Kinetics and Mechanism of Cationic Surfactant Adsorption and

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Langmuir 1998, 14, 2333-2342

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Kinetics and Mechanism of Cationic Surfactant Adsorption and Coadsorption with Cationic Polyelectrolytes at the Silica-Water Interface Edward S. Pagac,† Dennis C. Prieve, and Robert D. Tilton* Colloids, Polymers and Surfaces Program, Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received December 1, 1997. In Final Form: March 2, 1998 We used scanning angle reflectometry to measure the adsorption isotherm, adsorption kinetics, and desorption kinetics for cetyltrimethylammonium bromide (CTAB) surfactants on negatively charged silica surfaces. The initial adsorption rate increased with increasing CTAB concentrations between approximately 0.2 × cmc and 10 × cmc, displaying a discontinuous increase at the critical micelle concentration. The initial desorption rate was a monotonically increasing function of the bulk concentration of the surfactant solution from which the adsorbed layer was formed, both above and below the cmc. Combining equilibrium and kinetic information, we conclude that the adsorption mechanism and the structure of the adsorbed layer both change abruptly at the cmc. Below the cmc, monomeric surfactants adsorb to an extent that is consistent with a defective bilayer structure. Above the cmc, micelles adsorb directly to the surface, to an extent that is consistent with a close-packed monolayer of micelles. The adsorption rate was apparently limited by slow rearrangements within the adsorbed layer. CTAB adsorption was significantly hindered by coadsorption with polylysine, in terms of both the rate and extent of adsorption. The effect of polylysine on CTAB adsorption was very sensitive to the ionic strength and the order in which the surfactant and the polyelectrolyte were exposed to the surface. Different pathways to the same final bulk solution composition produced much different adsorption results. This demonstrates that coadsorption of CTAB and polylysine is inherently a nonequilibrium process dominated by kinetic traps. Although it had an overall hindering effect, coadsorption with polylysine did not alter the basic difference in CTAB adsorption mechanisms above and below the cmc.

Introduction Many commercially important complex fluid formulations are based on mixtures of surfactants and polyelectrolytes. These occur, for example, in paints and coatings, detergents, ore flotation media, printing inks, and drilling muds. In water-borne paints, for instance, surfactants are often added to control wetting and colloid stability, while water-soluble polymers, often polyelectrolytes or at least having polyelectrolyte blocks, are added for rheological control. While each component may be added ostensibly to control a single property, interactions among the various additives lead to cross-effects on fluid properties. Adsorption is a major determinant of complex fluid behavior. The dynamic surface and interfacial tensions that govern the wetting characteristics of multicomponent complex fluids are controlled by the coadsorption kinetics of all surface-active species. Also, the surface forces that influence the rheology and stability of particulate complex fluids are also strongly governed by coadsorption kinetics. When polymers or surfactants are added to stabilize a dispersion, anything that affects adsorption kinetics and/ or the composition of the adsorbed layer will affect colloidal stability. There has been a significant effort to understand surfactant adsorption and polyelectrolyte adsorption from single-component solutions.1-7 By comparison, research * Corresponding author. E-mail: [email protected]. † Current address: Coatings and Resins Research and Development, PPG Industries, Inc., Allison Park, PA 15101. (1) Parker, J. L.; Yaminsky, V.; Claesson, P. M. J. Phys. Chem. 1993, 97, 7706. (2) Somasundaran, P.; Lee, L. T. Sep. Sci. Technol. 1981, 16, 1475.

