Adsorption of Sodium Dodecyl Sulfate in the Presence of Poly

7306

Langmuir 2001, 17, 7306-7312

Adsorption of Sodium Dodecyl Sulfate in the Presence of Poly(ethylenimine) and Sodium Chloride Studied Using Sum Frequency Vibrational Spectroscopy Rosemary Windsor, David J. Neivandt, and Paul B. Davies* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK Received April 23, 2001. In Final Form: July 23, 2001 The ordering of sodium dodecyl sulfate, SDS, at the hydrophobic solid/aqueous solution interface has been studied as a function of the addition of the cationic polyelectrolyte poly(ethylenimine), PEI, and the univalent electrolyte sodium chloride (NaCl) to solution. The nonlinear optical technique of sum frequency vibrational spectroscopy (SFS) was employed to determine the ordering of the alkyl chain of the adsorbed surfactant. Sodium chloride was found to promote the onset of ordered SDS adsorption at lower bulk concentrations, a result interpreted primarily in terms of electrostatic shielding, with a minor contribution from salt-induced headgroup counterion condensation. Introduction of PEI to electrolyte-free SDS solutions was found to promote synergistic SDS adsorption through polyelectrolyte/surfactant complexation. Adsorption not only occurred at lower surfactant concentrations but also was more highly ordered than resulted from the addition of NaCl. The addition of NaCl and PEI together produced cooperative complexation of SDS and PEI and maximum ordering of the surfactant, which was independent of SDS concentration. This result is attributed to salt-induced enhancement of the strength of lateral surfactant alkyl chain interactions.

Introduction Surfactants find wide application industrially;1 however, formulations rarely contain a single surface active agent, and commonly multiple surfactants and/or polyelectrolytes are used in combination. Further, a simple electrolyte is often added to obtain the desired interfacial properties. While the adsorption of both surfactants and polyelectrolytes from their pure solutions has been widely investigated as a function of surface and solution conditions,2,3 less attention has been paid until recent years to adsorption from combined polyelectrolyte/surfactant solutions.4-7 Of the adsorption studies that have been performed from mixed solutions of polyelectrolytes and surfactants, the great majority have been performed on hydrophilic surfaces. Previous work performed in the authors’ laboratory has concentrated on adsorption at the hydrophobic solid/liquid interface including investigations of systems containing a cationic surfactant and various anionic polyelectrolytes.8 These studies are extended in the present work to investigate the polar orientation and the degree of chain conformational order of the anionic surfactant sodium dodecyl sulfate (SDS) at the hydrophobic solid/aqueous solution interface in the presence and absence of a cationic polyelectrolyte poly(ethylenimine) (PEI) and a univalent electrolyte (sodium chloride). * To whom correspondence should be addressed. E-mail pbd2@ cam.ac.uk. (1) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989. (2) Ingram, B. T.; Ottewill, R. H. In Cationic Surfactants; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1991. (3) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (4) Rupprecht, H.; Gu, T. Colloid Polym. Sci. 1991, 269, 506. (5) Somasundaran, P.; Cleverdon, J. Colloids Surf. 1985, 13, 73. (6) Neivandt, D. J.; Gee, M. L.; Tripp, C. P.; Hair, M. L. Langmuir 1997, 13, 2519. (7) Arnold, G. B.; Breuer, M. M. Colloids Surf. 1985, 13, 103. (8) Duffy, D. C.; Davies, P. B.; Creeth, A. M. Langmuir 1995, 11, 2931.

The structures of SDS and the repeat unit of nonprotonated PEI are represented schematically in Figure 1. Most studies of SDS adsorption at the solid/solution interface have been on hydrophilic surfaces, primarily mineral oxides.9-12 The onset of adsorption at hydrophobic interfaces typically occurs at higher solution concentrations of surfactant due to the absence of an electrostatic driving force. Hu and Bard13 examined SDS adsorption on hydrophilic aminoethanethiol and hydrophobic hexadecanethiol substrates. Adsorption on the cationic substrate was characterized by two distinct regions, the first up to SDS concentrations of 10-5 M being electrostatically driven monolayer formation followed at higher concentrations by hemimicelle or bilayer formation driven hydrophobically. On the hydrophobic substrate the onset of SDS adsorption occurred at higher concentration and resulted in a larger charge density at the interface, implying the formation of a more compact SDS layer. Day et al.14 have recorded the adsorption isotherm of SDS on hydrophobic graphitised carbon black. The isotherm was found to have two distinct steps. In the low concentration regime the surfactant was postulated to adsorb with its tailgroups in the plane of the surface. Upon further adsorption the monolayer reorientated to a more perpendicular configuration with the headgroups extending into the aqueous phase. Such a reorientation maximizes lateral hydrophobic interactions between surfactant alkyl chains and leads to an increase in the packing density. At full monolayer coverage the area occupied per SDS molecule was determined to be approximately 40 Å2 compared with 70 Å2 in the low concentration regime. (9) Somasundaran, P.; Kunjappu, J. T. Colloids Surf. 1989, 37, 245. (10) Dobson, K. D.; Roddick-Lanzilotta, A. D.; McQuillan, A. J. Vib. Spectrosc. 2000, 24, 287. (11) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90. (12) Xu, Z. H.; Ducker, W.; Israelachvili, J. Langmuir 1996, 12, 2263. (13) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5418. (14) Day, R. E.; Greenwood, F. G.; Parfitt, G. D. Proc. 4th Int. Congr. Surf. Activity 1967, 2, 1005.

