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Raman Spectroscopic Study of Sorption to CTAB-Modified Montmorillonite Matthew J. Dickey and Keith T. Carron* Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071-3838 Received September 15, 1995. In Final Form: March 1, 1996X The sorption of benzene, ethylbenzene, tert-butylbenzene, and quadricyclane onto CTAB-modified montmorillonite was studied. The CTAB was seen to be in a liquid-like form in the clay. It was also seen that quadricyclane was not stable in aqueous solutions and formed tricycloheptan-3-ol. Raman spectroscopy can be used to observe the sorption onto the organo-clay. The free-energy relationship for the sorption of the compounds studied on the organo-clay using an adsorption model was found. A deviation from ideal behavior was observed and a different model was developed that involved the molecular surface area of the surfactant and the molecular volume of the sorbate. If we assumed an absorption model, the agreement between our data and those predicted thermodynamically improved significantly.
Introduction Clays that are found naturally in the environment contain inorganic cations, e.g. Na+ and Ca2+. These ions cause the surface and interlayer spaces of the clays to be hydrophilic due to the cations being strongly hydrated. The presence of the cations is due to the negative charge created by deprotonated hydroxyl groups and charge deficiencies created by low valence substitutions. The number of cations in the clay is related to the structure of the clay and can be controlled with pH. Smectites, such as montomorillonite, tend to have very high cation exchange capacities (CECs) compared to other clays such as kaolinite. The large CEC of smectites arises from vacancies-created substitution of Al3+ for Si4+ and Mg2+ and Fe2+ for Al3+ in the clay lattice. The hydrophilic nature of the clays strongly affects their ability to sorb neutral organic materials. The hydrophilicity of natural clays can be modified and controlled by exchanging cationic surfactants for inorganic cations.1,2 Exchange of the surfactants of the form [(CH3)3NR]+ or [(CH3)2NRR′]+ with inorganic cations, where R is an alkyl hydrocarbon of 12 carbons or more, creates a hydrophobic clay.1,2 The surfactants are thought to form an organic phase which acts as a partition medium for extraction of neutral organic compounds. This modification causes the clays to sorb significantly larger amounts of hydrophobic organic compounds than clays without the modifier. For example, clays exchanged with cetyltrimethylammonium bromide (CTAB) have been shown to be 10-30 times more effective on a weight basis at sorbing neutral organic compounds than the sorption that occurs due to the natural organic material present in clays.3 Many researchers are studying the partitioning of organic compounds onto natural clays4-7 and organoclays.1-3,8-15 To date, most studies have been performed * Author to whom correspondence should be sent. X Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Boyd, S. A.; Mortland, M. M.; Chiou C. T. Soil Sci. Soc. Am. J. 1988, 52, 652. (2) Boyd, S. A.; Shaobai, S.; Lee, J. F.; Mortland, M. M. Clays Clay Miner. 1988, 36, 125. (3) Jaynes, W. F.; Boyd, S. A. Soil Sci. Soc. Am. J. 1991, 55, 43. (4) Johnson, R. L.; Cherry, J. A.; Pankow, J. F. Environ. Sci. Technol. 1989, 23, 340. (5) Subramanian, P.; Fitch, A. Environ. Sci. Technol. 1992, 26, 1775. (6) Mott, H. V.; Weber, W. J. Environ. Sci. Technol. 1991, 25, 1708. (7) Wang, X. O.; Thibodeaux, L. J.; Valsaraj, K. T.; Reible, D. D. Environ. Sci. Technol. 1991, 25, 1578. (8) Lee, J.; Crum, J. R.; Boyd, S. A. Environ. Sci. Technol. 1989, 23, 1365. (9) Faschan, A.; Tittlebaum, M.; Cartledge, F. Hazard Waste Hazard. Mater. 1993, 10, 313.