on adsorption from mixtures of surfactants and polyelectrolytes is at a fairly early stage.8-13 Because of the great practical importance of complex fluid mixtures, it is critical to understand the basic differences between adsorption from mixtures and from single-component solutions, and the degree to which lessons learned from single-component studies can be translated into practical application of mixtures. Some surfactants and polymers form complexes in solution, something that is certain to alter adsorption mechanisms. Other combinations of surfactant and polymer may be mutually repellent, but these molecules could be forced into close proximity at an interface if both are surface active. Here we compare the coadsorption behavior of a mutually repellent surfactant/polyelectrolyte combination to the adsorption of either species from a single-component solution. Previously,13 we observed that the cationic surfactant cetyltrimethylammonium bromide (CTAB) and the cat(3) Sober, D. L.; Walz, J. Y. Langmuir 1995, 11, 2352. (4) Marra, J.; Hair, M. L. J. Colloid Interface Sci. 1989, 128, 511. (5) Klein, J.; Luckham, P. F. Colloids Surf. 1984, 10, 65. (6) Dahlgren, M. A. G.; Waltermo, A° .; Blomberg, E.; Claesson, P. M.; Sjo¨stro¨m, L.; Torbjo¨rn, A° .; Jo¨nsson, B. J. Phys. Chem. 1993, 97, 11769. (7) Afshar-Rad, T.; Bailey, A. I.; Luckham, P. F.; MacNaughton, W.; Chapman, D. Colloids Surf. 1988, 31, 125. (8) Somasundaran, P.; Cleverdon, J. Colloids Surf. 1985, 13, 73. (9) Shubin, V. Langmuir 1994, 10, 1093. (10) Tilton, R. D.; Blomberg, E.; Claesson, P. M. Langmuir 1993, 9, 2102. (11) Argillier, J.-F.; Ramachandran, R.; Harris, W. C.; Tirrell, M. J. Colloid Interface Sci. 1991, 146, 242. (12) Neivandt, D. J.; Gee, M. L.; Tripp, C. P.; Hair, M. L. Langmuir 1997, 13, 2519. (13) Furst, E. M.; Pagac, E. S.; Tilton, R. D. Ind. Eng. Chem. Res. 1996, 35, 1566.

S0743-7463(97)01308-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/08/1998

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ionic polyelectrolyte polylysine coadsorbed from moderately high ionic strength (10 mM) solutions to form mixed layers on negatively charged silica surfaces. The layers contained CTAB and polylysine in amounts similar to those measured for each species adsorbing from its own single-component solution; that is, the total adsorption was approximately additive and neither component had a significant effect on the extent of the other’s adsorption. The major difference was that coadsorption kinetics were far slower than adsorption from either single-component solution. In this study, we further examine CTAB adsorption and its coadsorption with polylysine in aqueous solutions. The surface is negatively charged silica. We will pay particular attention to the key mechanistic roles of ionic strength and electrostatic interactions, as well as differences in the adsorption mechanisms above and below the critical micelle concentration (cmc). Much of the mechanistic insight will come from complementing equilibrium isotherms with adsorption and desorption kinetic measurements. We conduct all kinetic measurements with the optical reflectometry technique. Since the literature offers very few studies of surfactant adsorption or desorption kinetics on solid surfaces, we first discuss in detail CTAB adsorption from single-component solutions. This should provide a good basis for comparison to CTAB coadsorption with polylysine. For the rest of this paper, we discuss polylysine adsorption, coadsorption, and finally order-of-addition effects. The latter demonstrate that CTAB coadsorption with polylysine is essentially a nonequilibrium process dominated by kinetic traps.

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adsorption. When the interface is illuminated by a parallel (p)-polarized laser beam at an angle of incidence close to the Brewster angle

θB ) tan-1(nt/ni)

(1)

this change in the interfacial refractive index profile produces a significant change in the reflectivity of the interface. In eq 1 nt and ni are the refractive indices of the materials containing the transmitted and incident beams, respectively. Reflectivity data are expressed as the intensity reflection coefficient

Rp(θ) ≡ Ip(θ)/I0p

(2)

where Ip and I0p are the intensities of the reflected and incident p-polarized beams, respectively. By measuring the reflectivity profile Rp(θ) over a range of angles around θB, one may determine the optical properties of the layer, and from these one may calculate the surface excess concentration of the adsorbing species. To accomplish this, we regress measured reflectivity profiles according to a homogeneous adsorbed layer model. For this model, the reflectivity is determined by the optical average thickness (d) and refractive index (n) of the adsorbed layer. For the regression, we use the Abele`s matrix method23 to generate theoretical reflectivity profiles for any combination of d and n. Taken individually, the regressed n and d values are only meaningful within the confining assumption of a uniform layer. Furthermore, there is often significant experimental scatter in n and d values, but the surface concentration calculated from them

