Langmuir 1992,8, 2183-2191
2183
Fourier Transform Infrared Attenuated Total Reflection Spectroscopy Linear Dichroism Study of Sodium Dodecyl Sulfate Adsorption at the Al%Os/WaterInterface Using A1203-Coated Optics R. P. Sperline,’ Yuan Song,+and H. Freiser Strategic Metals Recovery Research Facility, Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received February 24,1992. In Final Form: June 8, 1992
Fourier transform infrared (FTIR)attenuated totalreflection (ATR)spectroscopywas used to characterize the surface excess and molecular orientation of sodium dodecyl sulfate (SDS)adsorbed at the Al2Odwater interface. A method is described for the quantitative IR-ATR determination of Gibbs’ surface excess on hydrophilic solids, in the presence of aqueous solutions (“in situ”). The method was used in an in situ linear dichroism (LD) study of the orientation of both the alkyl chains and OS03- head groups of adsorbed SDS on Al2O3. A1203 was present as a thin film (150-400nm) on the surface of ZnSe IR internal reflection elements. This approach makes full range IR-ATR adsorption measurements feasible for solids which absorb strongly in the IR region. Adsorption isotherms were determined for SDS (2 X 10” to 4 X 10-3 M,0.15 M NaC1) adsorption onto A 1 2 0 3 films. The adsorption densities found, 0.1-1.5 monolayer at pH 3.8,and 0.2-1.5 monolayer at pH 6.6, conform closely to literature values. Under conditions giving an average of 1.5 monolayers coverage, LD studies of the IR absorption bands of adsorbed OSOS-groups revealed an average angle between the group pseudo-& axis and the surface normal of ca. 43O. Under the same conditions, alkyl chains in adsorbed SDS exhibited an ordered packing with an angle of 45O between the extended chain axis and the surface normal, with the appearance of free rotation of the chain about the extended chain axis.
Introduction IR-ATR is very attractive for elucidation of surfactant adsorbate structures, in the presence of surfactant solution. The exact adsorbate structure is expected to depend on the hydrophobicity of the solid surface, the net surface charge, the solution ionic strength, and the surfactant structure and concentration.’ Spectroscopic studies of these structures must obviously be performed without drying the sample. IR-ATR allows (a) the in situ analysis of adsorption and (b) simultaneous quantitative and qualitative analyses of both the surfactant itself and any coadsorbed molecules. In the case of SDS adsorption on alumina, little spectroscopic evidence has been collected relating to the question of whether a “bilayer-island” structure exists at monolayer total coverage. It is unlikely that bilayers would exist on hydrophobic surfaces, but these bilayers (“admicelles”) are predicted to form on hydrophilic surfaces, particularly at lower pH (e.g., pH 3.8), where protons can compete with alkali counterions.lc Fluorescence probe work has demonstrated that at pH 6.5 and 0.1 M NaCl, surfactant aggregates exist on the surface, at lower than monolayer coverage,z but similar work at lower pH has not been published. This paper represents part of an effort to use IR-ATR and linear dichroism to distinguish between admicelles and monolayer “hemimicelle” structures at the Al~O$water interface. The wealth of literature, based predominantly on the spectra of dried adsorbate layers, suggests that adsorbed surfactants assume the structure of micelles or hemimi-
* Author to whom correspondence should be addressed.
+ Viiting Scholar from Fudan University, Shanghai,PRC. (1) (a) Harwell, J. H.; Hoekine, J. C.; Schechter, R. 5.;Wade, W. H. Langmuir 1985,1,251-262. (b) Yeskie, M. A.; Harwell, J. H. J. Phys. Chem. 1988, 92, 2346-2352. (c) Bitting, D.; Harwell, J. H. Langmuir 1987, 3, 500-511. (2) Chandar, P.; Somasundaran, P.; Turro, N. J. J. CoZZoid Interface Sci. 1987,117,31464.
celles, whether deposited from solution or by Langmub Blodgett techniques. The spectraof dried adsorbate layers show orientations similar to that found in crystals of surfactant. Adsorption isotherms give calculated surface coverages near the packing densities found in X-ray crystal structures. In the presence of surfactant solution,however, the actual structure could consist of a more loosely packed monolayer and a smaller component of bilayer. This would, on the average, give the coverage of a denselypacked monolayer. A very small lowering in packing density in the monolayer would allow chain motion along the long axis (molecular “director”) of an all-trans alkyl chain. If as suggested by the words “hemimicelle” and “admicelle”, the in situ adsorbate structure closely resembles that of free solution micelles, similar molecular motions should be observed. Lindman and Wennerstr6m reviewed the possibilities of motion in micellar s~rfactants.~ NMR data show that SDS molecules in micelles undergo rapid rotation about their long axes and slower rotation end-t~-end.~ Linear dichroism (LD) spectroscopy makes determination of the average orientation of surfactant molecules pos~ible.~ Frey and T a ” performed LD orientation analysis of the CHZand amide bands of peptides supported in lipid bilayers physically pressed onto a Ge internal reflection element (IRE) on the surface of liquid DzO, both in the presence of the DzO and after drying.6 LD analysis has been performed for the CHz groups of alkyl chains in Langmuir-Blodgett (LB)films deposited onto hydrophobic I R E S , ~for~ in situ examination of the (3) Lindman, B.; WennerstrBm, H. In Solution Behauior of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum: New York, 1982; Vol. 2, pp 3-26. (4) Johansson,A.;Lindman, B. InLiquid Crystals and Plastic Crystals; Gray, G. W., Wineor, P. A., Ede.; Ellis Horwood: Chichester, England, 1974; Vol. 2, pp 192. (5) (a) Frey, S.;Tamm, L. K. Biophys. J. 1991,60,922-30. (b)Cropek, D. M.; Bohn, P. W. J. Phys. Chem. 1990,94,6452-6467. (6) Maoz, R.; Sagiv, J. J . Colloid Interface Sci. 1981, 100, 46-96. (7) Kimura, F.;Umemura, J.;Takenaka, T. Langmuir 1986,2,96-101.
0743-7463/92/2408-2183$03.00/00 1992 American Chemical Society
Sperline et al.
