Infrared Reflection Absorption Spectroscopy (IRRAS) of Aqueous

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Infrared Reflection Absorption Spectroscopy (IRRAS) of Aqueous Nonsurfactant Salts, Ionic Surfactants, and Mixed Ionic Surfactants Alissa J. Prosser and Elias I. Franses* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907-2050 Received June 21, 2002. In Final Form: September 17, 2002 When a chromophore in solution is not surface active, it can still be detected via IRRAS at the air/water interface, down to the concentration level of 1-10 mM. This is demonstrated with new data on sodium sulfate and sodium methyl sulfate. With surface active solutes, like sodium dodecyl sulfate (SDS) and sodium dodecylsulfonate (SDSn), which adsorb from solution and form monolayers at the air/water interface, IRRAS primarily detects the monolayers up to about 10 mM, with little or no influence of bulk solution absorption. Above about 10 mM at 25 °C, the presence of monomers or micelles in solution may be detected, in addition to the monolayer. For both SDS and SDSn, the RA intensities, and thus the adsorbed monolayer densities, increase with their concentration and sodium chloride concentration (10 or 100 mM), as expected from the equilibrium surface tension data. The wavenumbers of the CH2 IR stretch bands indicate that the monolayers are not close-packed, consistent with the commonly reported minimum areas per molecule (ca. 40 Å2) determined from tensiometry. The polar group bands are not well-resolved and suggest little crystallinity. The splitting of the antisymmetric SO3 band, observed for SDS in the presence of salt, is not observed for SDSn or mixtures of SDS with SDSn. Adsorption from 50/50, by mole, mixtures of SDSn and SDS, or deuterated SDS, in water or 100 mM NaCl was also probed by IRRAS. Data from the polar group bands and the CH2 or CD2 bands indicate that the adsorbed monolayer is mixed and enriched in SDS.

1. Introduction The adsorption of soluble surfactants at an air/water interface is important in many applications, including spreading and coating flows, and lung surfactant formulations.1-5 The surface densities of soluble surfactant systems are typically inferred indirectly from surface tension data along with the Gibbs adsorption isotherm.6 In recent years, several experimental techniques have advanced so that surfactant adsorption at the air/water interface can be directly probed, rather than characterized from its surface tension behavior alone.7 Optical and spectroscopic techniques such as Fourier transform infrared spectroscopy (FT-IR) yield information on surface composition and conformation and, if the incident beam is polarized, molecular orientation.8-10 In this paper, infrared reflection absorption spectroscopy (IRRAS) is used to probe adsorbed layers of sodium dodecyl sulfate (SDS), sodium dodecylsulfonate (SDSn), and mixtures thereof at the air/solution interface. The effect of added inorganic electrolyte (NaCl), which affects the * E-mail: [email protected]. Phone: 765-494-4078. Fax: 765-494-0805. (1) Defay, R.; Prigogine, I. Surface Tension and Adsorption; Longmans, Green & Co Ltd: Great Britain, 1966. (2) Burdon, R. S. Surface Tension and the Spreading of Liquids; Cambridge University Press: Great Britain, 1949. (3) Goerke, J.; Clements, J. A. In Handbook of Physiology; Fishman, A. P., Macklem, P. T., Mead, J., Geiger, S. R., Eds; American Physiological Society: Maryland, 1986; Vol. 3, p 247. (4) Park, S. Y.; Hannemann, R. H.; Franses, E. I. Colloids Surf., B 1999, 15, 325-338. (5) Notter, R. H. Lung Surfactants; Marcel Dekker: New York, 2000. (6) Chattoraj, D. K.; Birdi, K. S. Adsorption and the Gibbs Surface Excess; Plenum Press: New York, 1984. (7) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New York, 1997. (8) Dluhy, R. A. J. Phys. Chem. 1986, 90, 1373-1379. (9) Mendelsohn, R.; Brauner, J. W.; Gericke, A. Annu. Rev. Phys. Chem. 1995, 46, 305-334. (10) Tung, Y. S.; Gao, T.; Rosen, M. J.; Valentini, J.; Fina, L. J. Appl. Spectrosc. 1993, 47, 1643-1650.

equilibrium surface tension and adsorption strongly, is also considered.11 There are few studies to date of IRRAS of adsorbed monolayers of soluble surfactants.10,12-14 This technique has been used often to probe spread insoluble monolayers, since the surface concentration is precisely known and there are no contributions to the IR signal from the subphase.15-17 However, most surfactant systems of interest are solutions, and it is interesting to know the effect of the subphase on the signal. To gauge the extent to which the bulk solution, containing surfactant monomers or micelles, contributes to the IRRAS signal, we also examine for the first time by IRRAS aqueous solutions of sodium methyl sulfate (SMS) and sodium sulfate (SS), ionic salts which are surface inactive, the second one definitely and the first one shown here to be so. In general, the reflectance absorbance (RA) depends on the thickness and complex refractive index of the adsorbed layer and on the substrate complex refractive index, as shown in Figure 1. When a solute is not surface active, there is no monolayer, and the value of RA depends only on the complex refractive index of the subphase. IRRAS data for surface inactive solutes yield information on the bulk absorptivity κ2 dependence on the solute concentration, κ2(c). Ideally, the information gained from this twolayer system could be used in the three-layer system to obtain information about the monolayer absorptivity κ1. (11) Simon-Kutscher, J.; Gericke, A.; Hu¨hnerfuss, H. Langmuir 1996, 12, 1027-1034. (12) Kawai, T.; Kamio, H.; Kon-No, K. Langmuir 1998, 14, 49644966. (13) Wen, X.; Lauterbach, J.; Franses, E. I. Langmuir 2000, 16, 69876994. (14) Meinders, M. B. J.; van den Bosch, G. G. M.; de Jongh, H. H. J. Eur. Biophys. J. 2001, 30, 256-267. (15) Flach, C. R.; Gericke, A.; Mendelsohn, R. J. Phys. Chem. B 1997, 101, 58-65. (16) Sinnamon, B. F.; Dluhy, R. A.; Barnes, G. T. Colloids Surf. 1999, 146, 49-61. (17) Wiesenthal, T.; Baekmark, T. R.; Merkel, R. Langmuir 1999, 15, 6837-6844.

