Langmuir 1993,9, 263-267
Fourier Transform Infrared Spectroscopic Study of the Adsorption of Cetyltrimethylammonium Bromide and Cetylpyridinium Chloride on Silica King-Hsi S. Kungt and Kim F. Hayes' Environmental and Water Resources Engineering, Department of Civil a d Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125 Received March 26,1992. In Final Form: August 24,1992 A Fourier transform infrared (FTIR) spectroscopic investigation of the adsorption of two cationic surfactants,cetyltrimethylammonium bromide (CTAB)and cetylpyridiniumchloride (CPC),on silica was conducted to identify the structural changes that occur as a functionof eurfactant surface coverage and state of wetness. In situ transmission and cylindrical intemal reflectance FTIR epectrometrystudieswere performed on silica self-supporting f h and colloidal suspensions, respectively. Spectroecopic results show that, in the presence of water, micelle-like, surfactant-aggregate clusters form on the silica surface, even at eurfactant surface coverage as low as 12 9%. Increasing the eurfactant sorption density reaults in an increeee in the size andlor the number of the eurfactant-aggregate dusters. In contrast, the etate of wetness appears to change the aggregation structure of sorbed eurfactants as a function of coverage. At low eurfacecoverage,adsorbed surfactant moleculeschange from the aggregated-clusterstate toa monomer state upon drying. During this drying process, the methylene chains of the surfactant molecules change from the aggregated state to a flat, parallel orientation on the eilica surface, preferring an association with the silica surface instead of with other methylene groups. Upon rewetting,the eurfactant molecules revert to their original aggregate-clusterform. For high surfacecoverage,no significantstructuralor orientational change is found during the drying and rewetting processes, presumably because the eurfactant molecules have reached monolayer or poesibly bilayer coverages, leaving insufficient room for sorbed surfactant molecules to rearrange upon drying. These results suggeetthat the spectra obtained from the preparation of dry surfaces are not necessarily representative of surfactant etructure and orientation in eitu.
Introduction Surfactants are currently being considered for their potential to retard the migration of hydrophobic organic contaminants (HOCs) in soh. Possible applications include in situ or ex situ surfactant modification of soil materials to provide an effective transport barrier zone. This treatment is based on the premise that surfactants sorbed to minerals will increase the hydrophobic organic content of the soil, thereby significantly enhancing the soil's sorptive capacity for HOCs. Recently, a substantial body of literature indicates that surfactant treatment of soil minerals is very effective at increasing their capacity for HOCS.'-~ However, less well known is how surfactant structure, degree of surfactant surface coverage, wetting and drying cycles, and the extent of HOC partitioning influence the relative hydrophobicity or stability of these coatings. A study conducted by Holeen et al.6 suggests that the relative hydrophobicity of a surfactant coating for HOCs depends on the extent of surfactant eurface coverage. Hence, in order to generate an optimal surfactant coating,it is necessarytodeterminehow surfactant surface coverage affects the hydrophobic properties and stability of the surfactant coatings. FTIR spectroscopic studies have been conducted to determine how surfactant surface structure may change as a function of changing solution conditions. Previous
* To whom all correspondence should be addressed.
+Current address: MS H824, INC-9, Los Alamoa National Laboratory, Los h o e , NM 87545. (1)Bouchard, D. C.; Powell, R. M.; Clark, D. A. J. Enuiron. Health
1988,A23,686. (2)Boyd, 5. A.; Lee, J.-F.; Mortland, M.M.Nature (London) 1988, 333,346. (3) Lee, J.-F.;Crum, J. R.; Boyd, S.A. Enuiron. Sci. Technol. 1989, 23,1366. (4) Srinivaaan, K.R.; Fogler, H. S. Clays Clay Miner. 1990,38,277. (5) Srinivaaan, K. R.; Fogler, H. S.Clays Clay Miner. 1990,38,287. (6) Hoben, T. M.;Taylor, E. R.; Seo, Y.-C.;Andereon, P. R. Enurron. Sci. Technol. lSSl,%, 1686.