Experimental Section Materials. We purchased crystallized CTAB (>99% purity) and poly-L-lysine hydrobromide, Mw ) 179 500 (degree of polymerization N ) 859, polydispersity index Mw/Mn ) 1.1), from Sigma. We purchased ultrahigh-purity (>99.9%) CTAB from Fluka and ACS-grade potassium bromide from Fisher. We deionized all water by reverse osmosis and further purified it with the MilliQ Plus System (Millipore). We conducted all experiments in air-equilibrated, pH 5.5-6.0 aqueous solutions. This pH is well below the pK of polylysine. We measured the refractive index increments for CTAB and polylysine using a BricePhoenix differential refractometer. We oxidized the silicon wafers (Lattice Materials Corp.) used for reflectometry at 1000 °C for 10 min in air, yielding 20-30-nm-thick oxide layers. We cleaned the oxidized wafers prior to each experiment following the procedure described previously.13 This treatment produces negatively charged surfaces that are completely wetted by water. Adsorption Measurements via Reflectometry. The instrumentation and experimental protocols for scanning angle reflectometry are described in detail in ref 13. We adsorbed surfactants and/or polyelectrolytes to oxidized silicon wafers from aqueous solutions undergoing fully developed laminar flow in a rectangular slit. The wall shear rate was 1.5 s-1, and the temperature was 25 °C in all experiments. Scanning angle reflectometry13-22 is based on the change in the interfacial refractive index profile caused by (14) Schaff, P.; De´jardin, P.; Schmitt, A. Langmuir 1987, 3, 1131. (15) Dijt, J. C.; Cohen Stuart, M. A.; Hofman, J. E.; Fleer, G. J. Colloids Surf. 1990, 51, 141. (16) Leermakers, F. A. M.; Gast, A. P. Macromolecules 1991, 24, 718. (17) Charron, J. R.; Tilton, R. D. J. Phys. Chem. 1996, 100, 3179.

Γ)

d(n - n0) dn/dC

(3)

is both more precise and independent of the model used to describe the adsorbed layer.14,24 Errors in n and d are mutually compensating. In eq 3, n0 is the refractive index of the bulk solution, and dn/dC is the refractive index increment of the adsorbing species. To interpret the reflectivity after adsorption, we employ a two-layer interface model (bulk silicon-oxide layeradsorbed layer-bulk solution). To determine n and d for the adsorbed layer, the thickness of the oxide layer (dox) must be known. To measure dox, we record Rp(θ) prior to adsorption and regress that data according to a single homogeneous layer optical model for the oxide layer using nox ) 1.46, the refractive index of amorphous silica. The refractive index of bulk silicon is nSi ) 3.882 + 0.019i at the helium-neon laser wavelength of 632.8 nm.25 See Figure 1 for sample reflectivity profiles and regressions. As a check on the instrument alignment, we measure dox both in air and under water and require agreement between the two before proceeding with any experiment. (18) Pagac, E. S.; Prieve, D. C.; Solomentsev, Y.; Tilton, R. D. Langmuir 1997, 13, 2993. (19) Mann, E. K.; Heinrich, L.; Voegel, J. C.; Schaaf, P. J. Chem. Phys. 1996, 105, 6082. (20) Mandenius, C. F.; Mosbach, K.; Welin, S.; Lundstro¨m, I. Anal. Biochem. 1986, 157, 283. (21) Tilton, R. D. In Polymer-Colloid Interactions: Techniques and Applications; Dubin, P., Farinato, R. Eds.; Wiley: New York, in press. (22) Charron, J. R.; Tilton, R. D. Langmuir 1997, 13, 5524. (23) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, The Netherlands, 1977. (24) de Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759. (25) Palik, E. D., Ed. Handbook of Optical Constants of Solids; Academic Press: London, 1985.