2184 Langmuir, Vol. 8, No. 9, 1992 orientation of stearic acid adsorbed from CC& onto Ge,9 and for examination of orientation in LB fiis of calcium stearate and amy1alcohol on A1203 (CH stretching region only).lo Umemuraet aL8andHigashiyamaandTakenaka11 also studied carboxylate head group orientations. Allara and Nuzzo performed analogous orientation analyses of fatty acids spontaneously adsorbed onto Ala03 films on Si by external reflection of the dried layers. They were able to determine the orientations of chain CH2, carboxyl head groups, and methyl, vinyl, and propargyl chain termini.12 Several other papers have dealt with methylene orientation in micellization and phase transitions of surfactant solutions, particularly alkyl sulfates and sulfonatee.13-1s Miller and Kellar have applied LD analysis to the IRATR spectra of sodium dodecyl sulfate adsorbed onto solid A1203 IREs, in the presence of aqueous solution.16 At the A1203 surface, interactions of head groups with the surface should result in band shifts and orientational ordering of the head groups, but in bilayer structures both bound and unbound head groups may be present. Hydrogen bonding, for example, should lock the head groups in position and lead to pronounced linear dichroism. In bilayers, some difference in orientation angle would appear between the bound and second-layer head groups. In previous studies,17bands due to terminal methyl groups showed solvatochromic shifts upon micelle formation. Similar changes are expected to differentiate between bilayer and monolayer adsorbate structures. We have demonstrated methods for quantitative, in situ analyses of adsorption a t both solid-liquidl8 and liquidl i q ~ i d ' interfaces ~*~ by I-R-ATR. These methods have led to dramatic advances in the science of mineral flotation21 and are beginning to be used in analyses of protein adsorption. Originally, these methods were limited to studies of adsorption at the surfaces of traditional IR transmissive IREs, but we are extending them to be applicable to coated IRESa t a level of accuracy previously unachievable.22 The first, necessary step in exploration (8) Umemura, J.; Kamata, T.; Knwni,T.; Tnkennkn,T. J. Phys. Chem. 1990,94,62-67. (9) Yang,R.T.;Low,M. J.D.;Haller,G. L.;Fenn, J. J . ColloidZnterjace Sei. 1973, U , 249-258. (10) Hdler, G. L.; Rice, R. W. J . Phys. Chem. 1970, 74,4386-4393. (11) Higashiynmn, T.; Tnkenaka,T. J.Phys. Chem. 1¶74,78,942-947. (12) (a) Allnra,D. L.; Nuzzo, R. G. Langmuir 1985,1,4652. (b) Same, refs 6-20. (c) Allnrn, D. L.; Nuzzo, R. G. Langmuir 1985,1, 52-66, and references therein. See also Soriaga, M. P.; Hubbnrd,A. T.J . Am. Chem. SOC. 1982,104,3937-3945. (13) (a) Mantach, H. H.; Knrthn, V. B.; Cameron, D. G. In Surfactants
in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Preas: New York, 1984; Vol. 1, pp 673-690. (b) Knwni,T.; Umemura, J.; Tnkennkn, T. Colloid Polym. Sci. 1984, 262, 61-66. (c) Kawni, T.; Umemura, J.; Tnkennkn,T. Bull. Znat. Chem. Res. Kyoto Univ. 1983,61,314-323. (d) Holler, F.; Calli4 J. B. J. Phys. Chem. 1989,93,2053-2068. (14) Weers, J. G.; Scheuing, D. R. In FTZR Spectroscopy in Colloid andZnterfaceScience;ACSSymp. Ser. 447;Scheuing,D. R., Ed.; Americau
Chemical Society: Wnshington, DC, 1990; pp 87-122. (15)Cross, W. M.; Kellar, J. J.; Miller, J. D. Preprints, XVZI Znternationul Mineral Processing Congress; Dresden, Sept 23-28,1991; pp 319-338.
(16) Miller, J. D.; Kellar, J. J. In Challenges in Mineral Rocessing; Sastry, K. V. S.,Fuerstennu, M. C., Eds.; Society of Mining Engineers: Littleton, CO, 1989; pp 109-129. (17) Umemura, J.; Cameron, D. G.; Mantch, H. H.J . Phys. Chem. 1980,84,2272-2277. (18) Sperline, R. P.; Murnlidhnran, S.; Freiser, H. Langmuir 1987,3, 198-202. (19) Sperline, R. P.; Freieer, H. Langmuir 1990, 6, 344-347. (20) Sperline, R. P.; Freiser, H. Soluent Extr. Zon Ezch. 1992,10,297312. (21) (a) Kellar, J. J.; Cross, W. M.; Miller, J. D. Appl. Spectrosc. 1989, 43,1456-1459. (b) Kellar, J. J.; Cross, W. M.; Miller, J. D. Appl. Spectrosc. 1990,44,1508. (c) Kellar, J. J.; Cross, W. M.; Miller, J. D. Sep. Sci. Technol. 1990,25, 2133-2155. (22) (a) Mielaarski,J.; Nowak,P.;Strojek, J. W. Znt. J. Miner. Process. 1983,11,303-317. (b) Kup, K. J.; Roberta, N. K. Colloids Surf. 1987, 24, 1-17.
of surfactant structures via IR-ATR is to demonstrate that evaporated thin films of A1203 behave like the A1203 powders used commonly for adsorption work. This paper reporta a successful method for quantitative surface excess determination of SDS on A1203 films evaporated onto ZnSe IREs. With this method, adsorption isotherms were determined for SDS adsorption onto the A1203 at two pH values. Quantitative reproduction of the published isotherms demonstrates the utility of the film method. Linear dichroism analyses for the sulfate and CH2 infrared bands are discussed. We demonstrate that certain spectral features are consistent with the formation of bilayers.
Experimental Section Materials. Sodium dodecyl sulfate (Fluka),as received, was dried under vacuum before use. Distilled water was used throughout; other reagents were reagent grade. All glassware was rinsed with dilute nitric acid and distilled water prior to use. Stock solutionsof 0.15 M NaCl were preservedwith0.1 g/L NaNs and had ca. 0.01 g/L AlaOs powder added in an attempt to prevent dissolution of the AlzOs f i i . pH was adjusted with dilute HC1. SDS eolutionswere prepared from these stocksolutionsby serial dilution. Solutions less concentrated than 8 X lo-' M in SDS were prepared twice in the same flask,without riding or drying, to equilibrate the solutionswith reagent adsorbed on the veesel WallS. Data Collection. IR spectra were acquired with a PerkinElmer 1800FTIRequipped with a narrow band mercury cadmium telluride detector, Perkin-Elmer CDS-3 software, and a Pneumatech -100 OF frost point compressed air dryer. Acquisition parameters were as follows: nominal resolution, 2.01 cm-l; Jacquinotstop,4; apodization,"medium"; acquisition,double beam; interferograms,double sided;phase, self-determined,128pointa; mirror velocity, 1.5 and 1.5; gain 1and 1;neutral density fiiters, none in the sample beam and "3" (20% transmission) in the reference beam; cycle, 3, 12, 25, 13, 1. The Model 1800 has a slow-scanning interferometer. Ten cycles totaling 250 sample and 250 reference scans were averaged per spectrum,requiring ca. 13 min collection time. The Ge optical fiiter caused sine waves (channel spectra) in the baseline, so was not used. No polarizer was used for quantitative adsorption measurements. Parametersappearingin Table I are from spectrasmoothedwith a 25 point (one point per cm-') CDS-3 routine, while data appearing in Tables I1 and I11 are from spectra smoothed with a 5 point routine. Internal reflection elementa (IRES)were 52.5 X 20.0 X 2.0 mm ZnSe parallelograms and trapezoids. A l a 0 8 was rf sputtered commercially by either CVI Laser, Inc., Albuquerque, NM, or Tucson Optical Research, Tucson,AZ,onto one flat side of each IRE. Film thicknesseswere rechecked periodically by obrvatim of the 3A/4 channel spectrum absorbance maximum in the W using a 1 2 O incidence specular reflectance device (HnrrickVRAPES/WW-BDG). The refractive index of sputtered A l a 0 8 was takentobe 1.62intheUV (valuefromTucsonOpticalRegeprch). In the IR, values of n for AlzOs were taken from Toon and Pollack." Before each background/sample acquisition wries, both the IRE and cell were cleaned by rinsing with water, ethanol,and CCC, then the IRE was exposed to an rf Oa plasma as previously described.'S
The clean IRE was immediately mounted in a stainleee steel, water-jacketed solution cell (Harrick ScientificMEC-ITC)kept at 28.0 OC by a circulatingwater bath. Thiiwas, also, the average temperature in the spectrometer sample compartment. Experimenta with nonadeorbing solutions had revealed that the two nonported corners of the sample cellacted likethe *dead-spaces" found in flow-injectionanalysis(FIA).The problem wna corrected by drilling two additional sampie flow ports, giving a port in each of the four comers. Solutionswere admitted to only the Alaoscoated side of the IRE, via Teflon tubing, and were retained by a Viton O-ring which had been rinsed with ethanol and CCl+ (23) Toon, 0. B.; Pollack, J. B. J . Geophys. Res. 1976,81,5733-5748.