10.1021/la020568g CCC: $22.00 © 2002 American Chemical Society Published on Web 11/01/2002

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experimental results are presented in section 3. Section 3.1 details the ATR results. The IRRAS results for surface inactive solutes are in section 3.2 while those for surface active solutes are in sections 3.3 and 3.4. The conclusions are summarized in section 4. 2. Materials and Experimental Methods

Figure 1. Schematics of models of IRRAS optics for surface inactive and surface active solutes.

Figure 2. Schematics of possible surfactant conformations probed by FT-IR.

This parameter depends on the bulk concentration c in the monolayer, κ1(c), or on the adsorbed surface density Γ, κ1(Γ). In practice, this suggests that, at a given bulk concentration, the surface density is related to the difference between the RA intensity of a surfactant system and the RA intensity of a similar surface inactive system for the same IR transition. No detailed modeling, or assumptions about the thickness and anisotropy of the adsorbed monolayer, is then required. It is well-known that the wavenumbers of the methylene stretching vibrations, in particular the antisymmetric stretch, which ranges from 2925 to 2915 cm-1, correlate with the packing density and the conformational order of the hydrocarbon surfactant tails.8,18 As the number of gauche conformers per chain (a measure of the “disorder” of the chains) increases, the wavenumber increases, as does the bandwidth.19,20 Figure 2 illustrates five possible surfactant configurations probed by various IR techniques. Bulk phase monomers and micelles can be probed using transmission IR21,22 or liquid cell ATR (attenuated total reflectance), as used in this study. Crystals are probed using films cast onto ATR plates,23 and adsorbed monolayers are probed using IRRAS. Section 2 describes the materials used, the experimental techniques, and the methods of data analysis. The (18) Gericke, A.; Hu¨hnerfuss, H. J. Phys. Chem. 1993, 97, 1289912908. (19) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: San Diego, 1990. (20) Hummel, D. O. Analysis of Surfactants: Atlas of FTIR-Spectra with Interpretations; Hanser: New York, 1996. (21) Scheuing, D. R.; Weers, J. G. Langmuir 1990, 6, 665-671. (22) Weers J. G.; Scheuing, D. R. In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D. R., Ed.; American Chemical Society: Washington, DC, 1991; Chapter 6, p 87. (23) Sperline, R. P. Langmuir 1997, 13, 3715-3726.

2.1. Materials. Sodium dodecyl sulfate (specially pure, >99%) was obtained from BDH Chemicals Ltd. (Poole, England). Deuterated sodium dodecyl sulfate (>98 atom %) was manufactured by Isotech Inc. (Miamisburg, OH) and was obtained from Aldrich Chemical Co. (Milwaukee, WI), as was sodium methyl sulfate (>97%). Sodium dodecylsulfonate (>99%) and 1-dodecanol (>99%) were obtained from Fluka Chemical Co. (Ronkonkoma, NY). n-Octane (>99%) was obtained from Sigma Chemical Co. (St. Louis, MO). Ethanol (dehydrated 200 proof) was obtained from Pharmco Products Inc. (Brookfield, CT). Sodium sulfate was received from Mallinckrodt Specialty Chemicals Co. (Paris, KY). Sodium chloride (>99.8%) was obtained from Fisher Scientific (Springfield, NJ). All materials were used as received. Surfactant solutions were prepared on a weight basis in Nalgene plastic containers to minimize possible ionic contamination from glass. The water was first distilled and then passed through a Millipore four-stage cartridge system consisting of an organic adsorption column, two mixed ion exchange columns, and an ultrafiltration unit. The resulting water had an initial resistivity of 18 MΩ‚cm. All SDS and SMS experiments were performed within 12 h after solution preparation to minimize the effects of hydrolysis. Solution surfaces were purified by aspiration.24-26 Some experiments were performed where dodecanol (C12OH), dissolved in octane, was spread at the air/water interface using a 50 µL syringe. Dodecanol dispersions were prepared by heating and then sonicating the aqueous preparation for a period of at least 1 h. Samples were tested immediately after sonication. 2.2. Fourier Transform Infrared Spectroscopy (FT-IR). 2.2.1. IR Instrument Used. A Nicolet Prote´ge´ 460 Fourier Transform infrared spectrometer equipped with a liquid nitrogen cooled MCT/A detector was used. The instrument was continuously purged with dry air from a Balston purge gas generator to minimize both water vapor and carbon dioxide within the sample chamber. Unpolarized radiation was used. Spectra (samples and backgrounds) were generally collected using either 512 scans at 8 cm-1 resolution or 1024 scans at 2 cm-1 resolution, Happ-Genzel apodization, and one level of zero filling (resulting in the same data spacing as if the spectra were taken at either 4 or 1 cm-1 resolution). The data acquisition time was either 4 or 16 min, depending on the resolution. The sample chamber was allowed to equilibrate for 10 min after the introduction of a sample prior to the collection of spectra. Spectra were taken at room temperature (22 ( 2 °C). 2.2.2. Attenuated Total Reflectance (ATR) Spectroscopy. Two germanium (Ge) internal reflection elements were used (Wilmad Glass, NJ), along with a custom-built ATR accessory (West Lafayette, IN) and a Harrick internal reflection liquid cell (Harrick Scientific Co., NY). Spectra were collected using 1024 scans at 2 cm-1 resolution. The baselines of the cast films were quite linear, but those for the liquid cell experiments were highly nonlinear. The baselines of the liquid cell ATR spectra were corrected using a commercial software package titled “ORIGIN”. ATR spectra are reported as plots of absorbance as a function of wavenumber. Cast films were prepared, first by dissolving the chemical of interest in ethanol and then spreading the resulting solution on an ATR crystal. The ethanol was allowed to evaporate in air, for ∼30 min before the spectral collection began. Enough material was spread to obtain a good spectrum, since the goal was to determine peak locations and general spectral characteristics, rather than absolute intensity values. (24) Lunkenheimer, K.; Miller, R. J. Colloid Interface Sci. 1987, 120, 176-183. (25) Lunkenheimer, K.; Retter, U. Colloid Polym. Sci. 1993, 271, 148-151. (26) Prosser, A. J.; Franses, E. I. Colloids Surf., A 2001, 178, 1-40.