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Figure 1. p-polarized reflectivity profiles before (b) and after (O) CTAB adsorption from 0.50 mM (0.55 × cmc) aqueous solution to the silica layer atop a silicon wafer. Curves are theoretical regressions with dox ) 29.1 nm for both curves. After adsorption, n ) 1.38 and d ) 8.95 nm, corresponding to ΓCTAB ) 2.8 mg/m2.

Figure 2. Square root of the Brewster angle reflectivity for p-polarized light scaling approximately linearly with surface concentration. Circles and squares refer to adsorbed layers formed at constant thickness and constant index, respectively.

In the case of multicomponent adsorption, each component i contributes to the optical properties of the layer according to13

d(n - n0) )

dn

∑i Γi dC

(4)

i

so a mixed layer of CTAB and polylysine is described by

d(n - n0) ) ΓCTAB

dn dn + ΓPL dCCTAB dCPL

(5)

The refractive index increments of CTAB and polylysine are nearly identical: dn/dCCTAB ) 0.15 cm3/g and dn/dCPL ) 0.16 cm3/g. Thus, when studying mixtures, we simply use the CTAB refractive index increment and relate n and d to a total surface concentration Γ as in eq 3, without distinguishing between CTAB and polylysine. In other words, we report Γ ) Γ CTAB + Γ PL for mixtures. Reflectometry does not provide molecular specificity, so accurate reporting of the total surface concentrations for these mixtures is only possible because of the similarity of the refractive index increments. We measure adsorption kinetics by monitoring Rp at the Brewster angle as a function of time.13,14,21 To a very good approximation, the difference Rp(θB,t)1/2 - Rp(θB,0)1/2, where Rp(θB,0) refers to the Brewster angle reflectivity of the oxide layer prior to adsorption, is proportional to the instantaneous surface concentration. To determine the proportionality constant, we perform a full scanning angle measurement after the adsorption plateau has been attained and relate the measured surface concentration to Rp(θB,plateau)1/2 - Rp(θB,0)1/2. This constant is then used to convert all measurements of Rp(θB,t)1/2 to Γ(t). The same proportionality constant also can be generated theoretically from the known oxide layer thickness as suggested by Figure 2. It is important to note that the linear approximation relating Rp(θB,t)1/2 - Rp(θB,0)1/2 to Γ(t) may deviate unacceptably for high surface concentrations and/or thick oxide layers. It is apparent in Figure 2 that deviation

Figure 3. Isotherms for CTAB adsorption (b) and CTAB coadsorption with 200 ppm polylysine with no added electrolyte (O). For coadsorption Γ ) Γ CTAB + Γ PL. The dashed line marks the cmc.

from linearity is not significant for the surface concentrations encountered in this study (approximately 0-3 mg/ m2) when dox ) 20 nm. Results and Discussion CTAB Single-Component Adsorption Isotherm. Figure 1 presents typical reflectivity profiles recorded before and after adsorption from a 0.50 mM solution of CTAB (0.55 × cmc) in water with no added electrolyte. The cmc of CTAB in pure water is 0.90 mM.13 The CTAB adsorption isotherm is plotted in Figure 3. CTAB adsorbs to a maximum surface concentration of 2.8-3.0 mg/m2 at and somewhat below the cmc. This corresponds to an occupied area of 22 Å2/CTAB, as would be expected for close packing of alkyl chains in a defective bilayer structure.26,27 (26) Garoff, S. Thin Solid Films 1987, 152, 49.