10.1021/la010505i CCC: $20.00 © 2001 American Chemical Society Published on Web 10/12/2001

Adsorption of Sodium Dodecyl Sulfate

Figure 1. Schematic representation of the structures of sodium dodecyl sulfate (SDS) and nonprotonated poly(ethylenimine) (PEI).

Recent interfacial investigations have raised the concern that many SDS surface studies may have been contaminated by dodecan-1-ol through hydrolysis of the surfactant. The effects of dodecanol on the adsorption properties of SDS at the hydrophobic solid/liquid interface have been studied by Ward et al.15 for the deuterated octadecanethiol/ aqueous solution interface, by Turner et al.16 for the polystyrene/aqueous solution interface, and by Lu et al.17 for the liquid/air interface. A common feature in adsorption from SDS solutions is the appearance of a pre-critical micelle concentration (cmc) maximum in the isotherm,18-20 the origin of which may well be preferential solubilization of dodecanol within SDS micelles, although alternative explanations such as surface roughness effects and structural changes in the layer brought about by excess SDS and counterion effects have been put forward. Further, the presence of dodecanol is observed to increase the degree of conformational order in SDS layers at concentrations below the cmc.15 Several workers have shown that the extent and rate of hydrolysis of SDS to dodecanol are proportional to the initial concentration of dodecanol in solution.21,22 Consequently, great care must be exercised to ensure that the SDS employed for adsorption studies is highly purified and, further, that the studies are performed within a time frame that prevents hydrolysis contributing significantly to the measured interfacial properties. In addition to investigating the adsorption of SDS in the presence of a univalent electrolyte, we have studied the effect of adding the polyelectrolyte poly(ethylenimine), PEI. This polyelectrolyte is cationic and contains primary, secondary, and tertiary amino groups. The degree of protonation of these amino groups is strongly pH dependent, and consequently the charge density of PEI varies with pH, the point of zero charge being in the range 10.811.00.23 Further, the solution concentration of PEI is known to affect the degree of protonation as elucidated by Winnik et al.24 The adsorption of PEI alone at the solid/ solution interface appears only to have been studied for (15) Ward, R. N.; Davies, P. B.; Bain, C. D. J. Phys. Chem. B 1997, 101, 1594. (16) Turner, S. F.; Clarke, S. M.; Rennie, A. R.; Thirtle, P. N.; Cooke, D. J.; Li, Z. X.; Thomas, R. K. Langmuir 1999, 15, 1017. (17) Lu, J. R.; Purcell, I. P.; Lee, E. M.; Simister, E. A.; Thomas, R. K.; Rennie, A. R.; Penfold, J. J. Colloid Interface Sci. 1995, 174, 441. (18) Brown, W.; Zhao, J. Macromolecules 1993, 26, 2711. (19) Montgomery, M. E., Jr.; Wirth, M. J. Langmuir 1994, 10, 861. (20) Wirth, M. J.; Piasecki-Coleman, D. A.; Montgomery, M. E., Jr. Langmuir 1995, 11, 990. (21) Czichocki, G.; Much, H.; Vollhardt, D. J. J. Colloid Interface Sci. 1983, 280, 109. (22) Nakagaki, M.; Yokoyama, S. J. Pharm. Sci. 1985, 74, 1047. (23) Radeva, T.; Petkanchin, I. J. Colloid Interface Sci. 1997, 196, 87. (24) Winnik, M. A.; Bystryak, S. M.; Liu, Z.; Siddiqui, J. Macromolecules 1998, 31, 6855.