SDS Adsorption at the AlzOslWater Interface The nonsolution side of the IRE contacted an aluminum-foil covered O-ring. Background solutions were introduced by gentle suction while tipping the cell to eliminate air bubbles. The filled cell was placed in a Harrick Scientific Model TMP-F-PO5 45’ twin parallel mirror reflection attachment and beam energy was maximized by alignment of the cell and mirrors. Alternatively, the cell was placed in a Harrick Scientific Model TMP-220 variable angle twin parallel mirror reflection attachment set at 45O. A long mirror arm in the TMP-22Oallowedthe use of trapezoid geometry IREs. By use of uncoated IREs, the incidence angles of the reflection attachments and numbers of solution sensing reflections in the cells were calibrated using the previous pr~cedure.~‘The TMPF-PO5 incidence angle was 45.0°, while the TMP-220 incidence angle was 44.1’. After the cell was assembled and filled with reference solution, the cell was allowed to equilibrate for 1h before collections of the background spectra were begun. During this period the 0rings compressed, as evinced by the time dependence of the 0ring absorption bands. After 1h, the O-ring absorbance became constant and was subtracted as part of the background. After collection of reference spectra, 10-20 cell volumes of sample solution were flushed through the cell by gentlesuction, displacing the reference solutions. Care was necessaryto prevent air bubble introduction. Molar absorptivities for the SDS IR bands were determined accordingto Beer’s law using a Wilmad 116-3circulartransmission cell with 25 X 2 mm CaF2 windows and Teflon spacers. Path lengths were determined from empty cell channel spectra. The SDS bands used for the surfaces excess analyses were the two v, OS03- bands at ca. 1200 and 1250 cm-’. The small change in the relative sizes of the two bands upon adsorption (in unpolarized ATR spectra) was ignored in the isotherm determinations. The bands were integrated together over the same integration range, 1320-1110 cm-l, as that used to obtain the integrated molar absorptivity for this band pair by transmission. Molar absorptivities of the individual bands (including the v, oso3- band at ca. 1060 cm-l) were determined from the amplitudes found by peak fitting of the transmission spectra. Polarized-light ATR spectra were obtained using a Double Diamond Ge polarizer (Harrick Scientific,99.0% efficient). With the plane of reflection within the IRE lying horizontally, an electric field parallel to this plane is obtained when the polarizer is oriented with the 0’ mark in a vertical direction. This orientation will be noted as “parallel”. Separate spectra were obtained, beginning 3 h after initial solution contact, for the 0, 90,180, and 270’ polarizer positions. The spectrometer was not sufficiently stable for equilibrations longer than about 6 h. Generally over 90% of the adsorption density observed after 3 h of exposure had been observed after 1 h of exposure, and virtually no change was observed between the spectra taken at 2 and 3 h. Longer equilibrations could not be examined because (a) the spectrometer did not remain sufficiently stable for more than 6 h and (b) the A1203 became continuously thinner during solution exposure, losing over 100 A (>5%) in 6 h. Data Reduction. Sample and reference spectra were subtracted with amplitude scaling of the reference spectra, to flatten the liquid water background. Residual water vapor bands were removed from the difference spectra by spectral subtraction, sometimes after shifting the water vapor spectra by *1 cm-1. Optical constants were interpolated from available data for Optical properties of the sputtered ZnSe,= H20,%and &&.23 AlzOs fibs were assumed to be the same as for crystalline A l 2 0 3 . By using the previously developed equations, it was possible to calculate the Gibbs surface excesses for adsorbed species in the presence of surfactant s o l u t i ~ n s . ~Specifically ~J~ (24)Sperline, R. P.;Muralidharan, S.; Breiser, H. Appl. - . Spectrosc. . 1986,40,~101&1022. (25)(a) Jamieson, T.H. Proc.-SPZE Int. SOC. Opt. Eng. 1983,430(9), 163-171. (b) Feldman, A.;Horowitz, D.;Wader, R. M.; Dodge, M. J. US. NTIS AD reDort #AD-A045095,1977,43 PP, identical to US.NBS Tech. . . Note 1979,No. &3,71 pp. (26)(a) Downing, H.D.; Williams, D. J. Geophys. Res. 1976,80,1656. (b) Pinkley, L.W.; Sethna, P. P.; Williams, D. J.Opt. SOC.Am. 1977,67, 494.