FTIR studies have shown that it is possible to obtain molecular-leveldetails of surfactant structure in micelles.' However, in situ FTIR studies of the structure of adsorbed surfactant has been limited due to sample preparation and sensitivity limitations. For example, in previous applications of diffuse reflectance infrared spectroecopy, samples have been dried,8*@ which, as this study demonstrates, can lead to changes in the structure of the sorbed surfactant. Also, when conducting in situ FTIR studies using a cylindrical internal reflectance (CIR) liquid cell and aqueous colloidal suspensions,the sensitivitymay not be adequate for low coverage studies. In addition, high background surfactant solution concentrations or surfae tant sorption to the CIR crystal can make background subtractionsand interpretation of resultaat low coveragee difficult. However, by choice of a surfactant which sorbs strongly and high surface area particles, these problems can be circumvented and CIR FTIR spectroecopy can be used effectively. Alternatively, by preparing self-supported films of colloidal particles, the sensitivity of FTIR spectroscopy can be greatly enhanced and the problems associated with the CIR accessory can be avoided. In this investigation, FTIR transmissionmode studies of d f - ~ u p p o r t h films g of silica and CIR FTIR studies of aqueous colloidal suspensions of silica have been conducted. FTIR spectra were obtained at various surfactant surface coveragesand surface wetneases. Silicawas chosen as the sorbent eince it is ubiquitous in soil and aquifer material and has a relatively strong affmity for cationic surfactants. Cetyltrimethylammonium bromide (CTAB) and cetylpyridiniumchloride (CPC), cationic eurfactanta, were seleded for these sorption studies. Both contain a 16-carbon methylene chain as hydrophobic moiety and a (7) Umemurr, J.; Cameron, D. 0.;Mantsch, H.H.J. Phy8. Chsm. 1980,84,2272. (8) Sides, R.; Yarwood,J.; Fox, K. Mikrochim. Acta [Wienl 1988, ZZ, 93. (9) Sivamohan, €2.; de Donato,P.; C a w , J. M.Longmuir 1990,6,897.
Q 1993 American Chemical Society
264 Langmuir, Vol. 9, No.1, 1993
hydrophilic quaternary ammonium group. CTAB was initially seleded for the entire study but was’found to be inadequate for the CIR experiments due to its relatively low affmity for silica. CPC was found to have a much greater affmity for silica than CTAB and, hence, was more suitablefor the CIR work. The effect of surfactant surface coverage on the structure of surfactant surface clusters was investigated by CIR. The transmission mode and self-supportingsilicafilmswere used for studyingthe effect of wetting and drying cycles on surfactant dispersion/ aggregation state on the surface.
Experimental Materials and Methods Surfactants and Silica. Cetyltrimethylammonium bromide (Cl&N(CH3)3+Br) and cetylpyridiniumchloride (ClsHa3Pyr+Cl-)were purchased from Fluca Chemical ! and were used as received. Co. with purity greaterthan 98% The critical micelle concentration (cmc) for CPC and CTAB are both about 9 X 1V M in pure waterlo. Fumed silica of 99.8% purity was obtained from Sigma. This high-purity silica was reported to have a mean particle size of 11 nm and a surface area of 255 m2/g. The silica was washed with diluted hydrochloric acid and hydrogen peroxide followed by exhaustive washing with Milli-Q (Millipore, Corp.) water to remove any metal or organic impurities. The high surfacearea, smallparticle size silica was found to be optimal for conducting CIR studies and for forming self-supporting films. Silica Self-supporting Film Preparation. Silica self-supporting films were prepared to conduct transmission FTIR experiments. These films are considered selfsupporting in that once they are prepared they can be mounted in the sample compartment of an FTIR spectrometer like a pressed KBr pellet. To prepare a selfsupportingsilicafilm,a small amount of a silica suspension (0.04 mL of 3 g/L) was transferred onto a smooth polyethylene sheet and then evaporated at room temperature to dryness. After the silica film was air-dried, the thin, self-supporting film was carefully separated from the polyethylene sheet and mounted in a holder for spectroscopic study. The holder was made by cutting a hole in cardboard sheets and inserting the supporting film between the sheets. Sorption Isotherms. CPC sorption isotherms were obtained by mixing in a centrifuge tube 5.52 mg of silica with CPC solutions ranging in concentration from 50 pM to 3 mM. The pH was adjusted by adding 0.1 N NaOH immediately after mixing. After the tubes were capped and rotated end-over-end at 25 “C for 24 h, a portion of thesuspension (around 2 mL) was removed from the tubes for spectroscopic measurements. The remainder of the suspension was monitored for the final pH and was then centrifuged to separate the silica from suspension. The supernatant was analyzed for unadsorbed CPC by UV spectrometry at the wavelength of 269 nm. The amount of sorption was calculated from the difference between initial and final concentrations. NaCl was used as backgroundelectrolyteto maintain an essentiallyconstant ionic strength at 0.1 M. The final pH was maintained at 5.2 f 0.1.
