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Coadsorption of Sodium Dodecyl Sulfate with Hydrophobically Modified Nonionic Cellulose Polymers. 1. Role of Polymer Hydrophobic Modification K. Derek Berglund,† Todd M. Przybycien,†,‡ and Robert D. Tilton*,† Departments of Chemical Engineering and Biomedical Engineering and Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received August 19, 2002. In Final Form: December 16, 2002 We examined coadsorption from mixtures of the anionic surfactant sodium dodecyl sulfate (SDS) with either hydroxypropyl cellulose (HPC) or hydrophobically modified hydroxyethyl cellulose (hmHEC). Coadsorption to nonselective, hydrophobic poly(dimethylsiloxane) (PDMS) surfaces was compared with coadsorption to negatively charged silica surfaces that were selective for the polymers. Optical reflectometry provided the total extent of adsorption as well as the adsorption kinetics. We referenced SDS concentrations against the critical association concentration (cac) and saturation concentration (csat) for solution-phase polymer/surfactant binding as determined by pyrene solubilization and fluorescence spectroscopy methods. Depending on the bulk SDS concentration, the total adsorbed mass could be either increased or decreased compared to the adsorbed mass attained from SDS-free polymer solutions. Above the cac, the total adsorbed amount decreased with increasing SDS concentration, for either polymer on either the silica or the PDMS surface. At high SDS concentrations with silica surfaces, adsorption of either polymer was completely prevented, leaving the surface bare, but the threshold SDS concentrations for this effect were well above the csat for the particular polymer. On the hydrophobic PDMS surface, only SDS adsorbed when its concentration exceeded the ordinary critical micelle concentration (measured in the absence of polymer). On either surface and at any SDS concentration where some polymer did adsorb, the total extent of adsorption was greater with the more hydrophobic hmHEC polymer than with the HPC polymer.
Introduction Mixed self-assembly and/or coadsorption of polymers and surfactants exert powerful controls over the macroscopic properties of complex fluids. Numerous examples occur in the manufacture and application of materials such as pharmaceutical suspensions or solid dosage forms, ceramics, paints, or coatings. The complexity of polymer/ surfactant interactions makes it difficult to rationally formulate multicomponent complex fluids. It is difficult to predictably control the structure of bulk solution selfassemblies or of adsorbed layers, the latter being especially challenging due to the common occurrence of persistent nonequilibrium states in adsorbed polymer layers. This typically necessitates empirical, trial-and-error approaches to industrial complex fluid formulation. Thus, there is a significant practical incentive to discern the mechanisms by which polymers and surfactants exert their control over fundamental interfacial phenomena. Coadsorption from a polymer/surfactant mixture depends on their mutual interactions, either mutually attractive or repulsive, as well as the selectivity of the surface for the adsorbing species. On a selective surface one of the individual components would adsorb, but not the other, from a single-component solution. Thus, we use a simple categorization scheme to organize polymer/ surfactant/surface systems:1,2 (I) the surfactant binds to the polymer, and the surface is selective; (II) the surfactant binds to the polymer, and the surface is nonselective; (III) * To whom correspondence should be addressed. E-mail: tilton@ andrew.cmu.edu. † Department of Chemical Engineering and Center for Complex Fluids Engineering. ‡ Department of Biomedical Engineering. (1) Braem, A. D.; Prieve, D. C.; Tilton, R. D. Langmuir 2001, 17, 883-890. (2) Velegol, S. B.; Tilton, R. D. Langmuir 2001, 17, 219-227.
the surfactant does not bind to the polymer, and the surface is selective; (IV) the surfactant does not bind to the polymer, and the surface is nonselective. Of course, each of these categories may be further divided as one sees fit, for example, whether a selective surface favors the polymer or the surfactant, whether one or more components are ionic, and so forth. The current investigation concerns the coadsorption of nonionic cellulose polymers and anionic surfactants on two surfaces with different selectivities. The interpretation of the coadsorption behavior is helped by also examining the bulk solution binding behavior. We consider two different hydrophobically modified cellulose polymers: hydroxypropyl cellulose (HPC) and hydrophobically modified hydroxyethyl cellulose (hmHEC). The surfactant is sodium dodecyl sulfate (SDS). A negatively charged silica surface is selective for the cellulose polymers; the anionic SDS is repelled from this surface. A nonselective surface is prepared by spin-casting a hydrophobic poly(dimethylsiloxane) (PDMS) film on a silicon wafer. Thus, we examine the roles of polymer composition and surface selectivity in two type I and two type II systems. Hydrophobically modified cellulose polymers have widespread industrial applications, both with and without alkyl sulfate surfactants, especially in the pharmaceutical, food processing, and paper industries. Interactions between hydrophobically modified cellulose polymers and anionic surfactants in aqueous solution have been studied extensively with a variety of experimental techniques, including ion selective electrodes,3,4 light scattering,5,6 equilibrium dialysis,7,8 calorimetry,6,9,10 fluo(3) Thuresson, K.; Soederman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909-18. (4) Ghoreishi, S. M.; Li, Y.; Bloor, D. M.; Warr, J.; Wyn-Jones, E. Langmuir 1999, 15, 4380-4387.