Langmuir, Vol. 8, No. 9,1992 2186
where A is the measured band absorbance, N is the number of solution sensing internal reflections, d,,3 is Beer’s law ‘effective path lengthmin the third phase (defined by Hansen as b d , d3 is the decay constant for the time average of the square of the electric field in the third phase as defined by Hansen (similar to the ‘penetration depth”), c is the molar absorptivitiy of the surfactant, c b is the bulk solution concentration of surfactant (M), and r is Gibbs’ surface excess (mol ~ m - 9 .Attenuation ~~ indices for the analyte phase (phase 3) were calculated from the molar absorptivities and the bulk concentrations, as the sum of the sample and solvent attenuation indices, at each wavelength. Wavelength was taken from the center of the integration range (1215 cm-1 for determinations of I’) and the optical properties of all phases were taken to be constants for each integrated band. Use of the full expressions for the expectation values for the squares of the electric fields allows automatic compensation of de,3and d3 for both absorption by the A1203 film and absorption caused by the aqueous final phase.27 Analyte absorption can reduce both b,tr and d3 when the attenuation index in the third phase exceeds ca. 0.01. Only the band height intensities are responsible for this effect, so the artifically large integrated molar absorptivities must be scaled down by some arbitrary factor, e.g., 1O00, before calculation of beeand d3. Use of the real (not scaled down) integrated molar absorptivitiesin eq 1then gives the correct resulta for the integrated total absorbance. Instrument polarization ratio and the correct averaging of the parallel and perpendicular components of de.3, required when no polarizer is used, were determined by published methods.” In the polarized spectra, subtraction of polarized reference spectra compensated for instrument polarization ratios. Spectra selected for band fitting were transferred to an IBMPC compatible computer.29 The Fortran peak fitting program,g0 modified by us to accommodate IR bands, used a MarquardLevinson least-squares method to fit Gaussian band shapes to the data by iterating band positions, widths, and intensities and a linear baseline. The program at this time does not provide variances for the fitted parameters, nor for other band types, e.g. Voigt shapes. After the 0-180’ and 90-270’ (polarizer angle) spectral pairs were checked for consistency,each spectrum was fitted to a series of Gaussian peaks. Residual liquid water background curvature was removed by the addition or subtraction of small amplitudes of the followingbands: (a) parallel polarization 824.3,980.9, and 1189.6 cm-’ of halfwidths 82.6,111.4, and 56.8 cm-’, respectively, and (b) perpendicular polarization 669.5,1063.9, and 1180.9 cm-l of half widths 159.6, 87.1, and 47.6 cm-l, respectively. These parameters were found by fitting the water background spectra directly. The band parameters were averaged for each perpendicular or parallel polarizer pair to yield the parallel and perpendicular parameters for further calculations. To obtain the LD absorbance ratio for each band, the calculated bulk solution absorbance was subtracted from the averaged experimental absorbance of the corresponding polarization, and the ratio of the results taken.
Results and Discussion Adsorption Isotherms. Figure 1 shows the surface excess results for SDS adsorption onto A1203 films from solutions containing 0.15 M NaC1, at pH 3.8 and pH 6.6, respectively. Included in that figure are points representing the surface excess results of Bitting and Harwell, obtained under similar conditions by solution depletion. At pH 3.8 the ATR isotherm results agree well with that of Bitting and Harwell. A t pH 6.6 the curve shapes are ~
-
(27)(a) Hamen, W. N.; Kuwana, T.; Osteryoung, R. A. Anal. Chem. 1966,38,1810-1821. (b) Hansen, W. N.J. Opt. SOC.Am. 1968,58,380390. (c) Hansen, W. N.Adu. Electrochem. Electrochem. Eng. 1973,9, 1-60. (d) Note that eq 55 in ref a has been corrected in ref c, eq 28. (28)Sperline, R. P.Appl. Spectrosc. 1991,45,677-681. (29)Sperline, R. P.Appl. Spectrosc. 1991,45,1046. (30)Lichtenberger, D. L.;Copenhaver, A. S. J. Electron. Spectrosc. Relat. Phenom. 1990,50,335-352.
Sperline et al.
2186 Langmuir, Vol. 8, No. 9, 1992 -8.5
I
1 Q
-9
-
= pH 3.8. ATR = pH
A.
a x
I -9.5 -
v)
m
X
w a,
8
r
-10-
pH 6.6, ATR = + pH 6.6, Bitting = x
5 Y v)
40 -10.5
-4.5
Q
x
I
-2.5
-3.5 Log [SDS (molar)]
Figure 1. Adsorption isotherma for SDS adsorption onto AlzOs in 0.15 M NaC1: ( 0 )28 O C , pH 3.8, IR-ATR results, this work; (m) 30 OC, pH 3.8, from ref IC;(+) 28 O C , pH 6.6,this work; (X) 30 OC, pH 6.6,from ref IC. similar, but they are offset by ca. 0.4 log l' unit. We attribute this offset to differences in surface ionization between these sputtered films and the y phase alumina powder used by Bitting and Harwell. At present, we do not know the structural phase of our sputtered A1203 films. The UV-visible index of refraction of the films reported by the supplier (1.62) is somewhat lower than that of sapphire (IZD = 1.761,suggesting an amorphous structure. A proportional reduction in n for A1203 in the IR would result in an increase in realcof only 6 % The relative contributions of interfacial and bulk surfactant species to the total absorption is given by the ratio of the two terms in eq 1. At 1.0 X l W 3 M SDS, bulk solution SDS contribution to the totalspectral absorbance is ca. 11%. At higher concentrations, this fraction increased and the calculated I? values became more irreproducible, thus setting an upper limit on the analyzable concentrations. At concentrations below l X lo4 M,at pH 6.6, the signal-to-noise ratios of the spectra did not allow accurate calculation of l'. At SDS concentrations below 1 X M, at pH 3.8, the results became more irreproducible, notably because of the presence of additional, nonnoise bands near those for SDS. The lower limit of concentration could perhaps be reduced by modification of the apparatus to improve signal to noise either by increasing the number of solution sensing reflections or by better matching of the instrument beam to the input port of the IRE. We could also calculate l' from the individual uaS and u, OS03- bands after band fitting, which removed some of the influence of bands due to adsorbed contaminants, but considerably more scatter in the isotherm curve8was seen. This variation in relative band intensities in the fitting resulta represents the influence of noise in the data points near the peak maxima. Contaminant band interference near the base of the SDS peaks was more readily removed by the fitting procedure than when near the SDS band maxima. No interference from the CH2 band progression was o b s e ~ e d . ~ Small changes in experimental conditions led to irreproducibility in l', and conditions had to be tightly controlled to obtain meaningful results. The general spectral background slope and curvature in the 1300-1000-~m-~ region are strongly affected by small changes in sample temperature. The absorbance of liquid water, and presumably ita dispersion, changes rapidly in this region. Equilibration of the freshly cleaned A1203filmswith liquid water for 1 h minimum was required to stabilize the baseline. Followingthe 0 2 plasma treatment, a very broad sigmoid wave slowly appeared between 1300and lo00cm-1.
.