FTIR SpectroscopicMeasurements. Infrared spectra were obtained using a Bio-Rad FTS-GOA FTIR spectrometer equipped with a liquid nitrogen cooled mercury cadmium telluride detector. Spectra of the samples were obtained with the coaddition of at least 256 and up to 2000 scans at a nominal resolution of 2 cm-1 (10) Mukerjee, P.; Myaele, K. J. Critical Micelle Concentrations of Aqueous Surfactant System; NSRDS-NBS 36; US Department of Commerce: Washington, DC, 1971.
Kung and Hayes
using a triangular apodization function. Spectra of aqueous samples were recorded by using a CIR acc888(vy or a demountable, 18 pm path length cell with CaFz windows (Spectra-Tech). The IR spectrum of the pure surfactant in the solid state was obtained by dissolving the surfactant in methanol, depositing the solution onto a KBr window, and then allowing the methanol to evaporate at room temperature. Infrared spectra of surfactant-coated silica were recorded using the CIR accessorywith zinc selenide or germanium crystals. While the type of crystal used did not affect the frequency positions of the IR bands, a greater sensitivitywas achieved with zinc selenide due to its lower refractiveindex. Hence, for sorption studies, the spectra obtained using the zinc selenide crystal are reported here. For the solutionmicelle measurements which did not require high sensitivity, the germanium crystal was used. Below the cmc, the zinc selenide and germanium crystals were both insufficiently sensitive to obtain reproducible spectra. For these measurements, the 18 pm path length cell with CaF2 windows was used. The final spectrum of the surfactant on silica was obtained by subtracting a previously obtained spectrum of a silica suspension under identical aqueous conditions except having no surfactant present. A spectral subtraction program supplied by Digilab was used for this background subtraction. A subtraction factor of between of 0.8 and 1.2 was typically employed to null out common features between the two spectra. A subtraction factor within this range never caused a peak shift of greater than 2 cm-I. In fact, the peak position which resulted from this subtraction procedure was usually insensitive (less than 1 cm-l shift) to the subtraction factor used. A similar subtraction method was also used for obtaining the final spectrum when using the demountable, sealed-precision path length cell. The peak position was located wing the screen cursor program supplied by Digilab. Others have reported that spectral shifts of less than 1 cm-I can be distinguished, regardless of instrument resolution, using similar background subtraction and peak location protoc~ls.~~ On the basis of our analysis of the subtraction and peak location techniques used in this study, shifta of greater than 2 cm-I were considered as a real change in the vibration energy. Spectra of CTAB-coated,self-supportingfilms of silica were obtained by introducing a drop of a CTAB solution onto a dry, self-supporting silica film. Samples were allowed to air-dry and spectra were collected at both wet and dry states. Relative quantities of sorbed surfactant on the silica film were controlled by increasing the surfactant solution concentration from 1.0 X 10-5 M up to the cmc. A blank silica film was prepared by subjecting a dried silica film to a water solution without surfactant. Final spectra were obtained by subtracting the spectrum of the uncoated silica film from the surfactant-coated selfsupporting silica film using the subtraction technique described above. Using this self-supporting film and transmittance FTIR, the sensitivity of the FTIR spectra was significantly enhanced compared to CIR studies at similar coverages. Spectra of surfactant monomers were obtained by placing a range of surfactant solution concentrations (5.0 x 10-4 to 8.0 X lo4 M), all below the cmc, in the 18 pm path length cell. Even though the monomer spectra had a relative higher signal to noise ratio and a much weaker band intensity compared to the other spectra obtained, (11) Scheuing, D. R. Fourier Transform Infrared Spect”py in Colloid and Interface Science. In Tronuform Infrared Spectroscopy in Colloid and Interface Science; ACS Symposium Seriea 447; American Chemical Society: Washington, DC, 1991.