10.1021/la026429g CCC: $25.00 © 2003 American Chemical Society Published on Web 02/12/2003
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rescence spectroscopy,3,7,8,11-16 nuclear magnetic resonance,3,16,17 and rheology.5,7,18-21 The binding has been found to proceed by the same mechanisms as for other cases of neutral polymer/surfactant binding.22 Generally, there is a surfactant concentration below which no cooperative binding between the polymer and surfactant takes place. This concentration, known as the critical association concentration, or cac, is typically less than the surfactant critical micelle concentration (cmc) and is virtually independent of polymer concentration and polymer molecular weight. The cac depends on temperature and the type of hydrophobic modification on the polymer.3,4,6,9,20 Increasing the surfactant concentration above the cac leads to the formation of micelle-like aggregates along the polymer chain, until the polymer becomes saturated with surfactant molecules. This occurs at a surfactant concentration, csat, that is strongly dependent on the polymer concentration, molecular weight, and hydrophobic modification and temperature. The structures created in the cellulose polymer and anionic surfactant systems near csat correspond to similar structures formed in other neutral polymer/surfactant aggregates, i.e., the string of pearls model. The polymer chain wraps around the surfactant molecules, with part of the polymer backbone becoming intertwined between the surfactant headgroups. Above csat, the only additional selfassembled structures that form when the surfactant concentration is increased will be free micelles in solution. Usually, the binding between a cellulose polymer and an anionic surfactant is strongly cooperative, with the polymer serving as a nucleation point for surfactant micellization along the polymer backbone. Yet, for the class of cellulose polymers that has more extensive hydrophobic modification, such as hydrophobically modified ethyl hydroxyethyl cellulose (hmEHEC), where the additional hydrophobic modification consists of pendant C16 groups, the binding has been found to proceed first through a noncooperative regime.3,6,9,17 This regime usually occurs at surfactant concentrations below the cac of the unmodified cellulose polymer/anionic surfactant system. Through calorimetry experiments, Thuresson and coworkers6 have shown that the only difference in SDS (5) Nystrom, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994-2002. (6) Thuresson, K.; Nystroem, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730-6. (7) Nilsson, S. Macromolecules 1995, 28, 7837-44. (8) Evertsson, H.; Nilsson, S.; Holmberg, C.; Sundeloef, L.-O. Langmuir 1996, 12, 5781-5789. (9) Singh, S. K.; Nilsson, S. J. Colloid Interface Sci. 1999, 213, 133151. (10) Singh, S. K.; Nilsson, S. J. Colloid Interface Sci. 1999, 213, 152159. (11) Winnik, F. M.; Winnik, M. A.; Tazuke, S. J. Phys. Chem. 1987, 91, 594-7. (12) Dualeh, A. J.; Steiner, C. A. Macromolecules 1991, 24, 112-116. (13) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785-9. (14) Medeiros, G. M. M.; Costa, S. M. B. Colloids Surf., A 1996, 119, 141-148. (15) Evertsson, H.; Nilsson, S. Macromolecules 1997, 30, 2377-2385. (16) Evertsson, H.; Nilsson, S.; Welch, C. J.; Sundeloef, L.-O. Langmuir 1998, 14, 6403-6408. (17) Evertsson, H.; Nilsson, S. Carbohydr. Polym. 1999, 40, 293298. (18) Thuresson, K.; Lindman, B.; Nystroem, B. J. Phys. Chem. B 1997, 101, 6450-6459. (19) Panmai, S.; Prud’homme, R. K.; Peiffer, D. G. Colloids Surf., A 1999, 147, 3-15. (20) Hoff, E.; Nystroem, B.; Lindman, B. Langmuir 2001, 17, 28-34. (21) Berglund, K. D.; Troung, M. T.; Garoff, S.; Przybycien, T. M.; Tilton, R. D.; Walker, L. M. Manuscript in preparation. (22) Goddard, E. D.; Ananthapadmanabhan, K. P. Interactions of Surfactants with Polymers and Proteins, 1st ed.; CRC Press: Boca Raton, FL, 1993.
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binding to ethyl hydroxyethyl cellulose (EHEC) or hmEHEC is the existence of this noncooperative regime for hmEHEC below the cac of EHEC. This regime is attributed to SDS binding with the hydrophobic C16 groups on the cellulose polymer.6 As is often observed for complexing polymer/surfactant systems, aggregation numbers (i.e., the numbers of surfactant monomers per micelle) for surfactant assemblies in cellulose polymer/surfactant aggregates are less than the aggregation numbers of free surfactant micelles.7,12,13,15 Because of its industrial importance, adsorption of cellulose polymers to the solid-liquid interface has received considerable attention over the past decade.23-37 These studies have shown that the adsorption of cellulose ethers is slow, often taking hours to reach an adsorption plateau.23,28,33,35 As is typical of most polymers, the adsorption of cellulose polymers is mostly irreversible over practical time scales of hours or days, suggesting the presence of persistent nonequilibrium states in the adsorbed layer. The adsorption behavior of cellulose polymers in the presence of surfactants has recently garnered attention as well.24,28,31,34-37 Joabsson and coworkers35 have shown that introducing SDS after preadsorbing hmEHEC (or EHEC) on silica only slightly affects the adsorbed amount, while the layer thickness varies considerably. In contrast, that same study found that introducing SDS after hmEHEC (or EHEC) adsorption on a hydrophobic surface greatly affects both the adsorbed mass and layer thickness. Lauten and co-workers have shown that the hydrodynamic layer thickness on polystyrene latex particles depends on the SDS concentration in an EHEC or hmEHEC solution.34 Likewise, the adsorbed amount of hmHEC on polystyrene latex particles was found to be highly dependent on the SDS solution concentration.28 In this paper, we determine how the differences in the type of hydrophobic modification influence the extent and kinetics of coadsorption on surfaces that differ in their selectivity. The second paper in this two-part series (following paper in this issue) emphasizes issues of pathdependent coadsorption and the structure of kinetically trapped layers prepared according to different processing histories. In this study, the key difference between a nonselective and a selective surface rests in the contribution of SDS/surface interactions to the overall adsorption energetics. For the nonselective hydrophobic PDMS surface, the SDS/surface interaction contributes favorably to the overall adsorption energy landscape, while for the (23) Malmsten, M.; Lindman, B. Langmuir 1990, 6, 357-64. (24) Claesson, P. M.; Malmsten, M.; Lindman, B. Langmuir 1991, 7, 1441-6. (25) Malmsten, M.; Claesson, P. M. Langmuir 1991, 7, 988-94. (26) Malmsten, M.; Lindman, B.; Holmberg, K.; Brink, C. Langmuir 1991, 7, 2412-14. (27) Pezron, I.; Pezron, E.; Claesson, P. M.; Malmsten, M. Langmuir 1991, 7, 2248-52. (28) Tanaka, R.; Williams, P. A.; Meadows, J.; Phillips, G. O. Colloids Surf. 1992, 66, 63-72. (29) Gau, C. S.; Yu, H.; Zografi, G. Macromolecules 1993, 26, 25249. (30) Malmsten, M.; Tiberg, F. Langmuir 1993, 9, 1098-103. (31) Yamanaka, Y.; Esumi, K. Colloids Surf., A 1997, 122, 121-133. (32) Hoogendam, C. W.; Derks, I.; de Keizer, A.; Cohen Stuart, M. A.; Bijsterbosch, B. H. Colloids Surf., A 1998, 144, 245-258. (33) Kapsabelis, S.; Prestidge, C. A. J. Colloid Interface Sci. 2000, 228, 297-305. (34) Lauten, R. A.; Kjoniksen, A.-L.; Nystroem, B. Langmuir 2000, 16, 4478-4484. (35) Joabsson, F.; Thuresson, K.; Lindman, B. Langmuir 2001, 17, 1499-1505. (36) Joabsson, F.; Thuresson, K.; Blomberg, E. Langmuir 2001, 17, 1506-1510. (37) Lauten, R. A.; Kjoniksen, A.-L.; Nystroem, B. Langmuir 2001, 17, 924-930.