Langmuir, Vol. 17, No. 23, 2001 7307

hydrophilic systems including hematite,23 silica,25 cellulose fibers,26 and particles of charged latex27 and zirconia.28 The amount adsorbed is determined by the PEI concentration, pH, and the ionic strength of the solution.29,30 Clearly, analysis of adsorption at the solid/solution interface from mixed SDS and PEI solutions must also take into account potential interactions between the two species. Van den Berg and Staverman first studied the interaction between SDS and PEI in bulk aqueous solution almost 30 years ago, interpreting their results in terms of the formation of uncharged polymer/surfactant complexes.31 Li and co-workers32 have reported on a combined electromotive force and isothermal titration calorimetry study of the system and found PEI to have a remarkable affinity toward SDS over the full pH range 2-10. The SDS concentration at which the onset of surfactant binding to the polymer occurred was found to decrease with decreasing pH, consistent with increased protonation of PEI at lower pH values and the dominance of the surfactant interaction by 1:1 charge neutralization electrostatics. Precipitation of the complex was often observed. Resolubilization was found to be possible at SDS concentrations greater than the cmc, attributed to interaction of micellar SDS with the SDS/PEI complex. The SDS/PEI complexation at high pH (where PEI is largely uncharged) was proposed to be driven by SDS micellar interaction with uncharged nitrogen atoms of the ethylenimine segments of the polyelectrolyte in an analogous manner to that in which SDS is believed to interact with oxygen atoms in poly(ethylene oxide). Conversely, Winnik et al.33 using fluorescent labeling experiments have suggested, in common with other workers,34,35 that hydrophobic interactions dominate the SDS/PEI interaction at high pH values. In this study sum frequency vibrational spectroscopy (SFS) is used to probe the alkyl chain conformation of adsorbed SDS. SFS is a nonlinear optical technique in which a visible laser beam of fixed frequency (ωVIS) and a tunable frequency infrared laser beam (ωIR) are overlapped, both spatially and temporally, on a surface and light with a frequency that is the sum of the two incident frequencies is emitted (ωSF ) ωVIS + ωIR).36 The SF signal is enhanced when the frequency of the infrared laser is in resonance with a vibrational mode of a molecule that is both infrared and Raman active and that is in a noncentrosymmetric environment. Consequently, detecting the SF light as a function of the infrared frequency produces a vibrational spectrum of interfacial molecules. On a molecular level, SFS in the C-H stretching region may be applied to the determination of the conformation of hydrocarbon chains at interfaces.37 Specifically, as in (25) Golub, T. P.; Skachkova, A. L.; Sidorova, M. P. Colloid J. Russ. Acad. Sci. 1992, 54, 697. (26) Petlicki, J.; van de Ven, T. G. M. Colloids Surf. A 1994, 83, 9. (27) Akari, S.; Schrepp, W.; Horn, D. Langmuir 1996, 12, 857. (28) Wang, J.; Gao. L. J. Colloid Interface Sci. 1999, 216, 436. (29) Horn, D. In Polymeric Amines and Ammonium Salts; Goethals, E. J., Ed.; Pergamon Press: Oxford, 1980. (30) Alince, B.; Vanerek, A.; van den Ven, T. G. M. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 954. (31) van den Berg, J. W. A.; Staverman, A. J. Recl. Trav. Chim. 1972, 91, 1151. (32) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 3093. (33) Winnik, M. A.; Bystryak, S. M.; Siddiqui, J. Macromolecules 1999, 32, 624. (34) Yui, T. S. T. I.; Abilov, Zh. A.; Pal’mer, V. G.; Musabekov, K. B. Issled. Ravnovesnykh Sist. 1982, 78. (35) Abilov, Zh. A.; Beisebekov, M. K.; Musabekov, K. B. In Reakzii v zhidkoi faze; Izd. KazGU: Alma-Ata, USSR, 1979; p 134. (36) Shen, Y. R. Principles of Nonlinear Optics; Wiley: New York, 1984. (37) Ward, R. N.; Duffy, D. C.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1994, 98, 8536.