Langmuir, Vol. 0, No.1,1993 265
Adsorption of CTAB and CPC on Silica
Table I. IR Band Assignments of the Methylene Chnin CH, Stretching Vibrations for CPC and CTAB state CPC (cm-1) CTAB (cm-1) monomer 2933,2861 2930,2860 2917,2849 solid 2916,2850 micelle 2923,2853 2923,2853
The FTIR spectra for the C-H stretching band region of CPC and CTAB in 0.01 M aqueous micelle solutions and in the solid state are shown in Figure 1. The two intense bands around 2920 and 2850 cm-l, in both CPC and CTAB spectra, were assigned to asymmetric and symmetric stretching vibration of C-CH2 from the methylene chain, respectively. For CTAB, the band at 2872 cm-l was assigned to C-CH3 symmetric Stretching vibration. The several weak bands appearing in the region of 2940 to 2960 cm-l in solid CTAB spectrum arise from C-CH3 asymmetric stretching and N-CH3 symmetric stretching vibrations. It is noted that the N-CHs stretching vibrations are not observed in the IR spectrum for CTAB in aqueoussolution. The disappearanceof N-CH3 is thought to be due to hydration of the quaternary ammonium.11 From Figure 1, it is seen that the CH2 stretching vibrational energies are higher in aqueous solutions than in the solid state. These energy shifts have been interpreted in terms of the structural order of the methylene chain group. For example, the frequencies, bandwidths, and integrated intensity of CH2stretching vibration bands have been shown to be correlated with the gauche/trans conformer ratio of methylene chains in micelles.llJ2 In particular, lower frequenciesand more narrow band widths have been found to result from a more ordered structure of the methylene chains, while increasingfrequenciesand widthe of these bands have been associatedwith increasing number of gauche conformers and disorder.11J2 These CH2 band shifta have also been found to be associated withgauche/trans transformations in studies in which the phase transition of a surfactant from solid to liquid state has been investigated.13J4 It is now generally accepted that the CH2 stretching absorption energies provide a measure of the degree of order/dwrder, compactness,and
crystallinity of the methylene chains in surfactant aggregate structures.1b17 For our samples,the higher vibrationalenergiesobserved in aqueous micelle solutions compared to the solid state suggesta a less ordered and compact structure (implying a higher gauche/trans conformer ratio) of the methylene chain for the micelle solution compared to the solid phase state. This indicates that the methylene chain changes from a more ordered, predominantly trans conformation in the solid phase13J4compared to a more disordered structure in the solution micelle phase.12 Furthermore, an even higher energy shift (Table I) for CPC and CTAB monomers below the cmc is observed. This is indicative of an even less structured,aqueousenvironmentcompared to the micelles and is consistent with interpretation from other studies that have shown a higher and constant position of CH2 vibrational stretching energies below the cmc for a series of alkyl chain monomers.18 Just as an increasing of CH2 stretching vibrational energies have been found to be associated with an increasingdisorderof the methylenechain,11-17differences in these vibrational energies may also be a reflection of the relative hydrophilic environments of the hydrophobic methylene chain.ls In the solid state, there is little or no water around the CTAB molecules and the methylene groups associate with each other. The relatively hydrophobic environment of the methylene groups in this solid state correlates with the relatively lower energies for the CH2 stretching vibrations. For CTAB molecules in aqueous solution below the cmc, the methylene groups are surrounded by water molecules. In this more hydrophilic environment, more gauche conformers may be formed in order to reduce the length of contact between the methylene groups and the water molecules. Hence, relatively higher vibrational energies are observed for the stretchingvibrationalmodes in the monomer state. Above the cmc, micelles are formed. In this case, the methylene groups experience a less hydrophobic environment than those in solid state, due to water penetration into the micelle core, but a more hydrophobic environment compared to the CTAB monomers below the cmc. Therefore, a middle range of CH2 vibrational energies and gauche/ trans conformer ratio would be expected. These observations and interpretations suggest that the relative hydrophobic state of sorbed surfactant molecules also correlateswith changesin the energyof the CH2 stretching vibrational modes. Research is currently underway to investigate the relative affinity of surfactant coatings for different hydrophobic probe molecules to verify this. Adsorption of CPC on Silica. Figure 2A shows the adsorption of CPC on silica at pH 5.2 in the presence of 0.1 M NaCl. From the shape of the sorption isotherm, CPC apparently has a high affiiity for the silica surface. Almost all the CPC in solution phase was adsorbed onto silica for sorption concentrations below 0.7 mmoUg. The
(12) Weem, J. 0.; Scheuing, D. R. Micellar ehape to rod transitions. In Tronsform Infrared Spectroscopy in Colloid and Interface Science,
19.3685. .., - - - - .