Polymer Hydrophobic Modification
Figure 1. Idealized schematic structure of HPC on the left and hmHEC on the right.
selective silica surface, SDS/surface interactions contribute unfavorably. This has a pronounced influence on the adsorption and desorption behavior on both surfaces, for either the HPC or the more hydrophobic hmHEC with C16 pendant groups. Experimental Section Materials. We obtained hydroxypropyl cellulose, designated HPC-SL, from Nippon Soda. According to the manufacturer, HPCSL had a molecular weight of approximately 60000; the average degree of substitution of propyl groups was DSpropyl ) 1.9 per glucose unit, and the molar substitution of propylene oxide groups was MSPO ) 2.1 per glucose unit. Using size exclusion chromatography with toluene as the solvent and linear polystyrene molecular weight standards, we determined the weight average molecular weight, Mw, for this HPC to be 94000 with a polydispersity index Mw/Mn of 2.0. Hydrophobically modified hydroxyethyl cellulose was obtained from Hercules with the product name of Polysurf 67. According to the manufacturer, molar substitution of ethylene oxide groups for this hmHEC was MSEO ) 2.5 per glucose unit, and the content of C16 linear alkane hydrophobic modifications was 0.53 wt %. Size exclusion chromatography (with the same solvent and molecular weight standards as above) indicated that Mw for our sample was approximately 400000 with an Mw/Mn of 5.4. A schematic of the structure of both polymers is presented in Figure 1. We purchased SDS from Fisher-Biotech (electrophoresis grade, >99% pure). Pyrene (99% pure) was purchased from Aldrich Chemical. We obtained PDMS as a gift from Rhodia Silicones, designated as Rhodorsil fluid 47 V 1000. We purchased Chromerge cleaning solution, sodium chloride, hydrochloric acid, sulfuric acid, and toluene from Fisher Scientific, sodium hydroxide pellets from EM Science, and RBS 35 detergent from Pierce. All reagents were ACS grade and used as received. Chromerge was prepared by mixing 25 mL of the Chromerge cleaning solution with 2.5 L of 36 N sulfuric acid. We purified all water by reverse osmosis followed by treatment with the Milli-Q Plus system consisting of ion exchange and organic adsorption cartridges from Millipore Corp. The pH of all solutions was unmodified from the air-saturated aqueous solution pH of 5.5-6.0. Optical reflectometry experiments were performed on optical grade silicon wafers purchased from Valley Design Corp. We first cleaned the as-received wafers by the concentrated acid procedure below, and then oxidized the clean wafers in air at 1000 °C for 20-25 min to generate 20-30 nm thick oxide layers. We measured the thickness of the oxide layer on each individual wafer via optical reflectometry immediately prior to each adsorption experiment. After oxidation, the wafers were cleaned by first washing them with RBS detergent and placing them under a low-pressure mercury vapor grid lamp for 30 min in the ultraviolet light ozone cleaner model no. 42 manufactured by Jelight Co. After these initial cleaning procedures the substrates were cleaned with concentrated acid in the following manner. The substrates were placed in the Chromerge solution for 30 min, followed by a 30 min soak in 6 N hydrochloric acid. They were then soaked in 10 mM sodium hydroxide for 30 min, yielding a completely water wettable, negatively charged silicon oxide surface. The substrates were thoroughly rinsed with water after each soak and were stored in water. They were rinsed profusely with water before being installed in the reflectometer flow cell
Langmuir, Vol. 19, No. 7, 2003 2707 and were never allowed to dry between the cleaning process and their use in the experiments. To generate hydrophobic surfaces, we spin-coated a PDMS film onto an acid-cleaned silicon wafer. Films were cast by depositing 1 mL of a 0.5 wt % solution of PDMS in toluene onto a wafer spinning at 3000 rpm for 1 min. This procedure was repeated three times for each wafer. The coated wafers were then placed in a vacuum oven at 60 °C for 24 h. This procedure resulted in polymer film thicknesses ranging from 15 to 20 nm as determined by optical reflectometry. Hydrophobic PDMS wafers were stored in air and were rinsed profusely with water prior to their use in reflectometry experiments. Sessile water drop contact angles on the PDMS films were greater than 90° and were stable after continuous immersion in water for well over 48 h. Determination of Critical Association and Saturation Concentrations in Solution. With pyrene as a molecular probe that strongly partitions from aqueous solutions into surfactant aggregates, we used both a fluorescence spectroscopy technique and an ultraviolet spectrophotometry technique to determine the cac and csat for SDS binding to hmHEC or HPC. The fluorescence spectroscopy technique exploits the wellknown environmental sensitivity of the emission properties of pyrene.11,38-41 Using the procedures detailed by Kim and coworkers,41 we detected the cac as the surfactant concentration at which the intensity ratio of the first to third monomer fluorescence emission bands, I1/I3, began to decrease in the polymer/pyrene/surfactant solution, owing to the decreased polarity of the pyrene microenvironment as it partitioned from the aqueous pseudophase into polymer/surfactant aggregates. Alternatively, we could also detect the cac as the surfactant concentration at which the intensity ratio of the excimer emission band to the first monomer emission band, denoted as Iex/Imon, began to increase. We recorded fluorescence spectra with 238 nm excitation, using either a Perkin-Elmer LS5B or a SPEX Fluorolog 3 fluorescence spectrometer. All experiments were conducted at room temperature that varied from 21 to 24 °C. For fluorescence measurements, aliquots of concentrated pyrene stock solutions in methanol were added to dry volumetric flasks. The methanol was evaporated in a vacuum oven, leaving behind the solid pyrene residue. Water, solid salt, polymer, and surfactant were then added to the vessel to achieve a 5 µM analytic pyrene concentration (total amount of pyrene per total volume, regardless of the phase). This is approximately 7 times the aqueous solubility limit of pyrene. These polymer/surfactant/ pyrene stock solutions were stirred in excess of 24 h before their use. Samples were prepared in sealed centrifuge tubes from the stock solutions, sonicated in excess of 12 h in the bath of a Branson ultrasonic cleaner (model 1200), and allowed to equilibrate to room temperature before the fluorescence spectra were recorded. The second test to determine the different binding transitions was a pyrene solubilization assay based on UV spectrophotometry. Different regimes of polymer/surfactant complexation were detectable since different types of aggregates display different solubilizing powers for a sparingly soluble chromophore, such as pyrene.41 Again following Kim and co-workers,41 we added an excess of solid powdered pyrene to 5 mL of a polymer/surfactant solution in a centrifuge tube. The samples were then sonicated in excess of 12 h. After sonication, the samples were allowed to equilibrate to room temperature before being centrifuged at 3000 rpm for 1 h to sediment any undissolved pyrene. Supernatants were diluted in 50 mM SDS/1 mM NaCl solutions to assay for the total solubilized pyrene concentration. Using a Varian Cary 300 Bio UV-vis spectrometer, the absorbance was measured at 336 nm in quartz cuvettes. Neither the polymers nor SDS has significant absorbance at 336 nm. We calculated the concentration of pyrene solubilized in the polymer/surfactant mixtures using the previously reported molar absorptivity of surfactantsolubilized pyrene, 2.06 × 10-5 M/cm.41 (38) Turro, N. J.; Baretz, B. H.; Kuo, P. L. Macromolecules 1984, 17, 1321. (39) Zana, R.; Lianos, P.; Lang, J. J. J. Phys. Chem. 1985, 89, 41. (40) Ananthapadmanabhan, K. P.; Goddard, E. D.; Turro, N. J.; Kuo, P. L. Langmuir 1985, 1, 352. (41) Kim, J.-H.; Domach, M. M.; Tilton, R. D. J. Phys. Chem. B 1999, 103, 10582-10590.