The wave could not be removed by flowing various solution through the cell, including organic solvents and dilute mineral acids and bases. This wave correeponds with the spectrum of the O-rings in contact with the plate. The metal foil masks supplied with the ATR flow cell can trap a thin layer of solution against the IRE. This layer is not readily flushed out when introducing the sample solution, so the masks were not used. The A1203 films can change significantly during an experiment. Contact with air bubbles seemed to contribute to film failures. The thickest A l 2 0 3 (0.4 pm)films failed by detachment, while in thinner filmsrapid thinning occurred in areas in contact with the bubbles, often perforating the filmswithin 3 h. In the absence of bubblee, the loss of A1203 from the films was more predictable. After exposure of the filmsto reference and SDS solutions for 4-6 h, filmsbecame measurably and uniformly thinner, losing some 50 A or 100 A when the solutions contained 0 or 0.15 M NaCl, respectively. Linear Dichroism Theory. The linear dichroic (LD) ratio for a band is defined as the amplitude ratio for two spectra taken of a single sample in perpendicular and parallel polarizer orientations (LD = A l / A i ) . Orientation angles for the functional groups were calculated by the method of Frey and Tamm,S from the LD of the two v, OSOS-bands, the one u, osos- band, the one 6 CH2 band, and the u, and us CH2 bands. Absorbance in the X direction is proportional to the expectation value of the square of the electric field in the X direction (Et2),to the square of the unit vector component (e.g. sin2 e) of the dipole change of the considered vibration (transition moment or TM) along the X direction, and to the magnitude of the T M this is true for each of the Cartesian components of the electric field. In the parallel polarization, interacting electric fields exist in both the X and 2 directions, but not in the Y direction, while in the perpendicular polarization, an interacting electric field exists only in the Y direction. For a random array of molecules and a given band, molecular TM vectors are equally distributed about the axes, so that
A,
(E;)
-I
All
(E:)
+ (E:)
(2)
Consider an adsorbed alkylated surfactant having a vibrational mode with its TM parallel to the director,which is inclined to the surface normal at an angle 8. When the molecules are associated in patches where the hydrocarbon chains are all aligned, TM vectors of the groups all have the same 2 vibrational component, although the various patches on the surface are oriented diffbrently in the XY plane. Averaging the interaction of the TM through all directions in the XY plane, the net fraction of the total moment along the Y direction is 1/2, and the net fraction along the X direction is 112. Again, in parallel polarization only the X and 2 fields are nonzero, and in perpendicular polarization only the Y field is nonzero, so that the LD ratio is given by
A ,, -= All,
(E:) sin2 (0)
(E:) sin2(e)
+ 2 ( ~ , 2cos2 ) (e)
(3)
or
If the Osos- groups in SDS are bound to the A1203 surface
Langmuir, Vol. 8, No. 9, 1992 2187
SDS Adsorption at the AlnOslWater Interface
a t a constant angle, u,(OSO~-) is an "axial" vibration and should obey eqs 3 and 4. Consider a vibrational mode in the same adsorbed surfactant, having its TM a t 90" to the molecular director. Assuming a free rotation about the director, the situation is noted in Frey and Tamm's equations by a = 90" (a= angle between molecular transition moment and the chain director) A --I,transv All,trmsv
(E;) (1+ cos2 (8))
(Ex2)(1+ cos2 (8)) + 2 ( E t )sin2 (8)
(5)
or sin (8)=
(
2((E;> - LD,r,,(E,2))
2LD,mv(Ez2)
+ (E;) - LDtrmsv(Ex')
)'"
(6)
Again, if the OSOS-groups in SDS were presented to the surface at a constant angle, the two nearly degenerate, orthogonal um(os03-)bands behave as if the group were freely rotating about the local pseudo-C3, axis and should obey eqs 5 and 6. These equations are valid for all vibrations exhibiting a cylindrical distribution about the axis of inclination (free rotation about the director). In an extended all-trans alkyl chain, both the CHZstretching modes and one CH2 scissor mode have TM vectors perpendicular to the long molecular axis (chain director). For eq 5 to hold, the CH2 groups must be free to rotate about the chain director. We have taken the view that the OSOS-group does not rotate with the CH2 groups, and because the two um modes are both normal to the pseudo-C3, axis, the OSO3- groups are spectroscopicallycylindricallysymmetrical unless specific binding occurs. If a molecule has no rotation about the chain director, different equations apply for the LD ratio. In the case where the bisectors of the CH2 groups (direction of us(CH2) and 6(CH2) TM vectors) lie exclusively in the plane of the surface (XY), none of them experience a Z direction electric field. The LD ratio would be given by the "locked horizontal" model (7) In the case where the bisectors of the CH2 groups form a plane which includes the 2 axis (the "locked vertical" model)
(E;) cos2(8) , = LD1," = A ~ ) (8) All (E;) cos2(8)+ 2 ( ~ , sin2
(8)
or
tan (e) =
(
(E;)
1
- LD(E,~) lI2
(9) 2LD(E,') One danger in the interpretation of angles above 50" is that this angle is near the so-called magic angle (cos2(8) = 1/3), at which the dichroic ratio for axially rotating molecules becomes identical with that of a randomly oriented ensemble of molecules. At low adsorbate coverages,nearlyrandom head group orientation (0-90" from normal) is a possibility. LD Analysis for OSOS-Bands. ATR spectra of SDS adsorbed on A1203 were obtained for 0.001-0.002 M SDS
solutions at pH 3.8 containing no NaC1, and at pH 3.8 and pH 6.6 in solutions containing 0.15 M NaC1. These conditions were chosen to maximize the formation of admicellar bilayers in the presence of salt, while retaining hemimicellar monolayers in its absence.1 Table I shows the effects of pH and ionic strength on the LD ratios of the three major OSO3- stretching bands (u, 1200,1245 cm-' and vB 1060 cm-l) of SDS adsorbed onto A12O3, alongwith parameters for transmission spectra of micellar and nonmicellar SDS solutions. Table I1shows additional similar spectra taken at 4 and 28 "C. Both Tables I and I1 show the LD ratios calculated for ATR spectra of a presumed random orientation of the oso3group at the interface. Figure 2 shows the parallel and perpendicular polarization IR spectra of SDS adsorbed onto A1203 from 1 X M solution, in the OS03stretching and CH2 bending regions. Tables I and I1 contain the ratios of band amplitude taken between the 1200-and 1245-cm-' bands, and between the 1200- and 1060-cm-1 bands (not LD ratios). In bulk solution, the transmission spectra show that the two bands of the Y, vibrations are more nearly identical than they are after adsorption onto A1203, except at very low coverage. These ratios are marginally larger under the assumed bilayer formation conditions (higher concentration, with added NaC1) than under the assumed monolayer formation conditions. The same comparison for the vB band is less conclusive because the bands are of different symmetry. These observations indicate a small loss in intensity for the 1245-cm-1 band upon adsorption. This will be discussed below. Equations 4 and 6 were solved for 8, the angle between the pseud0-C3~axis of OSOa- and the surface normal. To confirm the band assignments, both eqs 6 and 4 were applied to all bands. The combinations giving large tilt angles (ca. 60-70") were rejected as impossible, given the mainly compact monolayers expected under these conditions. The average angle between the pseudo-C3, OS03axis and the surface normal was found to be ca. 43" (42.3 f 1.3" for um Table I, 43.3 f 2.4" for um in Table 11, and 42.7 f 1.9" for all um), from the um OSOa- bands, while the one us band tended toward an angle near 48". A t 1X lo4 M, where the coverage is much lower, the angle 8 determined from the v,(OSOa-) bands was over 50°, suggesting a structure where some of the adsorbed head groups lie parallel to the surface. These results can be, and we believe are, caused by hydrogen bonding with the A1203 surface. The ratios of band amplitudes within one spectrum, at constant polarization, indicate a small loss in intensity for the 1245-cm-' Y, band upon adsorption. Hydrogen bonding with the A1203 surface would further lower the symmetry of the Os03- group, in addition to causing the bathochromic shift seen for the spectra under monolayer-only deposition conditions (Table I). For the 1060-cm-'ueband, binding of one of the three originally identical 0 atoms would cant the TM away from the original C3, axis, reducing the symmetry. A movement of the TM by only lo", in addition to the 43" inclination calculated from the other bands, would put the tilt calculated for the 1060-cm-1 band into the 50" range observed. LD Analysis of CH2 Bands. LD ratios for the combined scissor mode band of the CHZchain (6 CH2, 1465 cm-') were analyzed for two SDS solutions at 4 "C, 0.15 M NaC1, pH 3.8 and 6.6, and one SDS solution at 28 "C, 0.15 M NaC1, and pH 3.8. In addition, the u(CH2) region (2800-3000 cm-l) was analyzed for the 28 "C spectrum. Table I11contains the results of these analyses.