S0743-7463(95)00770-0 CCC: $12.00
on organo-clays using macroscopic methods. The method used by most researchers studying the organo-clays is a difference technique. The technique consists of adding a solution containing a known amount of the organic compound, shaking it for 12-24 h, centrifuging, and testing the remaining solution for the amount of organic compound that was not sorbed. This method provides sorption isotherms for single components and may provide data about the sorption of multicomponent mixtures if analysis of the supernatant is molecular specific. In this report, we will discuss a spectroscopic method that directly measures the organic compounds that are sorbed to the clay using Raman spectroscopy. Raman spectroscopy is an ideal method for studying sorption from aqueous solutions since water is an extremely weak Raman scatterer. The high surface area of the clays creates a large concentration of sorbed material relative to material in solution. The concentration difference is large enough to allow us to look at the clay itself and the organic compounds sorbed to it without interference from the water or sorbate species in the aqueous phase. One application of organo-clays is for use as liners at hazardous waste sites. Their effective use depends on our ability to predict their partitioning coefficients. The partitioning can be predicted by looking at a linear freeenergy relationship between the partition coefficients and the aqueous solubility of a class of sorbates.16 This relationship and knowledge of a compound’s solubility allows one to predict a partition coefficient. In this study, we found the free-energy relationship for sorption of benzene, ethylbenzene, tert-butylbenzene, and quadricyclane on a SAz-1/CTAB organo-clay. We found large deviations from ideal behavior for this system and have developed a model based on molecule volume that explains the deviation. Experimental Section Thiophene-free benzene was purchased from Fisher Scientific. Spectrophotometric grade hexane was purchased from J. T. Baker Inc. Ethylbenzene (99%), tert-butylbenzene (99%), quadricyclane (99%), and CTAB were purchased from Aldrich. These were used without further purification. The concentrations of the (10) Srinivasan, K. R.; Fogler, H. S. Clays Clay Miner. 1990, 38, 277. (11) Srinivasan, K. R.; Fogler, H. S. Clays Clay Miner. 1990, 38, 287. (12) Jaynes, W. F.; Boyd, S. A. Clays Clay Miner. 1991, 39, 428. (13) Jaynes, W. F.; Boyd, S. A. J. Air Waste Manage. Assoc. 1990, 40, 1649. (14) Lee, J.; Mortland, M. M.; Chiou, C. T.; Kile, D. E.; Boyd, S. A. Clays Clay Miner. 1990, 38, 113. (15) Smith, J. A.; Jaffe, P. R.; Chiou, C. T. Environ. Sci. Technol. 1990, 24, 1167. (16) Hansch, C.; Quinlan, J. E.; Lawrence, G. L. J. Org. Chem. 1968, 33, 347.
© 1996 American Chemical Society
Sorption to CTAB-Modified Montmorillonite benzene-, ethylbenzene-, tert-butylbenzene-, and quadricyclanesaturated water solutions were determined with a Hewlett Packard 5890 Series II gas chromatograph. SAz-1 montmorillonite was purchased from the Source Clay Repository (Clay Minerals Society, Columbia, MO). The native clay contains sand and other noncolloidal minerals that interfere with the mass determinations. These were removed by dispersing 2 g of the montmorillonite in 200 mL of deionized water and allowing it to stand several hours to settle out quartz sand and noncolloidal minerals. The suspension was decanted, filtered, and dried in an oven at 100 °C. We completely exchanged all native cations in the clay by mixing one cation exchange capacity (CEC) of CTAB with the clay. For the SAz-1 montmorillonite the CEC is 1.3 mol/kg.3 An amount of CTAB corresponding to the CEC was dissolved in water, added to the clay, and stirred for 4 h. The suspension was centrifuged, washed five times with deionized water, filtered, and dried in an oven at 100 °C. The samples were prepared by adding to 20 mg of the organoclay and stirring for 1.5 h in a sealed container 18 mL of an aqueous solution saturated with one of the sorbates. Preliminary experiments showed that equilibrium was reached within 1 h. The suspension was centrifuged leaving 4.3 mL of solution in the test tube, and the test tube was capped with an aluminum foil lined septum. Following centrifugation, spectra were taken to determine the isotherm. Following measurements with the saturated solutions, isotherms were formed by standard dilutions. The sample was diluted, shaken for 2 min, and centrifuged, and another spectrum taken. Preliminary experiments showed that equilibrium was reached within 2 min. Dilutions