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Adsorption Measurements. We measured adsorption and desorption using optical reflectometry. Principles and applications of this technique are described elsewhere.1,2,42-46 A detailed description of the scanning angle reflectometry technique that we used is available,46 and the particular instrument we used is that described by Furst and co-workers44 as modified by Velegol and co-workers.2 The instrument had a rectangular slit flowcell, and all materials that contacted the solution were stainless steel, Teflon, fluorinated elastomer O-rings, or acid-cleaned glassware. Solutions passed through the flowcell only once, without being recycled, to minimize the adsorption of surface-active impurities that might arise from the pumping system. All adsorption experiments were performed at a wall shear rate of approximately 1.0 s-1 and a constant temperature of 25 °C. After a clean wafer was mounted in the reflectometer flowcell, it was allowed to soak in 1.0 mM NaCl solution for 20 min to check for adsorption of surface-active impurities before polymer/surfactant solutions were introduced. Steady-state surface excess concentrations were determined from a scanning angle regression of the reflection coefficient for parallel polarized light, Rp(θ), after the adsorption plateau was attained. We evaluated the optical properties of the adsorbed layers using a homogeneous two-layer optical model, where the semiinfinite solution and bulk silicon were separated by an oxide layer of known refractive index and thickness and by the adsorbed layer of unknown refractive index and thickness. For the hydrophobic surfaces, the spin-cast PDMS film and the silicon oxide layer were treated as the same, given their similar refractive indices. The refractive indices at 632.8 nm are nSi ) 3.882 + 0.019i for bulk silicon and nox ) 1.46 for the silicon oxide layer. Using the Abele`s matrix method47 to numerically evaluate the two-layer model, we used χ2 minimization to fit the measured reflectivity profile Rp(θ) to obtain the average thickness, dav, and the average refractive index, nav, of the adsorbed layer. Although these parameters are model dependent and highly coupled, the effective optical thickness of the adsorbed layer, dav(nav - n0), where n0 is the refractive index of the bulk solution, is model independent. For multicomponent adsorption, the effective optical thickness is related to the surface excess concentration of each species, Γi, by
dav(nav - n0) )
∑Γ (dn/dC ) i
i
(1)
i
where dn/dCi is the refractive index increment of the adsorbing species i at the wavelength of interest. We measured the refractive index increments of hmHEC, HPC, and SDS at the heliumneon laser wavelength of 632.8 nm with a Phoenix Precision Instrument Co. differential refractometer. These are 0.139, 0.135, and 0.120 cm3/g, respectively. As reflectometry does not directly distinguish the relative amounts of each component in a mixed adsorbed layer, here we report both the effective optical thickness and the apparent polymer surface excess concentration calculated according to eq 1 using only the polymer refractive index increment. To measure adsorption and desorption kinetics, we simply monitored the reflectivity at the Brewster angle as a function of time and employed the approximation
Γ(t) ) R[Rp(θb,t)1/2 - Rp(θb,0)1/2]
(2)
where Rp(θb,0) is the reflectivity recorded at the Brewster angle before adsorption. The proportionality constant, R, is generated (42) Dijt, J. C.; Cohen Stuart, M. A.; Hofman, J. E.; Fleer, G. J. Colloids Surf. 1990, 51, 141-158. (43) Dijt, J. C.; Cohen Stuart, M. A.; Fleer, G. J. Adv. Colloid Interface Sci. 1994, 50, 79-101. (44) Furst, E. M.; Pagac, E. S.; Tilton, R. D. Ind. Eng. Chem. Res. 1996, 35, 1566-74. (45) Pagac, E. S.; Prieve, D. C.; Solomentsev, Y.; Tilton, R. D. Langmuir 1997, 13, 2993-3001. (46) Tilton, R. D. In Scanning Angle Reflectometry and Its Application to Polymer Adsorption and Coadsorption with Surfactants; Farinato, R. S., Dubin, P. L., Eds.; John Wiley & Sons: New York, 1999; p 331. (47) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1977.
Figure 2. (a, top) Fluorescence ratios I1/I3 (O) and Iex/Imon (9) for solutions containing 0.1 wt % HPC, 1.0 mM NaCl, 5 µM pyrene, and SDS. The inset shows the low concentration regime. (b, bottom) Pyrene uptake in solutions of 0.1 wt % HPC, 1.0 mM NaCl, and SDS. The lines are linear regression fits to the data in a given regime. numerically via the homogeneous two-layer optical model.46 This approximation is consistent with the full scanning angle procedure to within 4% for the current conditions.