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Table 1. Wavenumbers ((1 cm-1) of Vibrations in ATR Spectra SDS, aqueous SDS, solid 0.08 mM 0.8 mM 4 mM 8 mM νjA(CH2) νjS(CH2) νjA(SO3) νjA(SO3) νjS(SO3)

νjA(CH2) νjS(CH2) νjA(SO3) νjA(SO3) νjS(SO3) a

2917 2850 1248 1218 1085

2924 2853 a a a

2924 2853 1260 1232 1018

2924 2853 1260 1198 1061

2918 2850 1248 1216 1085, 1061

SDS-d, solid

SDSn, solid

C12OH, solid

2194b 2090b 1260 1219 1089

2918 2850 1209 1180 1067

2917 2848

80 mM 2920 2850 1248 1217 1083, 1061

SMS, solid

Na2SO4, solid

1258 1227 1070

1131 1102

Not measurable. b CD2.

min after the sample compartment was closed, to allow for sufficient purging and for surfactant equilibration. All spectra collected at a resolution of 2 cm-1 were reprocessed to 8 cm-1 for purposes of presentation. The location of the bands remained within the expected error of (1 cm-1; the intensity remained within the expected error of (0.0001. IRRAS spectra are reported as plots of reflectance absorbance (RA) as a function of wavenumber. RA is defined as -log(R/R0), where R0 and R are the reflectivities of the pure surface (background) and the film-covered surfaces, respectively. Spectra of water, or aqueous NaCl, were collected each day and were subtracted from the sample spectra. Despite the use of this procedure, it was not possible to remove entirely some of the bands due to water vapor in the resulting spectra. These water vapor bands do not interfere with the spectral regions of interest. The IRRAS spectra have been minimally corrected, even when the baselines were quite nonlinear. Baseline points at 3000 and 2800 cm-1 were used for the hydrocarbon region, at 2250 and 2000 cm-1 for the C-D region, and at 1300 and 1000 cm-1 for the polar group region. Using the ORIGIN software package to rigorously adjust the baselines between 3000 and 2000 cm-1 did not significantly affect the locations or intensities of the peaks of interest beyond the estimated error in their location. The polar group region (1300-800 cm-1) is so highly nonlinear that no attempts were made to rigorously correct the baseline in this region for the surfactants under study. ORIGIN was used to correct the baselines of SMS and SS. 2.3. Tensiometry. A KSV 5000 (KSV, Finland) equipped with a Wilhelmy plate connected to an electrobalance measures the surface pressure as a function of time. The instrument was used in a “modified” form: the Langmuir trough was removed, and a glass beaker was used as the sample vessel. The main reason for this was to reduce the solution volume required for a given measurement. The surface layers were aspirated prior to data acquisition.

3. Results and Discussion

Figure 3. ATR spectra of films cast from an ethanol solution (except for the C12OH film, see text) on a Ge plate. See Table 1 for peak assignments and wavenumbers for the most important peaks; further details can be found in ref 27. A dodecanol film was prepared by spreading directly onto the ATR plate liquid dodecanol (melting point 24-25 °C), which solidified at room temperature. Cast films of dodecanol from an ethanol solution showed some residual ethanol in the resulting spectrum. The wavenumber of the antisymmetric methylene stretch was at 2923 cm-1 rather than the value of 2917 cm-1 obtained by ATR for the pure chemical (see Table 1). Also, the bands in the spectral region between 1500 and 800 cm-1 were not clearly resolved (see Figure 3), indicating a lack of crystalline structure in the film cast from ethanol. 2.2.3. Infrared Reflection Absorption Spectroscopy (IRRAS). An in-situ monolayer/grazing angle external reflection attachment with a removable 10 mL teflon Langmuir trough (Graesby Specac Inc., U.K.) was used at an incident angle of 40° from the surface normal. The trough was used at the maximum surface area of 21 cm2. Samples were introduced into the trough using a 10 mL pipet, after which the surface layer was aspirated, removing less than 0.5 mL of solution. Spectra were collected 10

3.1. ATR Spectra of Pure Crystalline Surfactants, Molecular Solutions, and Micellar Solutions. We first obtained ATR spectra attributed primarily to bulk phases, to help interpret IRRAS spectra. The ATR spectra can be broken down into five major regions: the O-H stretching region, between 3500 and 3100 cm-1; the C-H stretching region, between 3000 and 2800 cm-1; the C-D stretching region, between 2250 and 2000 cm-1; the C-H bending/ wagging region, between 1500 and 1300 cm-1; and the S-O stretching region, between 1300 and 1000 cm-1. The ATR spectra for SDS, SDS-d, SDSn, C12OH, SMS, and SS are shown in Figure 3. The key peak positions are summarized in Table 1. Further details are given in ref 27. The C-H stretching regions for SDS, SDSn, and C12OH are quite similar: the methylene peaks are strong and sharp. All three spectra show a common peak centered at 1470 cm-1, which is identified as a methylene bending deformation. This peak is present as a doublet for C12OH and results from a crystal splitting effect.19 The corresponding peak for SDS and SDSn is a singlet, which is less intense (in comparison to the methylene peaks) than for C12OH and indicates less crystal symmetry. The sharp progression bands in the C12OH spectrum (1400-1300 and 1050-900 cm-1) are also indicative of a crystalline structure.28,29 These bands are not well resolved for SDS or SDSn. (27) Prosser, A. J. Thermodynamics of Equilibrium Adsorption and Surface Tension of Single and Binary Ionic Surfactant Systems. Ph.D. Thesis, Purdue University, expected 2002. (28) Park, S. Y. Effect of Dispersion State and Surface Composition on the Dynamic Surface Tension Behavior of Sparingly Soluble or Insoluble Surfactant Systems. Ph.D. Thesis, Purdue University, 1995. (29) Myrick, S. H. The Measurement and Interpretation of Low Dynamic Surface Tensions of Aqueous Long-Chain Alcohols. Ph.D. Thesis, Purdue University, 1999.