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Figure 1. FTIR spectra of (A) CPC and (B)CTAB in solid state (I) and in 0.01 M aqueous micelle solution (11).
the band positions could still be located relatively accurately and precisely. The CH2 asymmetric band position for the monomer was determined by visually locating the center of peak minima from transmittance using the s o h a r e cursor. On the basis of replicate analysis, the location of the center of the band was found to be independentof solutionconcentrationwith reproducibility of at least 2 cm-1. Results and Discussion FTIR Spectra Assignment of Methylene Chain.
ACS Sympoeium Series 447; America Chemical Society: Washington,
Dc,1991.
(13) Kellar, J. J.; Young,C. A.; Kuntaon, K.; Miller, J. D. J. Colloid Interface Sci. 1991,144, 381. (14) Nawlli, C.; Rebolt, J. F.; Swalen, J. D. J. Chem. Phys. 1986,82, 2136.
(15) Cameron, D. G.; Casal, H. L.; Mantsch, H. H. Biochemistry 1980,
(16) Lotta, T. I.; Laskkonen, L. J.; Virtanen, J. A,; Kinnunen, P. K. Chem. Phye. Lipide 1988,46, 1. (17) Lotta,T.I . ; V i e n ,J. A.;Kinnunen,P. K. J. ChemPhys. Lipids
----.--.
198A. 43.13.
(18) Umemura, J.; Mantach, H. H.; Cameron, D. G. J. Colloid Interface Sci. 1981, 83, 558.
266 Langmuir, Vol. 9,No.1, 1993
Kung and Haye8 (E)
(A)
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Figure 2. (A) CPC adsorption isotherm on silica at pH 5.2 in 0.1 M NaCl background electrolyte. (B) Same as part A, with scale expanded to show the low equilibrium concentration.
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Figure 3. CIR FTIR spectra of CPC adsorbed on silica. Data pointa correspond to samples at sorption levels of 0.2, 0.5, 0.7, and 0.8 mmol/g Si02 from Figure 2.