7308

Langmuir, Vol. 17, No. 23, 2001

this work, SFS may be used to elucidate the structure that surfactants adopt when adsorbed at the solid/liquid interface. In an all-trans hydrocarbon chain the constituent methylene groups lie in a locally centrosymmetric environment and hence are SF inactive. However, the presence of gauche defects in surfactant methylene chains breaks the local centrosymmetry, thereby making the resonances SF active. Terminal methyl groups of adsorbed surfactant alkyl chains have no local center of symmetry, and the strength of their sum frequency activity is determined by the degree of ordering of the surfactant chains themselves. The ratio of the strengths of the symmetric r+ stretching mode of the methyl group to the symmetric d+ stretching mode of the methylene group (r+/d+) has become widely used as a measure of surfactant ordering. The occurrence of a methylene d+ resonance in the absence of an r+ resonance indicates the presence of gauche defects in the methylene chain and an isotropic distribution of the methyl groups terminating the alkyl chains, consistent with an interfacial surfactant layer of low conformational order. An increase in the strength of the r+ resonance in relation to the methylene d+ resonance is indicative of a decrease in the extent of methyl group orientational isotropy and indicates a rise in the overall ordering of the surfactant tail, as expected for a moderately ordered surfactant layer. Finally, the observation of solely an r+ resonance indicates that the methylene groups are entirely trans in conformation and are therefore SF inactive. Further, the methyl groups must have little orientational dispersion, and consequently the surfactant layer must be very highly ordered. Such highly ordered surfactant monolayers have rarely been reported by SFS. Experimental Section Sum frequency spectra were recorded on the Cambridge nanosecond spectrometer. Briefly, a Nd:YAG laser is used to generate a pulsed visible beam (wavelength 532 nm, duration 8 ns, repetition rate 11.5 Hz) which is spatially overlapped on the surface with a tunable frequency infrared beam created by stimulated Raman scattering of a Nd:YAG pumped dye laser beam in a cell containing hydrogen at 34 atm. The visible beam is delayed through the use of a Herriot cell to ensure temporal overlap with the infrared beam on the surface. The SF signal is detected by a photomultiplier tube and integrated via a gated analogue boxcar. Spectra were recorded in the C-H stretching region (2800-3000 cm-1) at intervals of 2 cm-1. The beams impinged on the sample in a counterpropagating geometry (60° and 65° to the surface normal for the visible and infrared beams, respectively) in the PPP (sum frequency, visible, and infrared) beam polarization combination. Typically 60 complete scans were averaged to achieve a spectrum. Spectra were analyzed with a least-squares Levenberg-Marquardt fit to an equation based on the fundamental nonlinear properties of the interface and a Lorentzian description of the second-order susceptibilities.38 Spectra were recorded employing a Teflon or stainless steel liquid cell with an aqueous solution trapped between the hydrophobic substrate and a silica prism.39 The prism enabled the counterpropagating visible and infrared beams to enter the cell and the resulting SF emission to exit for detection by the photomultiplier tube. A micrometer was used to advance and retract the substrate (in relation to the prism). Hydrophobic substrates were prepared by adsorption of fully deuterated octadecanethiol (d-ODT) on gold-coated silicon wafers from methanolic solutions.40 Surfactant solutions were introduced to the cell in increasing concentrations. The solutions were allowed to equilibrate with the substrate retracted approximately 10 mm from the prism for a period of 30 min. The substrate was then advanced to within 1 µm of the prism, and the spectra were (38) Lambert, A. G. Ph.D. Thesis, University of Cambridge, 2001. (39) Bewick, A.; Kunimatsu, K.; Pons, B. S.; Russell, J. W. J. Electroanal. Chem. 1984, 160, 47. (40) Duffy, D. C. Ph.D. Thesis, University of Cambridge, 1996.

Windsor et al. acquired. After acquisition the substrate was withdrawn from the prism, the cell was rinsed thoroughly then filled with the next solution before being left to equilibrate. Each complete experiment was performed an average of five times. The SF flow cells and their components along with all glassware were cleaned by standard procedures41 employing 18.2 MΩ cm resistivity water. Sodium dodecyl sulfate (SDS) doubly recrystallized from ethanol was kindly provided by Unilever Research (Port Sunlight Laboratory). Fully deuterated SDS was obtained from MSN Isotopes (CDN Isotopes) Canada and was doubly recrystallized from ethanol prior to use. Stock solutions of SDS were prepared above the cmc of 8.3 × 10-3 M immediately prior to an experiment and diluted as required. The solution pH of mixed PEI/SDS solutions was adjusted to a value of 6.0 using AR grade hydrochloric acid. Poly(ethylenimine) (PEI), 750 000 molecular weight, was obtained from BASF and was used as received. At pH 6.0 the manufacturer quotes a nitrogen group protonation value of 30%, giving a calculated 2.1 charges per polyelectrolyte repeat unit. It should be noted however that due to the concentration dependence of the degree of protonation,24 this number should not be taken as absolute. Solution concentration is expressed throughout the paper as a fraction of the critical micelle concentration (cmc) of pure SDS for the surfactant, as parts per million (ppm) of the solution by weight for the polyelectrolyte and as moles per Liter (M) for NaCl.

Results and Discussion Adsorption from SDS Solutions. The polar orientation and degree of conformational order of SDS adsorbed at the hydrophobic d-ODT/water interface were investigated as a function of solution concentration of the surfactant. Sum frequency spectra in the methyl/methylene stretching region (2800-3000 cm-1) were recorded at SDS concentrations in the range 1/100 to 3 cmc and are given in Figure 2. A maximum of five resonances are observed in the spectra which are assigned to the C-H stretching modes of the alkyl chains of the surfactant.37 Both r+ symmetric (2873 cm-1, r+FR Fermi resonance at 2930 cm-1) and r- asymmetric (2963 cm-1) stretching modes arising from the terminal methyl group are observed. Additionally, the resonance at 2845 cm-1 arises from the d+ symmetric methylene stretching mode, while the broad band at 2910 cm-1 is attributed to a combination of the weak asymmetric methylene stretching mode and a Fermi resonance of the symmetric methylene mode. The fact that the resonances occur as “dips” in the spectra implies that the surfactant molecules are orientated with their polar headgroups extending into solution and their hydrophobic tails toward the d-ODT substrate. No discernible resonances are observed in the spectrum of SDS adsorbed from 1/100 cmc solution (Figure 2). This finding does not preclude the adsorption of SDS at concentrations lower than 1/100 cmc; however, it does imply that if SDS is present, it must either be lying with its alkyl chain in the plane of the surface or be completely conformationally disordered and consequently sum frequency inactive. Increasing the solution concentration to 1/30 cmc results in the observation of a d+ resonance and a weak r+ resonance. The small r+/d+ ratio is indicative of the presence of a large number of gauche defects in the methylene chain and a near isotropic distribution of the methyl groups terminating the alkyl chain, consistent with a comparatively disordered surface layer. The r+/d+ ratio and hence the degree of SDS conformational order is observed to increase with the SDS concentration as it is raised to 1/10 cmc, a finding that is attributed to an increase in the surface excess of SDS and a concurrent (41) Neivandt, D. J. Ph.D. Thesis, The University of Melbourne, 1998.