IRRAS of Chromophores

Since C12OH is a known contaminant of SDS, its presence could be spectroscopically identified by the presence of an O-H stretching band if the signal intensity is adequate. For crystalline C12OH, the O-H band is quite broad, centered at 3285 cm-1. The location and width of this band are affected by hydrogen bonding: the band shifts to higher wavenumbers and becomes sharper and weaker as hydrogen bonds are broken.19 There is no clear band in this spectral region for SDSn, as expected, but there is a small band visible in the SDS spectrum, centered at 3460 cm-1. This may be due either to a small amount of C12OH or to some water of hydration. The stretching vibration bands for alkyl sulfates and alkyl sulfonates have some differences (Figure 3). The antisymmetric SO3 stretch is the largest band of the respective spectra, as it is a combination of several overlapping peaks, but is generally visible as a double band.20 This band is broader for SDSn than for SDS. The location of this band is at 1210 and 1180 cm-1 for SDSn and at 1250 and 1220 cm-1 for SDS. The much less intense symmetric SO3 stretch is centered at 1065 cm-1 for SDSn and at 1085 cm-1 for SDS. The polar group region stretching vibration bands for SDS-d are in similar locations as those for SDS, but the antisymmetric SO3 band is clearly split, indicating a more crystalline structure, at least in these cast films. The C-D peaks are in the expected spectral region, centered at wavenumbers similar to those found for perdeuterated solid DPPC and perdeuterated solid hexadecanol.28 In general, νj(CD) ≈ 0.75νj(CH) and the intensity of a CD peak is half that of the corresponding CH peak.19 This isotope effect is well-known. The spectra for SDS-d, SMS, and SS are nearly flat in the region between 3500 and 2500 cm-1 (not shown). The CH3 stretching vibration band for SMS is not visible, being much less intense than that for SO3. The antisymmetric SO3 stretch band of SMS is quite broad (at 1227 cm-1). The relative intensity of the symmetric stretch band (at 1070 cm-1) with respect to that of the antisymmetric stretch is much larger than that for SDS. Additionally, the symmetric stretch band is shifted by approximately 20 cm-1 from the corresponding SDS band. The C-O-S stretch band is centered at 1000 cm-1. SS shows a large antisymmetric SO4 stretch band centered at 1120 cm-1. The inorganic sulfate spectrum is quite different from that of SMS and cannot be used to mimic SDS in the polar group region. The liquid cell ATR spectra for aqueous solutions of SDS were obtained (see Supporting Information, Figure S1). As shown for cast films, the antisymmetric SO3 band is twice as intense as the antisymmetric CH2 band. Below the cmc of SDS (ca. 8 mM), the antisymmetric CH2 band is centered at 2924 ( 1 cm-1 (Table 1). Above the cmc, the antisymmetric CH2 band is centered at 2920 ( 1 cm-1, indicating a more ordered conformation. At 8 mM, the wavenumber is about the same as that at 80 mM, indicating that some aggregates are present at the nominal cmc. The intensity (in terms of peak height or peak area) is proportional to the SDS concentration up to the cmc, but the slope decreases a lot beyond the cmc, by about 11 times. This decrease may be due to an increased electrostatic repulsion between the Ge surface (which is expected to be charged by itself or by adsorbed SDS) and the charged monomers and micelles as the ionic strength increases. This repulsion may deplete the region close to the plate to a concentration less than the bulk concentration. The general spectral characteristics above the cmc differ from those below the cmc; the C-H peaks become

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Figure 4. IRRAS spectra in the polar group stretching region for solutions of SMS (1, 5, 10, 20, 50, and 100 mM) and SS (0.1, 10, and 1000 mM). The spectrum for 1000 mM SS has been reduced by a factor of 10.

asymmetric and overlap more. Significant changes are also observed in the polar group stretching region. The spectrum for 8 mM SDS has a peak centered near 1150 cm-1, which is not observed at the other concentrations. Additionally, two peaks are seen for the symmetric SO3 vibration: a peak centered at 1060 cm-1 similar to that observed at 4 mM SDS, and a peak centered at 1080 cm-1 similar to that observed at 80 mM and in the cast SDS film. This result may imply that the spectrum probes not only bulk monomers but micelles, in addition to any adsorbed SDS. Further work is needed to analyze such data quantitatively. Nonetheless, there is a clear difference in the ATR spectra, in terms of the wavenumber and the band shape between a solution of monomers and a micellar solution. The above results imply that one can differentiate between crystalline, micellar, and bulk monomeric surfactant, and this should help the detailed interpretation of the IRRAS data. 3.2. IRRAS Spectra of Sodium Methyl Sulfate (SMS) and Sodium Sulfate (SS). To help address the issue of whether there are contributions from the bulk solution to the IRRAS intensities, the spectra of aqueous solutions of SMS and SS, two surface inactive sulfate salts, are considered. The IRRAS spectra of the polar group regions show clearly the presence of the salts (Figure 4). The three bands observed for a cast film of SMS are visible in the IRRAS spectra, but they are considerably broader and more asymmetric. The antisymmetric SO3 band is centered at 1200 cm-1, the symmetric SO3 band at 1055 cm-1, and the COS band at 999 cm-1. The peaks for the 1 mM solution are barely resolved, the limit of detection of SMS in bulk solution under the given spectroscopic conditions (1024 scans at 8 cm-1). The IRRAS spectra for SS show a single broad peak between 1150 and 850 cm-1, centered near 1000 cm-1, and are quite different from those for SMS. Being an inorganic salt, SS is undoubtedly surface inactive. The effect of SS on the surface tension of water (72 mN‚m-1) is small, actually increasing its value to 76 mN‚m-1 (at 20 °C) for a 1 M solution.30 The surface tension of aqueous SMS solutions as a function of the bulk concentration is shown in Figure S2 (see Supporting Information). Up to 20 mM, the effect of SMS on the surface tension is negligible; at higher concentrations, however, the tension decreases to about 60 mN/m, probably due (30) Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 61st ed.; CRC Press: Boca Raton, FL, 1980-1981.