were repeated until the sorbate signal was not observed. We found quadricyclane undergoes hydrolysis in the presence of water. A gas chromatograph/mass spectrum performed on the quadricyclane water solution identified the predominant product as tricycloheptan-3-ol. This compound is known and has been synthesized before from bicyclo-2-heptene.17 We were able to synthesize it by adding quadricyclane to water and refluxing the solution for 36 h. The product formed in the condensing tube was allowed to air dry in the tube and was washed out with ether. The ether was removed by evaporation and the remaining solid was identified as tricycloheptan-3-ol using GC-MS and nuclear magnetic resonance. The product was a white powder with a solubility of 0.27 M in water. It is important to note this greatly increased solubility in water when compared to the solubility of quadricyclane in water (2.62 mM). We did not observe sorption of norbornadiene, which is an isomer of quadricyclane. The Raman spectra were obtained with a Spectra Physics 2025 Kr+ laser operating at 647 nm. The detection system was a Photometrics CCD9000 system. The CCD was held at -102 °C. The detector was attached to an HR-320 (ISA) spectrograph with a 1200 g/mm ion etched grating blazed for 650 nm. The slit width was set at 30 µm. The laser power was set at 80 mW, and sample illumination was with a line focus produced by a 50-mm cylindrical lens (Melles Griot). The Raman spectrum was collected with a F1.8 Minolta camera lens and a ∼3:1 magnification for f-number matching with the spectrograph. Infrared spectra of the organo-clay were acquired with a Bomem MB 102 FTIR to obtain CTAB structural confirmation.
Results and Discussion Our method of surface analysis involved measuring Raman bands associated with the sorbate relative to Raman bands associated to the organo-clay. Figure 1 shows the Raman spectra of the sorbates and the strong Raman bands that were used as references. For benzene, ethylbenzene, and tert-butylbenzene, the strong ring mode around 1000 cm-1 was used. Quadricyclane has a strong ring deformation mode at 1077 cm-1. A complication in our study was that quadricyclane hydrolyzed to an alcohol in the presence of water. The Raman spectra of tricycloheptan-3-ol and quadricyclane are shown in Figure 2. (17) Roberts, J. D.; Trumbull, E. R.; Bennett, W.; Armstrong, R. J. Am. Chem. Soc. 1950, 72, 3116.
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Figure 1. Bulk Raman spectra of (a) benzene, (b) ethylbenzene, (c) tert-butylbenzene, and (d) quadricyclane showing bands used in sorption studies.
Figure 2. Bulk Raman spectra of (a) tricycloheptan-3-ol and (b) quadricyclane.
Relationships between layer charge and the interlayer expansion of clays by n-alkylamine hydrochlorides have been previously determined.18,19 Depending on the size of the organic cation and the mineral charge, the alkyl chains of the cations were predicted to form monolayers, bilayers, pseudotrimolecular layers, or well ordered paraffin complexes. We tried to determine conformation by comparison. An infrared spectrum in the C-H stretching region was taken for liquid hexadecane, solid hexadecane, and a KBr pellet of the organo-clay to determine the conformation of the organic modifier. The differences between solid and liquid hexadecane are due to differences in cis and trans confirmation between methyl and methylene groups in CTA+. It can be seen in Figure 3 that the peaks in the organo-clay that are due to CTA+ have positions and a peak ratio very similar to those of liquid hexadecane. Using this, we have concluded that CTA+ has formed a liquid-like complex in our clay that is not well ordered (paraffinic). This is in agreement with models based on basal X-ray diffraction spacings measured by Jaynes and Boyd.3 We developed models based on adsorption and absorption. The adsorption model was developed using fractional coverage, θ. In order to experimentally determine the fractional coverage, one needs to find an absolute coverage at a known solution concentration. We determined the coverage at saturation using GC analysis. The absolute amount of organic contaminant sorbed by the organo-clay was measured by stirring 18 mL of the organic saturated water solution with 20 mg of the organo-clay for 1.5 h and (18) Lagaly, G.; Weiss, A. In Proceedings of the International Clay Conference, Tokyo; Heller, L., Ed.; Israel University Press: Jerusalem, 1969; Vol. 1 p. 61. (19) Lagaly, G. Clays Clay Miner. 1982, 30, 215. (20) Rosen, M. J. Surfactants and Interfacial Phenomena; WileyInterscience: New York, 1978; p 46.