Results and Discussion HPC/SDS Complexation in Solution. Figure 2a displays the I1/I3 and Iex/Imon pyrene fluorescence intensity ratios for solutions containing 0.1 wt % HPC and 1.0 mM NaCl as a function of the SDS concentration. In the absence of SDS, a 0.1 wt % HPC solution yielded an I1/I3 value of 1.28. For comparison, pyrene has an I1/I3 ratio greater than 1.5 in a purely aqueous environment;41 therefore, this lower I1/I3 ratio indicates that there was already some interaction between pyrene and HPC without SDS. At low SDS concentrations (regime I in Figure 2a) both fluorescence ratios were fairly constant with average values of 1.28 and 0.15, for I1/I3 and Iex/Imon, respectively. These constant ratios indicated that the SDS had not yet created new hydrophobic domains into which the pyrene would partition. At approximately 1.5 mM SDS, the I1/I3 ratio began to decrease, indicative of pyrene partitioning into a less polar environment. As expected, the Iex/Imon ratio began to increase at this concentration, and shortly thereafter this ratio experienced a maximum. The maximum occurred when all the pyrene was fully solubilized in aggregates, and any further increase in surfactant concentration merely diluted a finite population of pyrene molecules over an increasing number of aggregates. This behavior is indicative of the onset of SDS association with
Polymer Hydrophobic Modification
HPC at a cac of approximately 1.5 mM SDS. At SDS concentrations above 20 mM, the I1/I3 ratio was fairly constant at 0.92, indicating that the pyrene microenvironment no longer changed as new aggregates formed. Also above 20 mM SDS, the Iex/Imon ratio continued to decrease as the average number of pyrene molecules per aggregate decreased continually. Compared to other systems where this fluorescence technique has been employed,41 the HPC/SDS system displayed some unusual fluorescence behavior above 5 mM SDS. The I1/I3 ratio experienced a local minimum, while the Iex/Imon Ratio experienced a local maximum. This same behavior of local extrema in the fluorescence ratios above csat was previously observed in an HPC/hexadecyltrimethylammonium chloride system by Winnik and coworkers.11 It was speculated that this behavior may be explained if pyrene were to preferentially partition out of the HPC/surfactant aggregate and into newly created free surfactant micelles at SDS concentrations above csat. The second maximum in Iex/Imon would be consistent with this behavior, if the partitioning into free micelles were strongly favored. Then a preferential enrichment of pyrene in free micelles would initially promote excimer formation, and further SDS concentration increases would again simply dilute the pyrene over an ever-increasing concentration of free micelles. The reason for the concurrent variation in I1/I3 is not clear, but it would suggest that the free SDS micelles presented a slightly more polar microenvironment for pyrene than the environment presented by HPC-bound aggregates (perhaps due to more extensive interaction with sulfate groups). If so, then hydrophobicity is not the sole driving force for pyrene partitioning into the micelles. While not conclusive, we suspect that the concentration at which Iex/Imon experienced its local minimum, approximately 5-6 mM, approximated the csat. For comparison, we note that the cmc of SDS in 1.0 mM NaCl solutions that contain no polymer is 7.9 mM (data not shown). This value for csat will be compared to the value obtained with the pyrene solubilization technique below. Figure 2b shows the pyrene solubility limit in solutions containing 0.1 wt % HPC and 1.0 mM NaCl as a function of increasing SDS concentration. Three distinct linear regimes are observed in the data. The slope of each linear regime can be interpreted as the solubilizing power of the aggregate type being created in each regime. The csat is indicated by a change in solubilizing power from a value characteristic of SDS/polymer aggregates to that corresponding to free SDS micelles. The solubilizing power of SDS micelles was measured previously by Kim and coworkers and ranges from 6.8 µM pyrene/mM SDS in the absence of salt to 7.9 µM pyrene/mM SDS in 100 mM NaCl.41 Between 1.9 and 2.9 mM SDS, the pyrene solubility stayed fairly constant at 1.5 µM. This is approximately twice the aqueous solubility limit of pyrene, indicating that HPC itself solubilized a small amount of pyrene. Between 2.9 and 4.2 mM SDS, the solubilizing power was 7.7 ( 0.3 µM pyrene/mM SDS, indicating the formation of micelle-like aggregates. Since this occurred below the cmc of SDS, these must have been polymer-associated aggregates. From 6.0 to 15.0 mM SDS, the solutions displayed a higher, constant value of the solubilizing power, 9.9 ( 0.1 µM pyrene/mM SDS. Although this solubilizing power was larger than expected for free SDS micelles, our solubilizing power corresponds well with a similar study of free SDS micelles after saturation of a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer.1 The difference in solubilizing power may be due to the higher purity of the SDS used
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Figure 3. (a, top) Fluorescence ratios I1/I3 (O) and Iex/Imon (9) for solutions containing 0.1 wt % hmHEC, 1.0 mM NaCl, 5 µM pyrene, and SDS. The inset shows the low concentration regime. (b, bottom) Pyrene uptake in solutions of 0.1 wt % hmHEC, 1.0 mM NaCl, and SDS. The lines are linear regression fits to the data in a given regime.
by Kim et al. Using a statistical t-test with an R of 0.05,48 the csat was determined to be 4.6 ( 0.1 mM SDS at 95% confidence. This is consistent with the csat estimated from excimer emission as noted above. hmHEC/SDS Complexation in Solution. Figure 3a displays the pyrene fluorescence intensity ratios for solutions containing 0.1 wt % hmHEC and 1.0 mM NaCl as a function of the SDS concentration. The qualitative behavior of this system was similar to that of the system containing HPC. Like HPC, the microenvironmental polarity in SDS-free hmHEC solutions indicated a pyrene association with hmHEC. Below 0.04 mM SDS, the I1/I3 ratio was fairly constant with an average value of 1.30 independent of SDS concentration, demonstrating that no surfactant aggregates were being formed. However, at 0.04 mM SDS the I1/I3 ratio began to decrease monotonically, indicative of the formation of polymer-associated SDS aggregates. As shown in the inset, a very broad maximum may be discerned in the Iex/Imon ratio between 0.02 and 0.05 mM SDS, indicating that the cac had been reached. After the I1/I3 ratio decreased monotonically above 0.04 mM SDS, another abrupt break in it occurred near 7.0 mM SDS, suggesting, as for HPC, that the csat had been reached and that SDS micelles were forming in solution. Above this SDS concentration the I1/I3 ratio was fairly constant with an average value of 1.03. Unlike that (48) Montgomery, D. C.; Runger, G. C. Applied Statistics and Probability for Engineers; John Wiley and Sons: New York, 1994.
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Table 1. Major Polymer/Surfactant Binding Transitions and Solubilizing Powers pyrene solubilization pyrene fluorescence polymer
cac, mM
csat for 0.1 wt % polymer, mM
HPC hmHEC
1.5 0.04
6.0 7.0
solubilizing power, µM pyrene/mM SDS regime II regime III 7.7 ( 0.3 0.9 ( 0.1
9.9 ( 0.1 9.8 ( 0.4
csat for 0.1 wt % polymer, mM
loading, g of SDS/g of polymer at saturation
estimated csat,a mM
4.6 ( 0.1 6.5 ( 1.0
0.9 1.9
3.1 0.7
a The saturation concentrations for 0.05 wt % HPC and 0.01 wt % hmHEC were estimated using the cac determined by pyrene fluorescence and the csat determined by pyrene absorbance at 0.1 wt % polymer.