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Prosser and Franses Table 2. Methylene Peak Locations for SDS Solutions in IRRAS Spectra c/mM

νjA(CH2)/cm-1

νjS(CH2)/cm-1

0 0 0 0 0 0 0 0 0 0 0

0.25 0.50 1.00 2.00 4.00 5.00 6.00 8.00a 10.00 20.00 50.00

2926 2925 2924 2924 2924 2924 2924 2924 2924 2922 2919

2859 2856 2856 2856 2854 2854 2854 2853 2853 2853 2849

10 10 10 10 10 10 10

0.63 1.00 1.25 2.50 5.00a 10.00 20.00

2927 2927 2926 2926 2925 2924 2921

2857 2857 2857 2856 2855 2855 2852

100 100 100 100 100 100 100 100

0.09 0.15 0.30 0.60 0.90 1.10a 2.30 4.50

2929 2929 2927 2926 2926 2926 2926 2924

2859 2859 2858 2857 2857 2856 2856 2855

cs/mM

Figure 5. Reflectance absorbance as a function of concentration for the SMS spectra shown in Figure 4.

a

Figure 6. IRRAS spectra in the hydrocarbon stretching region for adsorbed monolayers of SDS from the indicated subphase. Top, c ) 0.50, 2, and 10 mM, from top to bottom. Middle, c ) 0.63, 1.25, and 2.50 mM, from top to bottom. Bottom, c ) 0.15, 0.60, and 1.10 mM, from top to bottom. See Table 2 for wavenumbers of CH2 stretch bands.

not to any specific adsorption of SMS at the air/water interface but rather to a thermodynamic solution effect as a function of composition. This trend is similar to what is observed for aqueous methanol, ethanol, and methyl acetate solutions.30 That SMS is not surface active is further supported by the observed linear relationship between the RA intensity and the bulk concentration. Figure 5 shows the RA intensity (in terms of peak height) for the SO3 stretching vibrations. The C-O-S band intensities are similar to those for the antisymmetric SO3 stretch and are not shown. The broken lines are a best fit to the data points and to the origin (0, 0 point). For SS, the linearity between the absorbance and the concentration is less clear, either for peak heights or for integrated peak areas. 3.3. IRRAS Spectra of Sodium Dodecyl Sulfate, Dodecanol, and Sodium Dodecylsulfonate. 3.3.1. Spectra of SDS and Dodecanol. IRRAS spectra for several SDS concentrations in 0, 10, and 100 mM sodium chloride were obtained (Table 2). Figure 6 shows some select spectra in the C-H stretching vibration region. The concentrations shown were chosen mainly to show the increase in the RA intensity (|RA|) as a function of concentration, with little overlap between successive spectra. At c > 10 mM, the intensity of the bands continues to increase, and the bands become less symmetric.

cmc.

The antisymmetric methylene stretching vibration is centered at 2924 cm-1 for concentrations between 0.25 and 10 mM, and it shifts to lower wavenumbers at 20 and 50 mM, which are well above the cmc. A similar pattern is observed in the location of the symmetric methylene stretching vibration. The antisymmetric methyl stretch band is weak and is centered at 2960 cm-1. The symmetric methyl stretch is not visible. From the wavenumbers of the methylene bands below the cmc (Table 2), which are similar to that found for SDS monomers (section 3.1), the hydrocarbon chains in the SDS monolayers are inferred to be liquidlike, or disordered, with a large number of gauche conformers, and they are definitely less ordered than SDS micelles. Thus, we infer that the SDS monolayers are not close-packed. This inference is in agreement with calculations from surface tension data and the Gibbs adsorption isotherm. At the cmc, for all salinities examined, the surface density is found to be ∼4 × 10-6 mol‚m-2, which implies a surface area per molecule of 40 Å2.26 Simple alkanes, oriented in an all-trans configuration, have a minimum surface area per molecule between 18 and 20 Å2,31 while similarly oriented SDS molecules having a bulkier polar group would have a minimum surface area per molecule of 28 Å2, as calculated from the atomic radii and the bond angles.32 Above the cmc, the wavenumber of the methylene stretching vibrations decreases by 4 cm-1, indicating a contribution from the micelles to the IRRAS signal, since the monolayer density is expected to be the same. Indeed, the value of 2919 cm-1 for νjA(CH2) observed at 50 mM SDS is similar to that found from liquid cell ATR for an 80 mM SDS solution (Table 1). A general broadening and loss of symmetry of the bands is observed above the cmc. A broadening of the bands indicates a more liquidlike (31) Hiemenz, P. C. Principles of Colloid and Surface Chemistry, 2nd ed.; Marcel Dekker: New York, 1986. (32) Vold, M. J.; Vold, R. D. Colloid Chemistry; Reinhold: New York, 1964.