Adsorption of Sodium Dodecyl Sulfate

Figure 2. Sum frequency spectra in the PPP beam polarization combination (sum frequency, visible, infrared) of SDS adsorbed at the hydrophobic deuterated octadecanethiol (d-ODT) on gold/ aqueous solution interface as a function of surfactant concentration for 1/100, 1/30, 1/10, 1/3, and 3 cmc. The solid circles are the individual data points, and the solid lines are theoretical fits to the data. The spectra are displaced for clarity.

rise in the packing density of the surfactant. No discernible changes in the spectra are observed at SDS concentrations above 1/10 cmc, suggesting that the packing density of the surfactant at the interface has reached a maximum value, consistent with attainment of monolayer coverage. Importantly, no decrease in the degree of conformational ordering was detected between 1/3 and 3 cmc. This finding implies that the SDS employed was not contaminated with dodecan-1-ol as its presence at the interface is known to result in a decrease in the degree of surfactant conformational order as measured by SFS immediately prior to the cmc. The loss of order within the adsorbed layer is attributed to solubilization of the order imposing dodecanol in SDS micelles.15 Adsorption from SDS/NaCl Solutions. The SF spectra recorded for SDS adsorption at the hydrophobic d-ODT/solution interface from solutions ranging in concentration from 1/100 to 3 cmc in the presence of 0.1 M NaCl are given on the right of Figure 3 and are repeated on the left of Figure 4. For ease of interpretation solely the sum frequency data in the spectral region to be analyzed, that is, the r+/d+ stretching region (2830-2890 cm-1), are presented. The corresponding spectral fits were obtained by analyzing the spectra over the full frequency range recorded although again only the relevant spectral region is shown. The spectra of SDS adsorption in the absence of added electrolyte (Figure 2) are given again on the left of Figure 3 to facilitate comparisons. In contrast to adsorption of the surfactant in the absence of electrolyte where the onset of partially ordered adsorption was determined to occur between 1/100 and 1/30 cmc, a distinct d+ resonance is observed in the spectrum recorded at 1/100

Langmuir, Vol. 17, No. 23, 2001 7309

Figure 3. Sum frequency spectra (PPP) at the d-ODT/aqueous solution interface of (left) SDS adsorbed from pure surfactant solution as a function of concentration (reproduced from Figure 2) and (right) SDS adsorbed from surfactant solutions containing 0.1 M NaCl at the same SDS concentrations. The spectral range displayed has been reduced to the analysis region of 28302890 cm-1 for ease of comparison.

cmc SDS in the presence of electrolyte. It may be concluded therefore that the addition of NaCl to SDS solutions promotes the adsorption of SDS at lower solution concentrations, initially as a layer with low conformational order (no r+ resonance). Upon increasing the SDS concentration to 1/30 cmc an r+ resonance is observed in addition to the d+ resonance, consistent with an increase in the degree of conformational order of the adsorbed surfactant. As occurred in the absence of electrolyte, the r+/d+ ratio increases with SDS concentration to a maximum at 1/10 cmc, beyond which no change in the ratio is detected, a result again attributed to attainment of monolayer coverage. There are two primary mechanisms by which electrolyte promotes SDS adsorption at the hydrophobic surface. First, the addition of electrolyte results in the reduction of the effective headgroup charge of SDS experienced by other surfactant molecules within the adsorbed layer through non-site-specific electrostatic shielding. Second, increasing the free electrolyte concentration reduces the energetic penalty paid for the loss of counterion translational entropy upon condensation within the adsorbed surfactant layer. A further potential driving force for greater adsorption of the surfactant at higher electrolyte concentrations is that proposed by Zhang et al.,42 namely, an increase in the effective surfactant solution concentration brought about by the loss of free water through hydration of the added electrolyte. The later effect is expected to be weak at the comparatively low electrolyte concentration of 0.1 M. (42) Zhang, L.; Somasundaran, P.; Maltesh, C. Langmuir 1996, 12, 2371.

7310

Langmuir, Vol. 17, No. 23, 2001

Windsor et al.

Figure 4. Sum frequency spectra (PPP) at the d-ODT/ aqueous solution interface of (left) SDS adsorbed from 0.1 M NaCl solutions at SDS concentrations of 1/100, 1/30, 1/10, and 1/3 cmc (reproduced from Figure 3), (central) SDS adsorbed from solutions containing 100 ppm PEI at pH 6.0 at the same SDS concentrations, and (right) SDS adsorbed from solutions containing 0.1 M NaCl and 100 ppm PEI at pH 6.0 at the same SDS concentrations.