for the HPC/SDS system, the Iex/Imon ratio did not show a second maximum at higher SDS concentrations. We again used the pyrene solubilization assay to test the value of csat that we inferred from the fluorescence data. Those results are shown in Figure 3b for solutions containing 0.1 wt % hmHEC and 1.0 mM NaCl as a function of increasing SDS concentration (beginning at 4.0 mM). Two distinct linear regimes with finite, statistically distinguishable slopes were observed. The first regime had an extremely low but nonzero solubilizing power of 0.9 ( 0.1 µM pyrene/mM SDS. The second linear regime had a solubilizing power of 9.8 ( 0.4 µM pyrene/ mM SDS, indicating that SDS micelles were forming in solution. This solubilizing power of free SDS micelles in hmHEC solutions is similar to the value in HPC solutions. Using the statistical t-test as before, the csat was found to be 6.5 ( 1.0 mM SDS at 95% confidence. Discussion of Polymer/Surfactant Binding. Table 1 displays the major binding transitions observed using each technique as well as the solubilizing power in each regime for both polymer/surfactant systems. Compared to the literature, polymer concentrations of 0.1 wt % are rather low for the determination of cac or csat. We used dilute solutions for the polymer/surfactant binding studies so that the csat values for the even more dilute solutions used in the adsorption experiments could be reasonably estimated by assuming that the mass of bound SDS per mass of polymer at saturation is a constant. The choice of polymer concentrations was a compromise between problems imposed by more viscous, more concentrated solutions in the reflectometry flowcell and the diminished ability to distinguish binding transitions in more dilute solutions. As shown in Table 1, at the 0.05 wt % concentration used in HPC reflectometry experiments, we estimated the csat as 3.1 mM SDS, while we estimated the csat as 0.7 mM SDS for the 0.01 wt % concentration used in hmHEC reflectometry experiments. The cac values determined by the fluorescence technique were approximately 1.5 and 0.04 mM SDS for the HPC/ SDS and hmHEC/SDS systems, respectively. The significantly lower cac for hmHEC than for HPC is attributed to aggregate nucleation on the C16 pendant groups. These dilute polymer concentrations make it difficult to compare to literature values for saturation concentrations, where the polymer solutions are usually considerably more concentrated. Nevertheless, the cac is believed to be independent of polymer concentration, allowing us to compare cac results with literature values in a more straightforward way. Various authors report a range of cac values, from 1.0 to 2.4 mM SDS, for HPC and similar modified cellulose polymers, including HEC and EHEC.3,4,7,14,15 Our cac value for HPC falls within this range. Very few experimental studies have been made of the onset of cooperative binding between an anionic surfactant and the more hydrophobic hmHEC type of cellulose polymer. Using a combination of techniques, Thuresson and co-workers found that, at SDS concentrations as low as 0.1 mM, the surfactant was already bound
to hmEHEC.3 Using a gel swelling technique, Sjo¨stro¨m and Piculell determined the cac for their hmHEC/SDS system to be below 0.5 mM SDS.49 In contrast, Ghoreishi and co-workers used a surfactant-specific electrode technique and reported a much larger cac, 2.0 mM, for their sample of hmEHEC and SDS.4 Although our cac value for the hmHEC/SDS system is the lowest reported for this type of system, it is consistent with the first two reports above of upper bounds for the cac for a C16-modified cellulose. For the HPC/SDS system, the fluorescence and solubilization techniques resulted in saturation concentration estimates of 6.0 and 4.6 mM SDS, respectively, for a 0.1 wt % HPC concentration. The discrepancy between the csat values determined by the different techniques is most likely due to the unusual fluorescence behavior near csat. Therefore, for the HPC/SDS system the pyrene solubilization experiments most likely provided the more reliable estimate of the saturation concentration. Using the cac determined by fluorescence and the csat determined by solubilization, we calculate that 0.9 g of SDS is bound per gram of HPC at saturation. This loading corresponds well with the range of literature values from 1.0 to 1.3 g/g for the HPC/SDS system.4,10 For the hmHEC/ SDS system, the fluorescence spectroscopy and solubilization techniques resulted in saturation concentration estimates of approximately 7.0 and 6.5 mM SDS, respectively, at a 0.1 wt % hmHEC concentration. Again using the fluorescence results for an estimate of cac and the solubilization results for an estimate of csat, we calculate that 1.9 g of SDS is bound per gram of hmHEC at saturation. The difference in SDS loading at saturation is a consequence of the significant difference in hydrophobic modification between the two types of cellulose polymers. Since no prior studies of saturation concentration have been performed at these low polymer concentrations, we must linearly extrapolate back from the previously published high polymer concentration data to attempt a comparison. Doing this, we find that our estimate of csat for HPC/SDS at 0.1 wt % HPC falls a little below the range of extrapolated literature values, 5.4-8.6 mM SDS, for cellulose polymers such as HEC, EHEC, and HPC.4,6,9,13-15 Similarly extrapolating the existing literature data for hmEHEC to 0.1 wt % polymer, we find that our hmHEC saturation concentration falls within the extrapolated range of published csat values, approximately 5.6-10 mM SDS, for cellulose polymers such as hmEHEC.3,4,6 Adsorption from Single-Component Solutions. We measured the single-component adsorption of HPC and hmHEC onto both silica and PDMS surfaces in the presence of 1.0 mM NaCl. On silica, 0.05 wt % HPC and 0.01 wt % hmHEC solutions produced adsorbed layers with surface concentrations of 0.67 ( 0.05 and 0.91 ( 0.05 mg/m2, respectively. On the hydrophobic PDMS surface, (49) Sjo¨stro¨m, J.; Piculell, L. Langmuir 2001, 17, 3836-3843.
Polymer Hydrophobic Modification
Figure 4. Adsorption kinetics for 0.05 wt % HPC and 0.01 wt % hmHEC in 1.0 mM NaCl solutions on the hydrophobic PDMS surface. For comparison, two experiments appear shifted in time on the one figure.
Figure 5. SDS adsorption isotherm on the hydrophobic PDMS surface in 1.0 mM NaCl.
these same solutions of HPC and hmHEC adsorbed to surface concentrations of 1.09 ( 0.05 and 3.0 ( 0.3 mg/ m2, respectively. Adsorption plateaus were reached within 30 min for HPC on both surfaces and for hmHEC on silica. Samples of the polymer adsorption kinetics on the PDMS surface are displayed in Figure 4. On the hydrophobic PDMS surface, hmHEC adsorption reached approximately 95% of its final surface concentration after 3 h, requiring approximately 8 h to reach a steady surface concentration. The considerably slower hmHEC adsorption process is likely attributable to slow structural relaxations in the layer. Using optical reflectometry, it was previously found in our laboratory that SDS does not adsorb to silica under the range of surfactant and salt concentrations used in this study.1 The detection limit of our instrument for SDS adsorption is 0.05 mg/m2 or 960 Å2/molecule. A lack of adsorption, in fact a surface depletion, is expected, due to the electrostatic repulsion between the surfactant headgroup and the negatively charged silica surface. On the hydrophobic PDMS surface, SDS solutions containing 1.0 mM NaCl produced the adsorption isotherm shown in Figure 5. A slight maximum in adsorption occurred just before the cmc (7.9 mM determined by pyrene absorbance), most likely due to the presence of dodecanol in the adsorbed layer. Dodecanol is a common trace impurity found in SDS preparations.50 Above the cmc, dodecanol is prefer-
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Figure 6. Coadsorption from solutions containing 0.05 wt % HPC, 1.0 mM NaCl, and SDS. The data are displayed for the adsorbed mass on the PDMS surface (O) and the silica surface (b). The apparent HPC surface concentration shown on the right-hand axis was calculated via eq 1 using dn/dc for HPC, assuming only HPC was present in the layer.