2188 Langmuir, Vol. 8, No. 9, 1992
Sperline et al.
Table I. Spectral Parameters and Tilt Angles, SDS/AlaOa, 28 O C
ATR Spectra (cm-l) Darallel
half width
% bulkb
1201.5 1249.0 1060.1
27.1 21.4 11.1
6 7 3
1209.7 1250.6 1062.0
25.8 20.3 6.4
9 10 5
1208.6 1250.3 1061.6
27.0 19.0 8.4
15 17 9
v
23.3 19.8 9.7
1208.7 1248.7 1061.9
ampl. ratios" LDrandC 0.001 M SDS, No NaC1, pH 3.8
17 17 10
23.6 23.3 10.7
1200.3 1245.0 1059.8
(cm-l) a m ~ l . r a t i ~ * em Y
9 4 10
1.3 2.1
1197.8 1246.5 1058.3
LDmeasd 0.746 0.699 0.523
41 42
1.4 2.1
0.001 M SDS, 0.15 M NaC1, pH 6.6 1207.2 0.498 1249.1 1.5 0.478 1060.1 2.5 0.665
0.712 0.626 0.522
42 45
1.4 2.3
0.002 M SDS, 0.15 M NaC1, pH 3.8 1206.3 0.507 1249.1 1.5 0.497 1061.0 2.7 0.587
0.753 0.690 0.429
41 43
1.2 2.7
0.002 M SDS, 0.15 M NaC1, pH 6.6 1206.2 0.498 1246.8 1.4 0.478 1060.2 3.4 0.665
0.720 0.654 0.469
41 43
O.OOO1 M SDS, 0.15 M NaC1, pH 3.8 1199.7 0.498 1244.2 1.3 0.478 1059.2 1.3 0.665
0.551 0.454 0.693
51 57 53
1.1
2.4
micellized (cm-l)
half width
ce
1209.9 1246.5 1060.2
21.3 19.9 8.7
396 327 116
0.507 0.497 0.587
tilt (deg) transvd
1.4 2.9
Transmission SDectra ampl. ratio" not micell (cm-1)
(Ev2)/(Ex2)f
53
1.69 1.67 1.85
50
1.65 1.60 2.10
49
1.69 1.67 1.85
48
1.66 1.60 2.10
57 54 56
1.65 1.60 2.10
ampl. ratio"
half width
ce
1.1
18.2 19.9 9.8
425 398 179
1209.2 1244.1 1057.3
1.2 3.4
tilt (deg) axiale
2.4
Ratio of amplitude of 1208-cm-l peak, to peak. Percent of measured absorbance due to solution species. LD ratio (ALIA[)for presumed random orientation. Calculated for transition moment transverse to chain director. e For transition moment parallel to chain director. f LD ratio calculated for transition moment lying only in plane of A1203 surface. 8 Molar absorptivity, M-' cm-l.
Table 11. OSOS- Spectral Results, SDS/AlaOa, 4 and 28 OC ATR Spectra ampl ratid mode
v
(cm-l)
half width
parallel
LD perp
rand6
tilt (deg) meaed
@YmC
0.643 0.702 0.445
46 42
0.758 0.702 0.434
40 42
0.628 0.678 0.493
47 43
symd
0.001 M SDS, 0.15 M NaC1, pH 3.8,28 OC
v~s(OSO~-) Y~o(OSO~-) Yn(oso3-)
u,(OSOs-)
~~s(OSO~-) vs(OSO3-)
v,(oso3-) vs(OSO3-) v.s(OSOS-)
1208.5 1251.4 1062.7
25.8 20.2 7.1
1206.0 1251.2 1062.5
27.5 20.4 8.0
1206.7 1250.7 1060.6
26.8 19.6 9.9
mode v~o(OSOS-) VU(OSOS-) dOSO3-)
0.504 1.4 1.8
1.3 2.6
0.491 0.602
0.001 M SDS, 0.15 M NaC1, pH 3.8,4 OC 0.504 1.7 1.4 0.491 3.6 0.602 1.3 0.001 M SDS, 0.15 M NaC1, pH 6.6,4 OC 0.504 1.3 1.2 0.491 2.0 2.6 0.602
Transmission Spectra Used for Both 4 and 28 OC (Micellar) Y (cm-1) ef 1210.6 1248.4 1060.5
429 8 358 f 7 230 18
49
43
51
ampl ratio0 1.2 1.9
*
a Ratio of amplitude of 1210-cm-l peak, to peak. LD ratio (ALIA!) for presumed ramdom orientation. c Calculated for transition moment transverse to chain director. For transition moment parallel to chain director. e Molar absorptivity, M-l cm-l. f *Standard deviation of c.
Neither the 2896-, 1447-, and 1048-cm-' shoulders, nor the 1376-cm-1 CH3 deformation were considered. The 6(CH2) band is weak and suffers some interference from water vapor bands, despite continuouspurging and spectral subtraction. The v(CH3 bands, however, are unequivocal. Consider the structure of an all-trans (CH2)%chain lying at some angle to a surface onto which it is adsorbed. Both vas-and v,(CH2), and the 6(CHz) TM vectors lie transverse to the chain director (long axis). The u,(CH2) and 6(CHz) momenta lie in the plane of the C-C-C- atoms, while the v,(CH3 moment is normal to the C-C-C- plane. If the
adsorbate structure is such that the normal to the C-CC- plane is inclined to the surface (locked horizontal model), as in the known X-ray crystal structure of the SDS hemihydrate?l only the v,(CHz) moment will have a component normal to the surface (2direction). Both the v,(CH2) and 6(CHz) bands would exhibit LD ratios of (Ey2)/(Ez2) (eq7) between1.6and1.7,inourexperimental setup. (31) Coiro, V. M.; Mazza, F.;Pochetti, G. Acta Crystallogr.,Sect. C: Cryst. Struct. Commun. 1986,42,991.