adsorption maxima of 0.8 mmol/g was attained for an equilibrium concentration of CPC of about 0.5 mM. The sorption isotherm of Figure 2A is replotted on a log scale for CPC equilibrium concentration in Figure 2B. The isotherm exemplifies the typical characteristics for cationicsurfactant sorption on a negatively-chargedsilica surface. The common mechanistic interpretation is that in the region of low surfactant coverage (below the CPC solution concentration of 0.01 mM) the CPC molecules adsorb through an electrostatic attraction to the surface. In this region, a slope of unity is usually found, and it is generally thought that individual monomeric species are formingwhich are uniformly distributed acrosethe surface. At intermediate coverages (between 0.01 and 0.02 mM) wherethe slopeof the isothermplot increasesdramatically, it is generally thought that aggregate surfactant clusters are forming and that the increased slope occurs as a result of the favorablehydrophobicinteraction of the methylene chain groups juxtaposed to one another at this higher coverage. Finally, at high coverage, a plateau is reached indicating that the maximum surface coverage has been reached. This leveling off usually occurs near the cmc of the surfactant. Therefore, increasing the surfactant solution concentration results in an increasing number of solution micellesbut does not alter the adsorption density. The samples corresponding to the isotherm data points near 0.2, 0.5,0.7,and 0.8 mmol/g Si02 were examined spectroscopicallyusing CIR FTIR to identify the structure ofsorbed CPC on silica. Over this range of isotherm data, the surface coverage goes from 12 to 60% assuming that each surfactant headgroup occupies 0.26 nm2. As can be seen in Figure 3, FTIR spectra of sorbed CPC over the entire range of coverages have similar CH2 vibrational energies near 2925 and 2852 cm-I. These CH2 vibrational energies of sorbed CPC also have the same energy position as those found for solutionmicelles (TableI). These results
BlLAYER Figure 4. Schematic representation of the change in surface aggregate structureas increasingsorptiondensityon silica surfaca for monolayer and bilayer models.
indicate that a micelle-like aggregate structure forms on the silica surface, even at relatively low CPC surface coverage. Therefore, the FTIR spectra suggeat that the effects of increasing the sorption density of CPC on silica surface only resulta in an increase in the size or number of the micelle clusters rather than a dramatic change in their structure. A schematicrepresentation of the change in the surface aggregate structure as a function of d a c e coverage, based on these FTIR results, is given in Figure 4.
On the basis of past theoretical and thermodynamic studies'"22both monolayer and bilayer aggregateclusters have been proposed to form at relatively low coverages on charged mineral surfaces. From the M'IR study presented inthiswork, itimotposeibletoclearlydistinguiahbetween the monolayer (Figure 4A) or bilayer cluster (Figure 4B) structures because each may have similar CH2 vibrational frequenciesand the differenmamaybe unresolvablewithin the IR resolution of 2 cm-I used in this study. However, results from other in situ spectroscopic studies indicate that the formation of bilayer structures, even at low coverages is likely for these systems.~~" Effect of WetnessonCTABA m a t i o n . Theeffect of wetness on surfactant surface aggregationstructure was studied by transmission FTIR spectrometry using selfsupporting silica films. The FTIR spectra of CTAB on a silica self-supporting film at low surface coveragea under wet and dry surface conditions are shown in Figure 6. For the wet state (spectrum A), the CH2 stretching vibrational bands were found at 2925 and 2862 cm-l, suggesting that surfactant micelle clusters have formed on the surface. These resulta support those obtained from CIR studies discussedabovewhich show that micelle-likecluskm start to form even at low surface coverages. Upon removing water from the silica surface by air-drying, the CHZ stretching vibrational bands shift to higher energies at 2930 and 2858 cm-I (Figure 5B). Since these higher CH2 vibrational energies are also found for CTAB monomers in solution (Table I), we propose that the sorbed CTAB moleculesrsanangethemeelvesfromaneegregatedmi~e structure when the surface is wet to a diepersedmonomer state upon drying. During the drying process, the surfactant appears to change from a clustered state, with the methylene chainspointing predominantly into the solution phase, to a more dispersedstatewith separated monomers lying flat on the surface. As shown in Figure 5C, CH2 (19) Yeskie, M.A.; Harwell, J. H.J. phy8. C h m . 1988,92,2946. (20) Denoyel, R;Rouqueml, J. J. ColloidlnterfaceSci. 1991,119,655. (21) Somasundamn, P.; Kunjappu, J. T. Colloids Surf. 1989,97,246. (22) Cues, J. M.; Villieru, F. Langmuir 1992,8,1261. (23) Esumi, K.;Nwahama, T.; M m o , K. Colloids Surf. 1991, 67, 149. (24) Lee, E. M.;Simieter, A.; Thomas, R. K. Longmuir 1990,6,1031.
Langmuir, Vol. 9, No.I , 1993 267
Adsorption of CTAB and CPC on Silica
I I
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Wavenumber (cm ) Figure 1. FTIR spectra of CTAB on a self-supportingsilica f i i at low surface coverage for (A) wet surface, (B)dry surface, and (C) rewet surface.