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Figure 7. Same as Figure 6, but in the polar group stretching region.

Figure 9. Hydrocarbon stretching region for the indicated SDS samples.

Figure 8. Reflectance absorbance of νjA(CH2) as a function of concentration for the SDS spectral data summarized in Table 2. b, 4, and × indicate 0, 10, and 100 mM NaCl, respectively.

state, but the corresponding decrease in the wavenumber indicates a more ordered state. This apparent contradiction has been noted previously and has also been attributed to the sampling of the more-ordered micellar subsolution in the observed signal,12 in addition to the monolayer. The broad antisymmetric SO3 band is clearly visible (Figure 7) between 1250 and 1200 cm-1. At a given concentration, the intensity of this band is similar to that of the antisymmetric CH2 stretch. The symmetric SO3 band, expected near 1060 cm-1, was not clearly detected. The polar group region in the IRRAS spectra is quite different from that observed using ATR, mainly due to the highly nonlinear baseline. The RA intensity increases with concentration, but this increase is hard to quantify, since the antisymmetric SO3 band is a combination of several overlapping peaks and the location of the baseline is uncertain. Above the cmc, the RA intensity continues to increase, as does the bandwidth. The RA intensity ratio between the antisymmetric SO3 band and the antisymmetric CH2 band increases above the cmc. At 10 mM, this ratio is near 1, but at 50 mM (see Figures 9 and 10), this ratio is closer to 4. Additionally, the symmetric SO3 stretching vibration is more clearly visible. The effect of NaCl on a typical ionic surfactant such as SDS is to decrease the cmc and to increase the adsorbed surface density below the cmc.26 This is due to the compression of the diffuse double layer (counterion condensation), which screens more the electrostatic forces between the charged monolayer and the molecules in the

Figure 10. Same as Figure 9, but in the polar group stretching region.

bulk, and drives adsorption further.33 The effect of NaCl on the antisymmetric CH2 vibration is to change the band line shape, because of changes in the refractive index with salinity. This asymmetry may be the reason for the slight increase in the values for νjA and νjS as a function of salinity (Table 2). At each salt concentration, the wavenumber (33) Delahay, P. Double Layer and Electrode Kinetics; John Wiley & Sons: New York, 1965.

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significantly decreases at concentrations over 10 mM, irrespective of the cmc. In the case of 100 mM NaCl, the change in the value of νjA(CH2) at four times the cmc is 2 cm-1 less than the value at the cmc. When no salt is present, this change is 5 cm-1. The smaller difference in the presence of NaCl may be due to a decreased repulsion of the micelles as a result of the compression of the double layer at the higher salt concentration. The effect of NaCl on the S-O stretching vibrations is more significant. The symmetric S-O stretch is expected to be sensitive to the presence of counterions,22 but this band is not well resolved. However, as the salt concentration increases, the antisymmetric SO3 band splits (Figure 7), implying a structure closer to that of crystalline SDS (with possibly less water of hydration). The overall bandwidth is not affected, but the RA intensity at the cmc decreases with increasing salinity, suggesting a decrease in the adsorbed surface density or some change in the conformation of the adsorbed surfactant as the ionic strength increases. Although one can speculate that this decrease may be a result of an increase in the localized counterion binding, the explanation remains unclear. Figure 8 shows the values for |RA| at each salinity as a function of the bulk concentration. The line of |RA| versus log c is linear below the cmc, shows a plateau, or “break”, near the cmc, and then above the cmc increases linearly. This break may indicate a transition between detecting the adsorbed monolayer only at the cmc region and detecting the adsorbed monolayer plus some bulk solution, including micelles, since the surface density of the monolayer is expected to be constant above the cmc because the surface tension does not vary.34 This break is not observed when analyzing the intensities of the SO3 stretching vibrations. The overlap between the three data sets in Figure 8 is not as severe when the intensities are scaled by the intensity at the cmc. The normalized intensities show the expected trend of an increase in the relative RA intensity with concentration, because the surface density increases with increasing salt concentration. Figures 9 and 10 show the hydrocarbon and polar group stretching regions for SDS in cast films, in solution, and adsorbed at the air/water interface. Most of the major spectral characteristics have already been described. The hydrocarbon peaks broaden considerably from the sharp narrow bands in a cast film to the much weaker bands observed in solutions of monomers and micelles. The line shape observed from a 4 mM liquid cell ATR experiment is similar to that observed for a 4 mM IRRAS experiment, but the intensity values differ by an order of magnitude, as expected. The highly asymmetric CH2 bands observed for 50 mM SDS using IRRAS are quite different from those of the 80 mM micellar solution but are similar to those previously observed.12 This asymmetry is also observed in the polar group region. To test whether the presence of any dodecanol impurity (or hydrolysis product) can be detected from IRRAS, experiments were performed on spread layers of dodecanol, aqueous dodecanol dispersions, and mixtures of SDS and dodecanol (spectra are not shown here but can be found in ref 27). The solubility of dodecanol in water is ∼4 ppm at room temperature.35 For a spread layer of dodecanol on water with a total surface density equal to 25 × 10-6 mol‚m-2, the antisymmetric band is centered at 2918 cm-1, with an intensity (34) Sasaki, T.; Hattori, M.; Sasaki, J.; Nukina, K. Bull. Chem. Soc. Jpn. 1975, 48, 1397-1403. (35) Robb, I. Aust. J. Chem. 1966, 19, 2281-2284.