SDS Adsorption at the AlzOdWater Interface
cm-'
1550
Langmuir, Vol. 8, NO. 9, 1992 2189
985
Figure 2. In situ SDS/Al203IR-ATR spectra and band fitting, OS03- and CH2 bending region: both 0.001 M SDS in 0.15 M NaCl, pH 3.8. Parallel (upper traces) and perpendicular (lower data; (-1 fitted Gaussian curves, traces) polarizations: baseline, and s u m of curves. (-e)
Table 111. CH2 Spectral Results, SDS/AlzOs, 4 and 28 O C ATR Spectra LD tilt (deg) % b u l P randb measd trans+ 0.001 M SDS, 0.15 M NaCI, pH 3.8,28O C 9 0.419 0.524 47 2956.1 13.1 10 0.419 0.565 45 2924.2 12.3 9.2 9 0.412 0.552 45 2873.5 8.5 11 0.412 0.605 42 2853.1 17 0.462 0.600 45 1465.3 11.1 0.001 M SDS,0.15 M NaCl, pH 3.8,4 O C 9.5 3.6 0.462 0.808 35 1467.0 0.001 M SDS, 0.15 M NaCl, pH 6.6,4O C 9.8 3.7 0.462 0.693 40 1465.1
u (cm-l)
mode
u,(CHs) u,(CHz) u.(CHa) u,(CH2) 6(CHz) S(CH2)
6(CHz)
half parallel width
Transmission Spectra, 28 O C , also Used for 4 O C (Micellar) mode u (cm-l) mode Y (cm-1) edge
v,(CH3) v,(CH~) v,(CH3)
2956.0 2926.0 2872.7
196 11 592* 1 92 h 2
Y,(CH~) 2854.8 6(CHz) 1467.1
*
343 26 86* 18
*
Percent of measured absorbance due to solution species. LD ratio ( A J A I I )for presumed ramdom orientation. Calculated for transition moment transverseto chain director. Molar absorptivity, M-' cm-l. e &Standarddeviations of t. The observed LD ratios of 0.4-0.9 for all the bands (including OSO3-bands) indicate that all examined modes have a large 2 electric field component, precluding a 'locked horizontal" adsorbate structure like that of the solid hemihydrate. If we assume the adsorbed SDS to be undergoing rotation about the chain director, as in micellar s ~ l u t i o n , eq ~ . ~6 for a freely rotating "axially symmetric" molecule with a 90" angle between the TM and the director can be used for all transverse modes, including v,(CH2), v,(CH2), and 6(CH2). Tables I1and I11 show an excellent consistency of orientation tilt angle for all the transverse bands in the 28 "C spectra and fair consistency a t 4 "C. While the "locked vertical" model gives reasonable angles for the tilt of the chain director (ca 41"), it gives a much smaller tilt angle when applied to the 6(CH2) band (33-35") and is less consistent between experiments than the "axial rotation" model. Miller and Kellar observed the LD ratios of the CH2 stretching bands of SDS adsorption, on a solid A1203 IRE in the presence of aqueous solution and concluded that
3100 cm-' 2 700 Figure 3. In situ SDS/A1203IR-ATR spectra and band fitting, CHdCHs stretching region: both 0.001 M SDS in 0.15 M NaC1, pH 3.8. Parallel (uppertraces) and perpendicular (lower traces)
polarizations: (--) data, (-) fitted Gaussian curves, baseline, and s u m of curves. the adsorbed SDS assumed a random orientation.16 A reexamination of their spectral data reveals an LD ratio for random orientation of 0.65 and, for free axial rotation, tilt angles of 42O and 32" at 2.0 X lo4 M (LD = 0.95) and 1.0X l W 3 M (LD = 1.42),respectively. For that calculation we assumed an incidence angle of 60" in the commercial A1203 IRE. Simply calculating with an incidence angle of 45"did not account for the discrepancy. Orientation angles nearer to 40" support our conclusion that adsorbed SDS exhibits a reasonably well ordered interfacial layer on A1203 under these conditions. Adsorbate Structure. The spectra taken a t 4" did not include the v(CH2) region, and the 6(CH2)results were not sufficiently consistent to allow a conclusion about the reduction of motion at temperatures below the Krafft point. Mantsch et al.13aWeers and Scheuing,14and Cross et al.16 have examined the IR spectra of SDS phases and solutions at various temperatures and note characteristic changes in the spectra, particularly across the Krafft temperature (9-15 "C for SDS), signifyinga change in ordering of the alkyl chains. The presence of chain "kinks" and bends due to gauche and trans conformations in micellar SDS solutions has been analyzed by Holler and C a l l i ~and l ~ ~by Weers and Scheuing.14 Holler and Callis estimate each SDS molecule in a micellar solution to contain 0.68 kink states (gauchetrans-gauche), 0.77 double gauche states, and 0.36 bent end-methyl states. Examination of a space-filling molecular model reveals that in the kink state, 9 of the ll CH2 groups retain the orientation of v,(CH2) TM present in the all-trans conformation. In the double gauche state, all the CH2 groups but one retain roughly parallel v,(CH2) TM vectors. This general retention of the angle between the TM vectors and the averaged chain director means that these disorder mechanisms have a relatively small effect on the tilt angles found in LD analyses of adsorbed SDS. Examination of a space-filling molecular model also reveals that in the bent end-methyl state, the TM of the v,(CH3) is within 30" of being transverse to the chain director, and in the all-trans state, it is ca 35" off the chain director. In the gauche-gauche state, the v,(CH3) TM is also transverse to the all-trans chain director. In all states the v,(CH3) TM vectors have strong components transverse to the all-trans chain director. Thus we expect both v(CH3) bands to behave similarly to the CH2 bands.