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Figure 6. Proposed schematic representation of CTAB structurelorientationon silica at low surface coverage for wet and dry states.
stretching vibrations of the adsorbed CTAB molecules
shift back to their original low energy values (at 2926, 2862 cm-1) when water is reintroduced onto the surface.
Apparentlyupon rewetting, the dispersed monomers revert to the original aggregate clusters. A schematic representation of pomible CTAB aggregation structures at the silica/solution interfacial region for the wet and dry surface conditions at low surface coverage is shown in Figure 6. For the wet stage, the methylene chains form an aggregate structure due to the favorable hydrophobic interactions in the presence of water. However, upon drying, dispersed monomers apparently favor an association with silica surface compared to other methylene chains. Another possibility is that the sorbed surfa&anta may move apart upon drying due to the repulsive interactions of the polar cationic head groups that are lea well shielded as water is removed. In either case, the higher vibrational energies found for the dry,low coverage state are consistent with a more disordered structure (higher gauche/traneconformer ratio) indicating leea surfactant clustering and a less hydrophobic CTAB environment on the silica surface. At very high CTAB coverage no significant differences in the vibrational energies were found between the wet and dry samples (Figure 7A,B). However, slightly lower CH2 stretching vibrational energies were found at the higher coveragea (2923 and 2862 cm-l) compared to the low coverage systems suggating that sorbed CTAB was approaching a complete monolayer or bilayer on the silica surface. T h e lower CH2 stretching vibrational energies are consistent with a decrease in the gauche conformer content of the clusters and represent a more structured, hydrophobic aggregate for the higher coverage compared to the lower coverage system. Figure 7C showe the effectsof subtracting the wet state CTAB spectrum from that of the dry state spectrum at high surface cowrages. When thisprocedure is performed, it is seen that the spectrum of sorbed CTAB in the dry state at high coverageis very similarto the CTAB spectrum
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Wavenumber (cm ) Figure 7. FTIR spectra of CTAB at high surface coverage on silica at (A) wet stage and (B)dry states and (C) difference spectrum after the drying process.
for the solid-state CTAB (compare vibrational energies in Table I and Figure 7). This reault indicates that at high CTAB loading, the silica surface is probably covered by a least a monolayer and perhaps a bilayer, and that some solid form of CTAB is also forming upon drying. That no monomer forms on the surface upon drying is probably a result of little or no "free" surface for CTAB to disperse to at high surface coverages.
Conclurions The resulta of CIR and transmission FTIR spectroscopic studies on aqueous oxides suspensions and self-supporting f h ,respectively, lead to the following conclusions for CTAB and CPC sorption on silica: (1) Sorbed CPC and CTAB molecules form aggregate clusters on silica surface in aqueous solution, even at low coverage (12 76 1. (2) At low surfactant coverage, upon drying, surfactant molecules change from aggregated clusters to dispersed monomers with the methylene chain lying flat on the silica surface. However, surfactant molecules revert to their original aggregate form upon rewetting. (3) For high surfactant coverages, no significant shifts in the CH2 stretchingvibrational energiesare found during the drying process. On the basis of the resulta of the subtraction of the wet and dry spectra, the presence of solid-state surfactant has been identified, suggesting that part of the sorbed surfactant is crystallized on the surface as the silica surface is dried. (4) The relative shifts in CH2 vibrational energies can be used to assess the relative hydrophobic properties of sorbed surfactant with lower energies reflecting a more structured and hydrophobic environment. (5) Spectra obtained from the preparation of dry surfaces are not necessarily representative of the structure and orientation of sorbed surfactant in situ. Acknowledgment. Funding for this research was provided by the Office of Research and Development, US. EnvironmentalProtection Agency under Grant R-815750 to the Great Lakesand Mid-Atlantic Hazardous Substance Research Center, and the National Science Foundation (NSFGrant BSC-8968407). Partial funding of the r-ch activity of the Center is also provided by the State of Michigan Department of Natural Resources. The content of thispublication does not necessarily represent the viewe of these agencies.
