of K. Perhaps the simplest way to account for the nonconstancy of K is to use eq 1, with 00 replaced by EE. Representing the entire integrand byJIXp), eq 1 would then appear as
I
A C ETON lT91LE /AC ETON E
// 0
Figure 11. Wavenumber of maximum IR absorption by acetone for the acetone-acetonitrile system, plotted against the mole fraction of acetonitrile in the external solution.
of the other two-solvent systems. Though not anticipated prior to experimentation, the data indicate that vm varies linearly with log(x,), x, pertaining to the external liquid phase.
The lower limit of integration, xpiwould be identified with the mole fraction of the polar component at an infinite distance from the clay surface while the upper limit of integration,xpf, together with E and K would apply at some arbitrary distance from the clay surface. An additional equation is still needed in order to complete the connection between E, K,and x,. This is provided by an equation introduced in part 1: K = K,,
+ ( K p- K,,)xp