Figure 7. Coadsorption from solutions containing 0.01 wt % hmHEC, 1.0 mM NaCl, and SDS. The data are displayed for the adsorbed mass on the PDMS surface (0) and the silica surface (9). The apparent hmHEC surface concentration shown on the right-hand axis was calculated via eq 1 using dn/dc for hmHEC, assuming only hmHEC was present in the layer.
entially solubilized by the SDS micelles in solution. Above the cmc, the adsorption plateau was 1.15 ( 0.05 mg/m2. This corresponds to an effective optical thickness of 0.138 nm, and an occupied area per molecule of approximately 42 Å2. This is consistent with monolayer adsorption since a close-packed SDS monolayer would have a surface area per molecule of 36 Å2 on the basis of the size of the surfactant headgroup. Compared to polymer adsorption, SDS adsorption to the hydrophobic PDMS surface was rapid, establishing an equilibrium surface concentration within 1 min at all SDS concentrations examined. SDS adsorption on PDMS was always completely reversible. Coadsorption of Cellulose Polymers and SDS on Silica. Figures 6 and 7 display the coadsorption “isotherms” for the HPC/SDS and hmHEC/SDS systems, respectively, on the silica surface. These are plots of the effective optical thickness dav(nav - n0) of the mixed adsorbed layer as a function of the SDS concentration while the bulk polymer concentration was held constant. The HPC and hmHEC concentrations were fixed at 0.05 (50) Ward, R. N.; Davies, P. B.; Bain, C. D. J. Phys. Chem. B 1997, 101, 1594-1601.
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Berglund et al.
and 0.01 wt %, respectively, and the NaCl concentration was fixed at 1.0 mM. Coadsorption from polymer/surfactant mixtures was in all cases irreversible on silica, as will be discussed in more detail in the companion paper in this issue. Therefore, all data points shown in the figures were from the first exposure of clean surfaces to the polymer/surfactant mixtures. Adsorption plateaus in the coadsorption experiments on silica were achieved within 20-30 min for both polymer/surfactant systems. This was not significantly different from the single-component adsorption for both polymers on silica. On silica, the total adsorbed amount of the HPC/SDS complex experiences a maximum of approximately 0.85 mg/m2 (0.115 nm effective optical thickness) at SDS concentrations between 0.5 and 1.0 mM. It then decreases sharply with increasing SDS concentration. In contrast, the total adsorbed amount of the hmHEC/SDS complex does not experience a maximum. It decreases gradually from 0.9 mg/m2 (0.125 nm effective optical thickness) as the SDS concentration is increased. Above 10 mM SDS the HPC/surfactant complex does not adsorb, whereas SDS concentrations of 25 mM are required to prevent hmHEC/SDS complex adsorption on silica. At all SDS concentrations, the hmHEC/SDS system adsorbs to a greater extent than the HPC/SDS system. The total adsorbed mass on silica depends on a balance of competing forces at the interface. The major driving force for adsorption is the net attraction of the polymer segments to the surface. Surfactant binding hinders polymer adsorption through a variety of mechanisms. Bound surfactant decreases the adsorption driving force by shielding the hydrophobic residues on the polymer, effectively increasing its favorable interaction with the solvent. Likewise, repulsive electrostatic interactions occur between the polymer-bound surfactant molecules and the surface. Another more subtle effect could occur if surfactant molecules are present in the adsorbed layer. These molecules would decrease adsorption through lateral repulsive interactions between bound surfactant molecules on neighboring polymer chains or different segments on the same chain. Therefore, as more surfactant molecules bind to the hydrophobic polymer, the polymer adsorption is reduced. The maximum surface coverage for the HPC/SDS system on silica occurs at a surfactant concentration near the cac measured by pyrene fluorescence. The difference in the total adsorbed mass values for HPC adsorption from an SDS-free solution and for HPC coadsorption with 1.0 mM SDS could simply represent the additional mass of SDS in the coadsorbed layer. Alternatively, additional mass could also adsorb if the footprint of the polymer/ surfactant complex were smaller than the footprint of the polymer in the absence of SDS. Recent theoretical approaches have indicated that a dimensional collapse of the polymer should be expected at the onset of surfactant and polymer association.51,52 There is experimental evidence for this dimensional collapse. Evertsson and Nilsson have measured a maximum in microviscosity and rigidity in EHEC/SDS clusters formed close to or slightly higher than the cac using fluorescence probe techniques.15 For hmHEC cross-linked gels, Sjo¨stro¨m and Piculell measured a deswelling of the gels at low SDS concentrations before the onset of the cooperative SDS binding.49 Thus, the enhanced adsorption could be due at least in part to the smaller footprint of the HPC/SDS complex at the interface. A similar maximum in the total adsorbed mass is not observed with the hmHEC/SDS system on silica.