2228 Langmuir, Vol. 12, No. 9, 1996
Figure 3. FT-IR spectra of (a) solid hexadecane (b) organoclay, and (c) liquid hexadecane showing that CTA+ is in a liquidlike form when exchanged to the clay.
Dickey and Carron
Figure 5. Raman spectrum of tricycloheptan-3-ol and quadricyclane sorbed to the organo-clay. The inset shows the reaction that occurs when quadricyclane is in water.
Figure 4. Raman spectrum of benzene sorbed from a saturated aqueous solution to the organo-clay.
determining the amount of organic contaminant remaining in solution. A blank was measured by repeating the above but without the organo-clay to take into account evaporation from solution, adsorption to the glass, etc. The numbers were subtracted to give the amount of organic contaminant adsorbed by the organo-clay. The crosssectional area of the adsorbed organic contaminant was calculated using the total amount of organic contaminant and its density. We assumed the organic molecule was spherical. This allowed the cross sectional radius of each molecule to be calculated using the density. This radius was then used to get the cross-sectional area of each molecule, which was then multiplied with the total number of sorbed organic molecules to give the total cross sectional area of the sorbed organic contaminant. Once the absolute amount sorbed was determined, we used Raman measurements for relative sorption changes. The model of the sorption process we are proposing assumes that CTA+ takes a cylinder-like form when it is exchanged to the montmorillonite SAz-1. The amount of CTA+ exchanged to the 20 mg of organo-clay used can be estimated using the CEC of the clay. Using a basal spacing of 22.9 Å (ref 3) as the height of the cylinder and 46 Å2 (ref 20) as the cross sectional area, the surface area of CTA+ can be calculated. θ is determined by dividing the total crosssectional area of the adsorbed organic contaminant by the total surface area of CTA+. The isotherms of benzene, ethylbenzene, and tertbutylbenzene on the organo-clay were made using dilutions prepared from saturated aqueous solutions and relative Raman measurements. Figure 4 shows the Raman spectrum of benzene sorbed from a saturated aqueous solution onto the organo-clay. Since quadricyclane is not stable in water, a different approach was used. We found that, in the time it took to saturate the water
Figure 6. Isotherms of (a) quadricyclane, (b) benzene, (c) ethylbenzene, and (d) tert-butylbenzene on the organo-clay.
solution with quadricyclane and separate out the quadricyclane that did not dissolve, sufficient tricycloheptan3-ol formed to develop a significant coverage on the clay. This can be seen in Figure 5. We alleviated this problem by eliminating the step of making the saturated solution and instead shortened the experiment by preparing a solution of known concentration. The isotherms (Figure 6) were plotted, and the slopes were determined. Since the isotherms are linear, the slopes are taken to be equivalent to the partition coefficient. The solubility and the partition coefficient can be used to form a linear free energy relationship. The linear freeenergy relationship for adsorption is
-log(S) ) log(K) +
µ°sat - µ°i 2.3RT
(1)
where S is the solubility, K is the partition coefficient, µ°sat is the chemical potential of the saturated solution, and µ°i is the chemical potential of the interface.21 We performed this analysis for each of the organic compounds (Figure 7). Using an adsorption model, we found a slope of 1.52, which is significantly larger than the predicted 1.00. The difference between the predicted slope and the observed slope correlated with the size of the sorbate. If we use the K predicted by eq 1 in a saturable sorption equation (Langmuirian), we find (21) Mullen, K.; Carron, K. Anal. Chem. 1994, 66, 478.