Adsorption from SDS/PEI Solutions. The effect of the addition of 100 ppm poly(ethylenimine) on the degree of conformational ordering of SDS at the hydrophobic surface was studied through the collection of SF spectra as a function of SDS concentration (central spectra of Figure 4). For experiments performed in the presence of PEI there is a possible contribution to SF spectra from the methylene groups of the polyelectrolyte. To determine whether such a contribution existed, spectra in the C-H stretching region of adsorption from solutions containing solely PEI and from mixed deuterated SDS/protonated PEI solutions were recorded (not shown). No resonances were observed in the spectra, indicating that the methylene groups of the polyelectrolyte are SF-inactive, most likely due to an absence of macroscopic ordering of PEI at the interface. Consequently, all methyl (groups which are absent in the polyelectrolyte) and methylene resonances are assigned to the surfactant. Strong r+ methyl and d+ methylene resonances are observed in the spectrum recorded at 1/100 of the SDS cmc, indicating the presence of a comparatively ordered SDS layer. This is in contrast to SDS adsorption behavior from pure surfactant solutions where no adsorption was observed at this concentration (Figure 2). Similarly, although adsorption was observed from 0.1 M electrolyte containing SDS solution at 1/100 cmc, it was considerably less ordered (left spectral series of Figure 4). It may therefore be concluded that the addition of PEI acts to enhance the adsorption of ordered SDS molecules at the hydrophobic d-ODT surface. The mechanism by which PEI promotes SDS adsorption is through complexation of the polyelectrolyte and the surfactant in solution, resulting in synergistic adsorption of the complex. Because of the high linear cationic charge density of PEI (an average of approximately 30% of the nitrogen atoms charged at pH 6.0), the driving force for complexation with the anionic surfactant has a significant electrostatic component, in addition to an entropic gain which arises due to the release of condensed counterions of the polyelectrolyte upon surfactant binding. Hydrophobic interactions commonly observed between surfactant alkyl chains and methylene backbones of polyelectrolytes and more importantly inter-

alkyl chain surfactant interactions are expected to provide additional entropic driving forces for polyelectrolyte/ surfactant complexation.43 Complexation of an SDS molecule with PEI results in a loss of charge from both the surfactant and an amine group on the polyelectrolyte. Consequently, the hydrophilicity and hence the aqueous solubility of the complex is lower than the sum of the individual noncomplexed components. Additionally, the hydrophobic content of the complex is increased by the inclusion of the methylene tail of the surfactant. The combined effect of these two factors is synergistic adsorption of the surfactant and the polyelectrolyte at the hydrophobic d-ODT/solution interface in the form of a complex. The onset of adsorption occurs at a surfactant concentration below that at which adsorption from solely SDS solution is expected due to the fact that both the hydrophobicity and insolubility of the complex are greater than the sum of the constituent SDS and PEI species. In common with adsorption from pure SDS solutions and from SDS solutions containing added electrolyte, the r+/d+ ratio is observed to increase with SDS solution concentration up to 1/10 cmc, beyond which no change is observed. The maximum r+/d+ ratio obtained for adsorption from SDS/PEI solutions is however higher than obtained in either of the previously examined systems. Since the polyelectrolyte/surfactant complexation is at least partially driven by charge neutralization, the packing density of SDS at the interface may be expected to be regulated by the linear charge density of PEI and the solution concentration of SDS. Calculation of the mole ratio of SDS molecules in solution to the number of PEI charge units reveals that if all available surfactant is complexed with the polyelectrolyte, then charge neutralization of PEI will occur at 1/12 cmc, assuming 30% of the polyelectrolyte amino groups are protonated. Clearly, this value is consistent with the SDS concentration at which the maximum in the r+/d+ ratio is observed. The increase in the r+/d+ ratio and hence the degree of conformational (43) Malovikova, A.; Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1984, 88, 1930.