2190 Langmuir, Vol. 8, No.9, 1992
X-ray crystal structures of the 2:1,5l 1:1,32and 8133 SDS hydratee show planes of OS03- head groups, with straighb ened (CHZ)~ chains angling away from the 0803- planes. The chains are always parallel. The angle between the chain director and the OS%- plane depends on the degree of hydration; in the SDSHzO = 81 structure, the chains are nearly vertical ( 7 9 O ) to the planes of osof, while in the hemihydrate the chains lie at 46.4O from the ososplane (43.6O off normal). In the monohydrate, one of the two unique SDS ions is slightly bent near the OS03- end, but the chains are otherwise parallel and lie at ca. 3 9 O off the OSOa-plane(s). In the hemihydrate, the C-0-S bonds form a continuation of the all-trans CHZstructure, with an angle of 42.6O between the S-O (next to carbon) bond and the sulfate plane normal. Unlike the expected single plane of oso3- groups expected in the fiit layer of SDS near the A l z 0 3 surface, the SDS monohydrate and the 81 structures have both two planes of OSO3- in each lamellar layer of molecules. In these considerations,we have argued that the bending and kinking of the alkyl chains result in LD ratios indistinguishable from those arising from an adsorbate structure composed completely of all-trans alkyl chains. It is possible that the packing of surfactant molecules on the A1203 surface is slightly higher than that in micelles, due to (a) the lack of curvature and (b) immdbilizing hydrogen bonding of the head groups to the Also3surface. This more dense packing would reduce the occurrence of chain bend and kinks, leading, in fact, to a nearly all-trans adsorbate structure, with free rotation about the chain directors. Improvements in the signal to noise to allow observation of bands due to gauche chain conformations, followed by a conformation analyses of the type done by Holler and CallislM would settle this issue. Evidence for the existence of "bilayer-islands" may lie in the hypsochromic shift in the 1210-and 1245-cm-I bands when conditions change from those favoring monolayers (low concentration, low ionic strength, or higher pH) to those favoring bilayers (Table I). Comparison with the micellar and nonmicellar transmission spectra suggests that os&- groups a t the A 1 2 0 3 surface exhibit the 1201-cm-' band position, while os@- groups freely solvated by water, as in micellar and nonmicellar solution, exhibit a band at 1208-1210 cm-'. In "bilayer islands", the 0803- groups in the second adsorbate layer would necessarily be solvated by water, giving rise to increased absorbance at 1208-1209 cm-l. In these analyses, only region; two Gaussianswere used to fit the 1200-1250-~m~~ the noise in the spectra did not justify using more. It appears, however, that in spectra of bilayers two bands from the first layer of adsorbed SDS and two bands from the second layer are actually present. Growth of the second-layer bands is seen as shifts in the two bands used to fit the overall spectrum. Conclusions To our knowledge, these results represent the first in situ determinations of orientations of adsorbed surfactant head groups. In part, the significanceof this method to the study of adsorption at hydrophilic surfaces is that it allows quantitative analyses on solids which would otherwise be too opaque in the longer wavelength ranges. We have explored the technique of coating a standard hydrophobic internal reflection element (IRE)with a very thin f i i (ca. 100-400 nm) of a hydrophilic oxide. The (32)Coiro, V. M.;Manigramw, M.;Mazza, F.;Pochetti,G. Acta Crystallogr., Sect. C: Cryat. Struct. Commun. 1987,43,850-864. (33) Sundell, 5.Acta Chem. S c a d . Ser. A 1977,31,799-807.
Sperline et al.
oxide film is sufficiently thin to allow penetration of the ATR evanescent field to the oxide/solution interface and provides a hydrophilic surface for adsorption. The work of Miller and Kellar,lsin usingIRES made from solid pieces of mineral fluorapatite, has demonstrated that there is sufficient application to justify improvements k the methodology. The present coated IRE method is a major improvement,particularlyfor the study of adsorption onto less IR transparent solids. By use of a polarizer, the method now permits the determination of LD ratios, in situ, for surfactants adsorbed onto oxide surfaces in the lower IR region (below 2000 cm-I). Previously, this kind of analysishad to be performed either on dried fiis,or f i i deposited via the LangmuirBlodgett technique, neither of which is representative of in situ conditions, or on unsuitable solid IRE materials (mostly hydrophobic solids). The LD results for SDS/AlzOsadsorption indicate that, in addition to the os03 groups being tilted with respect to the surface normal, in the fiist layer of adsorbed SDS, there is a lowering of symmetry and a bathochromic shift in the OsoQ group, Suggestive of bonding of one or two (not three) of the sulfate oxygenswith the alumina surface. This effect is probably due to hydrogen bonding of the SO3 with surface AlOH groups. The similarity between LD ratios for the mutually orthogonal ves(CHz) and v,(CHz) bands (and the WHz) band) in the same spectrum demonstrates that at 28 OC, there is free or nearly free rotation about the CH2 chain axis in adsorbed SDS. The tilt angle found by in situ IR-ATRis nominally 45O, in agreement with the known X-ray crystal structure of SDS hemih~drate.~'This conclusion was also reached by Cross et al. by comparieon of the alkyl CH stretching spectra of the hemihydrate and the spectra of SDS adsorbed on solid A1203 IREh at adsorption densitiesgreater thanca. 1.5X 1Wl0mol cm-z.16 The near equality of the tilt angles found by in situ IRATR for the OS03-groups with the respectiveangles found in the crystal structure reinforces the idea. A further conclusion to be drawn is that the tilt angle of the second adsorbate layer is nearly identical with that of the first layer. Whether the chain axes of the second layer are parallel with those of the fiit layer or are similarly tilted but form a chevron pattern with the first layer cannot be decided from these data. This work is part of a continuing effort to find spectroscopicevidence for or against the formation of "bilayer-islands" of adsorbed surfactants. The shift of the v,&so3-) bands to higher energy and the margiaally lower relative intensity of the 1248-cm-l band, under bilayer conditione, support the notion of "bilayer-islands" of adsorbed SDS in the presence of NaC1. These experiments indicate that more careful definition of the differences between adsorption in the presence and the absence of added electrolyte is in order. Experiments are underway to examine the effect of changing from Na+ to Li+in the added electrolyte. We are pursuing further improved temperature studies in an attempt to observe a change in orderipg withii adsorbed surfactant layers. Decreased thermal disorder should favor formation of bilayers (and more mamive phase separation, which must be avoided). In some surfactqt systems,the "chain-melting" disorder transition occurs above room temperature. We are currently engaged in conducting in situ IR-ATR linear dichroism studiesof alkyl benzenesulfonatee(ABS). Dodeoyl ben"sulf0nat.e adsorbs strongly onto Also3 under the same conditions as does SDS and exhibits even more pronounced LD ratios than does SDS. The OS03-
SDS Adsorption at the AlzOsl Water Interface
head group is rigidly connected to the chain in linear alkyl benzenesulfonate surfactants, so a more accurate check on the head group orientation angles could be made by comparison of the LD results for the head group and the C6H4 group. Preliminary results indicate that dodecyl benzenesulfonate bonding with the alumina surface is pH dependent, similar to that of SDS. Further experiments are underway to explore whether this bonding affects the orientation of the hydrocarbon chains in these, and other,
Langmuir, Vol. 8, No. 9,1992 2191
surfactants, by simultaneous analyses of the head group and alkyl group vibrational spectra.
Acknowledgment. We thank the National Science Foundation, Engineering Division, for its support (Grant CTS-9006874). We also thank Dr. S. Roberta,Department of Biochemistry, University of Arizona, for assistance with molecular graphics programs and for helpful discussions.