Sorption to CTAB-Modified Montmorillonite
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Figure 8. Plot showing the linear dependence of the θc/θm deviation on the molecular volume of the compounds studied. Figure 7. Free-energy diagram showing the relationship between solubility and partitioning of the organic molecules studied onto the organo-clay.
model the sorption process as absorption and use surface concentrations instead of surface coverage. The equation we used was
Table 1. Parameters and Experimental Values Used in Eq 1 molecule benzene quadricyclane ethylbenzene tert-butylbenzene
solubility (M)
K
θc
0.0211 14 0.47 0.00262 24 0.065 8.79 × 10-4 101 0.214 -4 1.74 × 10 223 0.118
θc )
θm
1022 × volume θc/θm (cm3/molecule)
0.156 3 0.0141 4.6 0.0305 7 0.00813 14.5
K1.52C 1 + K1.52C
K′ )
1.5 1.67 2.04 2.57
(2)
where K1.52 is the corrected partition coefficient and C is the concentration of the sorbate. The correlation between measured surface coverage (θm) relative to the predicted surface coverage (θc) and the molecular volume of the sorbate is given in Table 1. It can be seen that the deviation increases with molecular volume. A plot of θc/ θm versus molecular volume is shown in Figure 8. This plot shows a linear dependence of the deviation on molecular volume. We have interpreted the positive correlation to mean that large molecules have fewer sorption sites available to them due to steric hindrance. Assuming that the nonideality of the slope of the linear free-energy diagram takes into account the volume of molecules other than those in this study, then the solubility of a molecule in water can be used, along with the linear free-energy diagram, to predict the partition coefficient of the molecule on the SAz-1/CTAB organo-clay. The intercept of eq 1 is related to the driving force to reach 0.5 monolayer coverage. For our data, we found a y-intercept of 0.155. The sign of the intercept determines if 0.5 monolayers will be reached; a negative intercept indicates 0.5 monolayers will be reached while a positive intercept indicates 0.5 monolayers will not be reached. This result is consistent with our observation of coverages less than 0.5 in our isotherms. Our analysis described above depends on the model that we use for the sorption process. We initially assumed an adsorption model using surface areas; however, one could
χ1 χ2
(3)
where χ1 is the mole fraction of the organic contaminant in water and χ2 is the mole fraction of the organic contaminant in CTA+. The number of moles of these compounds was previously determined in the adsorption model. When we used eq 3 to calculate the partition coefficient, we found that eq 1 gave us a slope of 1.13 and an intercept of -2.18. Since eq 1 is a thermodynamic relationship, deviations from a slope of 1 can be assumed to be errors in the model. In this case, the sorption process appears to stem from an absorption into the CTA+ layer on the surface. An example of an application of eq 1 to an adsorption model for a well structured, self-assembled monolayer system can be found in ref 21. In this work we found the slope to be 0.997 with the adsorption model. Conclusion We have demonstrated the utility of Raman spectroscopy for the study of sorption at modified clays. The use of a Raman band associated with the modifier as an internal standard allowed us to find sorption isotherms. The model for sorption was tested using a linear freeenergy relationship between the sorption coefficient and the solubility. A deviation from ideal behavior was observed, and a model was developed that involved the molecular surface area of the surfactant and the molecular volume of the sorbate. If we assume an absorption model, we find much better agreement between our data and that predicted thermodynamically. This leads us to the conclusion that the sorption of organic compounds onto CTAB-treated montmorillonite is best describe as an absorption process. Acknowledgment. The authors gratefully acknowledge the support of the AFOSR through grant number 93-NA-187. LA9507702