Adsorption of Sodium Dodecyl Sulfate

order of SDS with surfactant concentration may then be explained by a decrease in the average separation distance of surfactant molecules along the PEI backbone as the amount of complexed surfactant increases. Consequently, the packing density of the surfactant (present in the form of an SDS/PEI complex) at the interface would be expected to rise with SDS concentration until charge neutralization occurred at 1/12 cmc. Further evidence for SDS/PEI complex formation was the observation of solution turbidity (which did not occur in the absence of the polyelectrolyte). SDS/PEI solutions showed an onset of turbidity in the range 1/40-1/35 cmc. As the surfactant concentration was raised, the degree of turbidity increased. In the concentration range examined no precipitation or solution reclarification was observed. Turbidity in mixed polyelectrolyte/surfactant solutions is indicative of strong interactions between the two species and the formation of macroscopic aggregates often leading, at sufficient surfactant concentrations, to precipitation. An additional series of sum frequency spectra of the SDS/ PEI system were recorded covering the region from solution clarity at 1/40 cmc, through the turbidity point at approximately 1/35 cmc, and up to the predicted PEI charge neutralization point of 1/12 cmc. The spectra showed a monotonic increase in the r+/d+ ratio with SDS concentration over the entire range examined. It may be concluded therefore that surfactant complexation with the polyelectrolyte occurs beyond the concentration at which turbidity first appears and in fact continues up to the SDS concentration calculated to result in complete PEI charge neutralization, 1/12 cmc. Interestingly, while in some oppositely charged surfactant/polyelectrolyte systems raising the surfactant concentration above the point of charge neutralization results in a loss of turbidity or resolubilization of the complex, for the PEI/SDS system we observed no effect on the adsorbed layer above 1/10 cmc, although concentrations greater than 1/3 cmc were not employed. Somewhat surprisingly, it is noted that even at 1/100 cmc SDS, where the average separation between surfactant molecules along the polyelectrolyte chain would be predicted from a purely electrostatic argument to be approximately 10 charge units, a high degree of order in the adsorbed SDS layer is observed. The implication of this result is that the driving force for polyelectrolyte/ surfactant interaction must contain, in addition to the charge neutralization component which would give rise to a statistical distribution of surfactant along the polyelectrolyte, a considerable degree of cooperativity. Cooperative polyelectrolyte/surfactant interactions derive from the entropic benefit of surfactant molecules binding to the polyelectrolyte at sites neighboring those already containing bound surfactant molecules in order to maximize inter-alkyl chain interactions. Adsorption from SDS/PEI/NaCl Solutions. Addition of 0.1 M NaCl to composite SDS/PEI solutions resulted in the adsorption of highly conformationally ordered SDS at the hydrophobic solid/liquid interface, as evidenced by the series of spectra on the right of Figure 4. The SF spectrum recorded at 1/100 cmc has very strong methyl and methylene resonances which are of comparable strength. The r+/d+ ratio is considerably larger than the maximum observed not only for adsorption from pure SDS and SDS/NaCl solutions but also for the SDS/PEI system in the absence of added electrolyte. Further, in contrast to the previous systems studied, the r+/d+ ratio is invariant with SDS concentration over the entire range studied. This result implies that in the presence of both PEI and

Langmuir, Vol. 17, No. 23, 2001 7311

NaCl the adsorbed conformation of SDS at concentrations as low as 1/100 cmc is comparable to that at 1/3 cmc. The addition of electrolyte typically reduces the binding affinity of surfactants to polyelectrolytes in oppositely charged systems, resulting in an increased surfactant concentration at which binding begins (the critical association concentration, cac).44 This occurs for two reasons: first, the shielding of attractive electrostatic interactions, and second, a decrease in the translational entropy gain derived from polyelectrolyte counterion release upon surfactant binding. However, with the addition of electrolyte the degree of cooperativity of the polyelectrolyte/surfactant interaction rises,45 that is, as discussed above, the entropic driving force for surfactant molecules to bind to the polyelectrolyte at sites neighboring those already containing bound surfactant. This results in localized regions along the polyelectrolyte backbone with a maximum degree of surfactant complexation interspersed with regions of little or no surfactant complexation. Adsorption of such an SDS/PEI complex would be expected to result in a surfactant layer at the hydrophobic surface which is highly ordered, even when the surfactant concentration is low. As the mole ratio of surfactant to charge units of the polyelectrolyte is raised through the addition of more surfactant, the localized regions of surfactant complexation along the polyelectrolyte expand in size at the expense of the number of noncomplexed sites. Importantly, no change in the degree of alkyl chain conformational order at the hydrophobic interface is expected since the only change is to the number of surfactant molecules present not to the overall order which is already at a maximum. Since the strength of sum frequency generation is highly dependent upon the degree of interfacial order of the species being probed, no change in the experimental r+/d+ ratio is observed with increasing the surfactant concentration from 1/100 to 1/3 cmc. The high degree of cooperativity of the SDS/PEI interaction in the presence of NaCl not only gives rise to the invariance of the r+/d+ ratio as a function of surfactant concentration described above but also, we postulate, is responsible for the higher degree of conformational order observed in comparison to the same experiments performed in the absence of added electrolyte. The cooperative interaction is driven by the entropy gain derived from the loss of ordered water structure around the surfactant tail upon its removal from solution. The addition of sodium chloride to aqueous solutions increases the strength of hydrophobic interactions between species giving rise to the well-known “salting out” effect. For the SDS/PEI system stronger inter SDS alkyl chain interactions are therefore predicted to occur in the presence of added electrolyte, and consequently a higher degree of conformational order is expected. In an attempt to support this hypothesis, an additional experiment was conducted where 100 ppm PEI, 1/100 cmc SDS solutions with progressively higher NaCl concentrations (0.1-4 M) were equilibrated with the hydrophobic substrate. The r+/d+ ratio rose consistently with the ionic strength of solution as predicted if the salting out effect is regulating the degree of interfacial SDS alkyl chain conformational order. We suggest therefore that under carefully selected conditions sum frequency vibrational spectroscopy may be employed to give a direct measure of the degree of surfactant interalkyl chain cooperativity in polyelectrolyte/surfactant complexes. (44) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (45) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642.