Prosser and Franses

Figure 11. IRRAS spectra for (1) adsorbed monolayers of 20 mM SDS, (2) 20 mM SDS-d, and (3) a 20 mM SDS/SDS-d mixture of 50/50 mol %.

of 6.1 × 10-3, while the symmetric band is centered at 2850 cm-1, with an intensity of 3.9 × 10-3. This total surface density is four times greater than that of a single close-packed monolayer for an all-trans hydrocarbon chain.31 For a spread layer with a total surface density equal to 6.4 × 10-6 mol‚m-2, the locations of the methylene peaks do not change, but the intensities decrease to 2.8 × 10-3 and 1.8 × 10-3, respectively. The locations of the methylene peaks suggest that these dodecanol layers are crystalline under these conditions. The O-H stretching vibration of dodecanol is not visible within the noise of the intense water vapor peaks between 4000 and 3300 cm-1. The locations and intensities of the CH2 bands for a 100 ppm dispersion of dodecanol are, at 2918 cm-1, 5.2 × 10-3 and, at 2850 cm-1, 4.0 × 10-3. These values are similar to those observed for the spread layer of 25 × 10-6 mol‚m-2 dodecanol. For a 1000 ppm dispersion, the intensities increase to 26 × 10-3 and 18 × 10-3, respectively. Mixtures of SDS, below and above the cmc, with added 10 ppm of dodecanol resulted in spectra that were indistinguishable from those of SDS alone at the same concentration. Although the methylene peaks are due to both SDS and dodecanol, the wavenumber for the antisymmetric CH2 band was 2924 cm-1 at 6 mM SDS and 2922 cm-1 at 18 mM SDS. These values are similar to those shown in Table 2 for aqueous SDS. This suggests that small amounts of dodecanol have no discernible impact in the reported IRRAS spectra of SDS, despite the known strong effect on the surface tension.36 IRRAS experiments on deuterated SDS-d yield spectra similar to those for regular SDS. Figure 11 illustrates the C-H, C-D, and S-O stretching regions for 20 mM SDS, 20 mM SDS-d, and a 50/50 mixture of the same total molar concentration. These three spectra nearly overlap in the polar group region. The antisymmetric peaks are nearly identical. The intensities for the mixture C-H and C-D stretching bands are roughly half those of the pure solutions. There is no significant change in the location of the peaks in the pure solution versus that of the mixture. Thus, the adsorbed monolayer is also a 50/50 mixture of the two surfactants, as expected. Hence, if SDS-d is substituted for SDS in mixtures with SDSn, it behaves as regular SDS (see section 3.4). 3.3.2 Spectra of SDSn. The hydrocarbon stretching region in Figure 12 is similar to that for SDS (Figure 6). As the bulk concentration increases, |RA| increases (Table (36) Vollhardt, D.; Emrich, G. Colloids Surf. A 2000, 161, 173-182.

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Figure 12. IRRAS spectra in the hydrocarbon stretching region for adsorbed monolayers of SDSn from the indicated subphase. Top, c ) 0.89, 1.78, and 3.56 mM, from top to bottom. Middle, c ) 0.44, 0.89, 1.78, and 3.56 mM, from top to bottom. Bottom, c ) 0.22, 0.44, and 0.89 mM, from top to bottom. See Table 3 for wavenumbers of CH2 stretch bands.

Figure 13. Reflectance absorbance of νjA(CH2) as a function of concentration for the SDSn spectral data summarized in Table 3. b, 4, and × indicate 0, 10, and 100 mM NaCl, respectively.

Table 3. Methylene Peak Locations for SDSn Solutions in IRRAS Spectra c/mM

νjA(CH2)/cm-1

νjS(CH2)/cm-1

0 0 0 0

0.89 1.78 3.56 7.11a

2925 2924 2924 2915

2856 2856 2855 2849

10 10 10 10

0.44 0.89 1.78 3.56

2925 2924 2924 2923

2857 2856 2856 2855

100 100 100

0.22 0.44 0.89

2924 2924 2924

2856 2855 2855

cs/mM

a

Above solubility; see text.

3). The same three bands are visible: νjA(CH2) centered at 2924 cm-1, νjS(CH2) centered at 2856 cm-1, and νjA(CH3) centered at 2960 cm-1. These wavenumbers suggest a high degree of disorder in the adsorbed monolayer and are similar to those for SDS monolayers. The SDSn concentrations examined are near or below the solubility, with the exception of the 7.11 mM at no NaCl. Crystallites are observed visually as long slender rods near the surface. SDSn does not micellize at room temperature, since the Krafft temperature is near 35 °C.37 From the decrease in the wavenumber and the rather large increase in |RA|, it is clear that some floating crystals were sampled. The polar group region is also similar to that for SDS, but the antisymmetric SO3 stretch is clearly shifted to lower wavenumbers (see Supporting Information and ref 27). For SDSn, this peak is centered between 1200 and 1150 cm-1. The symmetric SO3 stretch is clearly visible for SDSn and is centered at 1040 cm-1. The effect of NaCl on the intensity of the antisymmetric CH2 vibration is summarized in Figures 13 and 14. As the salt concentration increases, the RA curves shift to lower concentrations. This is quite different from what was observed for SDS (Figure 8) but is similar to what is observed for the surface densities calculated from the surface tensions.38 The antisymmetric SO3 band for SDSn (37) van Os, N. M.; Haak, J. R.; Rupert, L. A. M. Physico-Chemical Properties of Selected Anionic, Cationic, and Nonionic Surfactants; Elsevier Publishing Company: The Netherlands, 1993. (38) Prosser, A. J.; Franses, E. I. Submitted.