On silica, adsorption is prevented for both polymers at SDS concentrations that are much larger than the estimated csat (see Table 1). Although all polymers in the bulk are saturated with bound surfactant when the bulk SDS concentration exceeds csat, the SDS is electrostatically depleted from the interfacial region. This would shift the local polymer/surfactant binding equilibrium toward more dilute solution behavior. Prevention of adsorption would be expected when the local SDS concentration near the interface is sufficient to saturate polymers in close proximity to the interface; in other words, there is a “surface csat”. Without fully modeling the nonequilibrium polymer/surfactant layers, we simply use Gouy-Chapman theory to approximate the electric field near the charged silica surface53 and assume that the local surfactant concentration obeys the Boltzman distribution. When calculating the Debye length from the total ionic strength, we also assume that the unbound surfactant concentration does not exceed the measured cac. We take 1 nm as an arbitrary but reasonable cutoff distance at which to calculate the local SDS concentration. Using the streaming current apparatus described in the companion paper in this issue, the bare silica ζ potential is -129.5 mV in 0.1 mM NaCl. Assuming that the surface has a constant charge and equating the ζ potential with the surface potential, then we may calculate the silica surface potential at other ionic strengths. A surfactant concentration of 10 mM is required to prevent adsorption in the HPC/SDS system. With this surfactant concentration, a surface potential of -46.9 mV, and a 1.0 mM background 1:1 electrolyte concentration, the SDS concentration is 2.0 mM at a distance of 1 nm from the silica surface. For the hmHEC/SDS system adsorption is not prevented until the bulk surfactant concentration reaches 25 mM. Using this surfactant concentration, a surface potential of -73.1 mV, and a background 1:1 electrolyte concentration of 1.0 mM, the SDS concentration 1 nm from the silica surface is approximately 2.1 mM. The consistency of this simple calculation with the estimated csat in Table 1 supports the argument that adsorption persists well beyond the bulk csat due to electrostatic depletion of SDS from the charged surface. Coadsorption of Cellulose Polymers and SDS on Hydrophobic PDMS. Figures 6 and 7 also display the coadsorption isotherms for HPC/SDS and hmHEC/SDS on the nonselective, hydrophobic PDMS surface. The polymer and salt concentrations were the same as in the experiments with the silica surface. For the HPC/SDS system, the adsorbed amount experienced a maximum of 1.5 mg/m2 (0.19 nm effective optical thickness) at 1 mM SDS, slightly below the cac. It then decreased with increasing SDS concentration until approximately 6.0 mM SDS. As on silica, this maximum can be attributed at least in part to the presence of SDS in the adsorbed layer and/or a dimensional collapse of the polymer near the cac. Above 6 mM SDS, the surface concentration remained constant with an average value of 0.95 mg/m2 (0.13 nm effective optical thickness). For the hmHEC/SDS system the adsorbed mass decreased monotonically from 3.4 mg/ m2 (0.46 nm effective optical thickness) until a lower plateau of 1.0 mg/m2 (0.14 nm effective optical thickness) was reached at approximately 8 mM SDS. Recall that 20-30 min was required to reach steadystate coadsorption on silica, regardless of the SDS concentration. In contrast, on the hydrophobic surface,
(51) Diamant, H.; Andelman, D. Europhys. Lett. 1999, 48, 170-176. (52) Groot, R. D. Langmuir 2000, 16, 7493-7502.
(53) Israelachvili, J. N. Intermolecular and Surface Forces; 2nd ed.; Academic Press Inc.: San Diego, 1992.
Polymer Hydrophobic Modification
Figure 8. Adsorption kinetics for solutions of 0.05 wt % HPC, 1.0 mM NaCl, and SDS (2) and 0.01 wt % hmHEC, 1.0 mM NaCl, and SDS (4) on the hydrophobic PDMS surface.
adsorption kinetics depended strongly on the SDS concentration as shown in Figure 8. For the HPC/SDS system below 5 mM SDS, approximately 20-50 min was required to reach 95% of the adsorption plateau, but adsorption was considerably more rapid above 5 mM SDS, taking less than 1 min to establish 95% of the final extent of adsorption. Surface saturation times were as long as 4 h for the hmHEC/SDS system at low SDS concentrations. Yet, above 8 mM SDS, the time required to establish an adsorption plateau was again approximately 1 min. Above 8 mM SDS in both polymer/SDS systems, both the surface concentrations (Figures 6 and 7) and the rapid adsorption kinetics (Figure 8) indicate that the adsorbed layers are SDS monolayers that contain little or no polymer. The adsorbed mass and kinetics are identical to the results for single-component SDS adsorption on PDMS in the absence of polymer. Above csat, the free surfactant activity becomes sufficiently large that it competes effectively for adsorption to the hydrophobic surface. Furthermore, the hydrophobic residues that are mainly responsible for the adsorption of these hydrophobically modified celluloses to PDMS have been completely solubilized by SDS at these high concentrations since the csat has been reached. Conclusions In this study we measured the cac and csat for mixtures of two types of cellulose polymers with SDS, in parallel with adsorption measurements at a selective and a nonselective surface. We found that the onset of cooperative association between SDS and the cellulose polymer in bulk solution occurs at a much lower SDS concentration for hmHEC than for HPC. Furthermore, the greater
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hydrophobicity of hmHEC with respect to HPC correlated with an increased SDS loading on the polymer at saturation. The hmHEC polymer bound 1.9 g of SDS/g of polymer at saturation, whereas the HPC polymer bound 0.9 g of SDS/g of polymer. The bulk surfactant concentration controls the total adsorbed mass for both of these polymer/surfactant systems, on both the charged, selective silica and the hydrophobic, nonselective PDMS surfaces. For the HPC/ SDS system, the adsorbed mass experiences a maximum below the cac on both surfaces. For the hmHEC/SDS system at low SDS concentrations the adsorbed mass for the polymer/surfactant complex is approximately the same as the adsorbed mass of the polymer in the absence of SDS. Increasing the SDS concentration beyond the cac decreases the adsorbed amount on both surfaces. On silica there exists a regime at high SDS concentrations where hmHEC/SDS and HPC/SDS solutions produce no detectable adsorption. This threshold SDS concentration is considerably higher than the estimated csat for binding in bulk solution, an effect that we attribute to a shift in the local degree of polymer/surfactant binding, owing to an electrostatic depletion of surfactant from the charged interface. Since the hydrophobic surface is nonselective, competitive adsorption occurs between the cellulose polymer/SDS complex and free surfactant. Above csat, the free surfactant activity becomes sufficiently large that it dominates adsorption to the hydrophobic surface. The hmHEC/SDS system adsorbs to a greater extent than the HPC/SDS system on both surfaces and at all SDS concentrations. This is especially evident on the hydrophobic surface where at low SDS concentrations the hmHEC/SDS adsorbed layer has almost 2.5 times as much mass as the HPC/SDS system. On silica, the HPC/SDS adsorbed mass experiences a much steeper decline with increasing SDS concentration than does hmHEC/SDS. Likewise on silica, we find that HPC/SDS adsorption is prevented at moderate SDS concentrations while adsorption of the hmHEC/SDS system is only prevented at SDS concentrations larger than 25 mM. While the adsorption kinetics on silica are approximately the same, hmHEC/ SDS takes a much longer time to reach plateau adsorption on the hydrophobic surface than does HPC/SDS. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research, to the National Science Foundation (Grant No. CTS9623849), and to the Pennsylvania Infrastructure Technology Alliance. We also thank Jeff Pyun of the Carnegie Mellon Department of Chemistry for assistance with the size exclusion chromatography measurements. LA026429G