7312

Langmuir, Vol. 17, No. 23, 2001

In common with SDS/PEI solutions prepared in the absence of NaCl, turbidity was observed to occur above a threshold surfactant concentration. In the presence of 0.1 M NaCl the SDS concentration corresponding to the onset of solution turbidity was approximately 1/30 cmc (in comparison to 1/40-1/35 cmc in the absence of added electrolyte). The fact that the threshold concentration is higher is consistent with the generally observed phenomenon of dampened electrostatic interactions between oppositely charged polyelectrolytes and surfactants in the presence of salt. The observation of the onset of solution turbidity at only a marginally higher SDS concentration in the presence of added electrolyte has a further significance: it implies that despite the differences in the spatial distribution of SDS along the polyelectrolyte backbone and the alteration of the structure of the species adopted at the hydrophobic surface that results, little change occurs in the macroscopic solution properties of the complex. This does not however imply that conformational changes of the polyelectrolyte/surfactant complex in solution do not affect the structure of the species that are formed at the interface. Little literature exists on the conformational nature of the solution species formed between SDS and PEI. To the authors’ knowledge, Winnik et al.33 have performed the only study using steady-state fluorescence measurements of pyrene-labeled PEI. However, the label site acted as a hydrophobic seed for SDS interaction, thereby making extrapolation of the results to nonlabeled PEI difficult. In a linear polyelectrolyte/oppositely charged surfactant system the formation of micellar-like surfactant aggregates with the polyelectrolyte acting as a polycounterion is typically observed. Precisely what occurs when the polyelectrolyte is branched is unclear. If, however, micellar-like aggregates of SDS/PEI do form in solution in the present study, then they must undergo significant rearrangements/breakage upon adsorption at the hydrophobic surface in order to give the conformationally ordered layers detected by sum frequency vibrational spectroscopy. Conclusion We have employed sum frequency vibrational spectroscopy (SFS) to probe changes in the ordering of sodium dodecyl sulfate at the hydrophobic solid/liquid interface induced by the addition of NaCl and PEI. It was determined that a moderate concentration (0.1 M) of electrolyte promotes the onset of ordered adsorption of the surfactant at a lower bulk concentration than occurs in its absence. This observation is attributed to electrostatically favorable shielding of inter-surfactant headgroup repulsions and second to an increase in the degree of counterion condensation within the adsorbed layer. The addition of poly(ethylenimine) at 100 ppm had a pronounced effect on the adsorption behavior of SDS. In

Windsor et al.

common with the simple electrolyte, the polyelectrolyte promotes ordered SDS adsorption at a lower surfactant concentration than is observed in its absence. PEI is, however, more effective in imposing order on the adsorbed surfactant than NaCl. The mode of action of the polyelectrolyte is believed to be primarily electrostatic. Complexation of SDS with the protonated amine groups of PEI in solution results in synergistic adsorption of the complex at the hydrophobic solid/solution interface. The degree of conformational order and hence interfacial packing density of the surfactant was found to increase with SDS concentration due to a monotonic decrease in the average separation distance between complexed SDS molecules along the polyelectrolyte chain as the surfactant concentration is raised. In addition to the charge neutralization driving force for complexation, a degree of cooperativity was determined to exist in the SDS/PEI interaction. Solution turbidity, attributed to the formation of macroscopic SDS/PEI aggregates, began in the surfactant concentration region 1/40-1/35 cmc. The degree of conformational order of SDS rose in parallel with the measured turbidity up to the predicted point of PEI charge neutralization at 1/12 cmc. Addition of NaCl to PEI containing solutions resulted in a greater degree of conformational ordering of the surfactant than was observed in its absence. Furthermore, the degree of SDS ordering was found to be independent of the surfactant concentration over the range investigated in this study. This was attributed primarily to the added electrolyte dampening electrostatic polyelectrolyte/surfactant interactions (a supposition that is supported by the observation of a rise in the surfactant concentration at which solution turbidity first occurs) through shielding and counterion condensation, while promoting a much higher degree of cooperative complexation. Consequently, localized regions of high surfactant density were formed along the polyelectrolyte backbone, resulting in an orientation of surfactant at the interface that was both very high and, importantly, independent of SDS concentration. It is postulated that the high degree of surfactant ordering can be explained by an increase in the strength of intersurfactant alkyl chain interactions mediated by addition of the electrolyte. Acknowledgment. The authors thank Drs. A. M. Creeth and J. Hines of Unilever Research (Port Sunlight Laboratory) for helpful discussions and the latter for the gift of the recrystallized SDS. Drs. M. S. Johal, E. W. Usadi, and A. M. Briggs are acknowledged for experimental assistance. R.W. thanks the EPSRC and Unilever Research for a CASE studentship. D.J.N. gratefully acknowledges the Oppenheimer Fund of the University of Cambridge for the award of a Research Fellowship. LA010505I