Figure 14. Reflectance absorbance of νjA(CH2) as a function of concentration for the 50/50 SDS/SDSn spectral data summarized in Table 4. b and × indicate 0 and 100 mM NaCl, respectively.

shows no signs of splitting as the salinity increases, as was observed for SDS. 3.4. Spectra of SDS/SDSn Mixtures. The IRRAS spectra for 50/50 (mol/mol) mixtures of SDS/SDSn at 0 and 100 mM NaCl were obtained (see Supporting Information). The peak locations and intensities are summarized in Table 4. The hydrocarbon stretching region is similar to that for SDS and SDSn alone. There is no apparent affect of NaCl on the spectral shape. The polar group region has a broad antisymmetric SO3 band (centered between 1250 and 1200 cm-1), which is closer to that observed for SDS than it is to that for SDSn, indicating that the monolayer may be enriched in SDS. There is no evidence, however, of this band splitting as the salinity is increased, as was observed for SDS alone. The symmetric SO3 band is not visible, which again is closer to the case observed for SDS than to that for SDSn. Above the cmc, in the absence of NaCl, solutions of 50/ 50 SDS/SDSn remain clear, while, in the presence of 100 mM NaCl, white granular precipitates are clearly visible. This difference in solution behavior is evident in both the wavenumber and the intensity of the antisymmetric methylene stretch. Below the “cmc”, νjA(CH2) is 2924 cm-1 (2925 cm-1 with added NaCl), but above the “cmc”, νjA(CH2) is 2922 cm-1 (2919 cm-1 with added NaCl). Also, above the cmc in the presence of NaCl, the RA intensity is much

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Table 4. Methylene Peak Locations for 50/50 SDS/SDSn Solutions in IRRAS Spectra ct/mM

νjA(CH2)/cm-1

νjS(CH2)/cm-1

0 0 0 0 0 0 0 0

0.16 0.31 0.63 1.25 2.50 5.00a 10.00 20.00

2929 2923 2926 2924 2924 2923 2923 2922

2857 2857 2857 2856 2856 2853 2856 2852

0.63 1.25 2.50 5.00a 10.00 20.00

100 100 100 100 100 100 100 100 100

0.02 0.04 0.08 0.16 0.31 0.63 1.25a 2.50 5.00

2925 2925 2925 2925 2925 2924 2925 2923 2919

2854 2863 2856 2857 2855 2854 2854 2853 2850

11. This further suggests that the adsorbed monolayer is enriched in SDS-d, or SDS, by about 20%.

cs/mM

a

Table 5. Methylene Peak Locations for 50/50 SDS-d/SDSn Solutions in IRRAS Spectra

cmc/solubility; see text.

Figure 15. IRRAS spectra for adsorbed monolayers of 50/50 SDS-d/SDSn solutions from the indicated subphase and ct ) 0.63, 1.25, 2.50, and 5.00 mM, from top to bottom. See Table 5 for wavenumbers of CH2 and CD2 stretch bands.

greater than typically observed for micellar solutions, indicating the detection of some crystallites. It is not possible to reliably determine the composition of the monolayer from the SDS/SDSn spectra from these data, because the chains for both surfactants yield overlapping peaks. For this reason, we use deuterated SDS and a 50/50 SDS-d/SDSn mixture (Figure 15). The peak locations and intensities are summarized in Table 5. Since both C-H and C-D stretching bands are visible, it is clear that both SDS-d and SDSn are present in the adsorbed monolayer, as expected. The antisymmetric SO3 bands for SDS-d/SDSn are similar in terms of shape and relative intensities to those for the SDS/SDSn mixture (see Supporting Information). The symmetric SO3 bands are more clearly resolved. The ratio of the intensity of the antisymmetric CH2 stretch to the intensity of the antisymmetric CD2 stretch is about 1.4. For a 50/50 mixture, this ratio is expected to be near 2, as estimated from Figure

ct/mM νjA(CH2)/cm-1 νjS(CH2)/cm-1 νjA(CD2)/cm-1 νjS(CD2)/cm-1

a

2926 2926 2925 2926 2925 2924

2859 2859 2857 2855 2855 2853

2200 2198 2198 2198 2197 2195

2095 2095 2093 2094 2094 2094

cmc.

Conclusions This article is a systematic examination of the crystalline structure, solution, and adsorption behavior of a given surfactant system using FT-IR techniques. From the ATR studies we observe that νjA(CH2) ≈ 2924 cm-1 for monomers, versus νjA(CH2) ≈ 2920 cm-1 for micelles and νjA(CH2) ≈ 2917 cm-1 for solid crystals. This trend corresponds to the shift from more disordered or liquidlike to more crystalline or solidlike conformations. For SDS above the cmc (8 mM), there is a general lack of symmetry observed in the liquid ATR peaks, as well as a higher degree of overlap between neighboring peaks. The absorbance is linear with the concentration between 0.08 and 8 mM, but at 80 mM the measured intensity is an order of magnitude less than expected. A linear (or log-linear) relationship is also observed between the reflectance absorbance and the concentration for SDS, SDSn, and mixtures thereof below the cmc. However, above the cmc, a second linear relationship is observed, due to contributions to the RA intensities from monomers in solution and from micelles. From the IRRAS studies on SMS and SS, surface inactive salts, it is clear that bulk monomers do contribute to the reflectance absorbance, because they affect the absorption coefficient of the subphase. We also infer that IRRAS data for soluble surfactants can be interpreted without consideration of bulk effects for concentrations up to at least 5 or 10 mM, irrespective of the value of the cmc. However, at higher concentrations, bulk effects affect the observed absorbances and can also be detected from a shift of the methylene bands to lower wavenumbers. Acknowledgment. This research was supported in part by the National Science Foundation (Grants CTS 96-15649 and 01-35317) and by the National Institutes of Health (Grant HL 54641-02). Supporting Information Available: In one figure, we report ATR spectra of aqueous SDS for the concentrations 0.0880 mM. In another figure, we report the equilibrium surface tensions of aqueous SMS for the concentrations 0-100 mM. In a third figure, we report IRRAS spectra of aqueous SDSn in the polar group region, for 0, 10, and 100 mM NaCl. Finally, in two other figures, we report the IRRAS spectra of adsorbed monolayers of 50/50 mol/mol mixtures of SDS/SDSn at 0.31-5.00 mM for no salt and 0.02-1.25 mM for 100 mM NaCl. This material is available free of charge via the Internet at http://pubs.acs.org. LA020568G