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At concentrations well above the cmc, the CTAB adsorption isotherm decreases to a plateau of 1.5-1.6 mg/ m2. We also found such an adsorption maximum at the cmc for CTAB in 10 mM KBr solutions.13 In that case the average surface concentration was approximately 2.6 mg/ m2 at the cmc (0.20 mM) and approximately 2.0 mg/m2 at higher bulk concentrations. The amplitude of the adsorption maximum is thus considerably smaller in the presence of added electrolyte. In the next few paragraphs we offer and evaluate several possible explanations for the CTAB adsorption maximum. Adsorption maxima have in the past been attributed to the presence of trace surface-active impurities in the surfactant sample. These impurities would adsorb below the cmc but would be solubilized in micelles above the cmc (just as surface tension minima are explained). A similar argument can be offered in terms of a polydisperse surfactant. In some cases reported in the literature, additional surfactant purification was found to decrease the amplitude of the adsorption maximum but could not completely eliminate it.28 We note that none of our CTAB samples displayed a surface tension minimum. More importantly, neither repeated CTAB purification by two different procedures (ether-induced precipitation from solution in methanol or cold crystallization from aqueous solution) nor use of an ultrahigh-purity CTAB sample had any detectable effect on the adsorption isotherm. If the CTAB samples were not the source of impurities, another candidate could be the water or some soluble residue on the walls of the flow system. To check this, we flowed water over a bare oxide surface in the reflectometer. There was no change in reflectivity during 24 h of continuous flow. So, if there were indeed some trace surface-active impurity in the water, it was distinguished by only being able to adsorb cooperatively with CTAB, without being able to adsorb by itself. One of the arguments in favor of the trace impurity hypothesis is that the adsorption maximum has been seen on macroscopically flat surfaces but is rarely seen in solution depletion measurements. The explanation is that adsorption under conditions of extremely large surface area/volume ratios in solution depletion experiments causes all the impurities to be depleted from solution. When the impurities are averaged over a tremendous surface area, their effect on the adsorption isotherm is unnoticeable. Consider an alternative explanation for the different behaviors observed for high surface-to-volume experiments versus low surface-to-volume experiments. The measurements of adsorption maxima on macroscopically flat surfaces have been made by optical methods. Whereas solution depletion methods typically assay the residual surfactant ion concentration in the supernatant, reflectometry and ellipsometry directly report the total adsorbed mass per unit area, manifested as the interfacial optical impedance. This total surface concentration includes both surfactants and any bound counterions. Perhaps the degree of counterion binding in the adsorbed layer is greater below the cmc than above it. Hypothetically, if the same number of surfactant ions were adsorbed above the cmc as below but for some reason fewer counterions were bound above the cmc, the optically measured surface concentration would be less above the cmc. If in the execution of a solution depletion experiment one used an assay that detected only surfactant ions or was only weakly (27) Rennie, A. L.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Langmuir 1990, 6, 1031. (28) Arnebrant, T.; Ba¨ckstro¨m, K.; Jo¨nsson, B.; Nylander, T. J. Colloid Interface Sci. 1989, 128, 303.

Pagac et al.

sensitive to counterions, this effect would go unnoticed. For CTAB, the ratio of molecular weights of CTA+ to CTAB is 0.78. Thus, the maximum counterion effect that could result (for a fixed amount of adsorbed CTA+) would be to increase the apparent surface mass concentration by 28% for complete counterion binding as compared to no counterion binding. While significant, this counterion binding effect alone cannot fully explain the adsorption maximum measured for pure water. Note that the refractive index increment we use in this study is based on the mass concentration of CTAB, not CTA+. While on the subject of the technique-specific aspects of optical adsorption measurements, we note that unaccounted optical anisotropy could produce artifacts in the interpretation of reflectivity data. By measuring the polarization rotation upon reflection, we ruled this out, as there was no significant optical anisotropy either above or below the cmc. For ionic surfactants, the equilibrium concentration of monomeric surfactant ions passes through a maximum at the cmc.29-32 This has also been offered as an explanation for ionic surfactant adsorption maxima.33 This decrease in monomeric surfactant ion activity is a consequence of incomplete counterion binding to micelles and would be most severe in the absence of added electrolyte. Accordingly, the CTAB adsorption maximum was more pronounced for measurements conducted with pure water than for experiments conducted in the presence of 10 mM KBr. This explanation is more consistent with our findings than is the trace impurity hypothesis. Nevertheless, the experimentally measured decrease in monomeric CTA+ activity above its cmc is fairly gradual,29 not nearly as sharp as our adsorption isotherm would suggest. This leads us to seek still another explanation. Consider the possibility that micelles adsorb above the cmc at the expense of monomeric surfactants. How might this affect the packing in the adsorbed layer and thereby affect the extent of adsorption? CTAB micelles have aggregation numbers of approximately 80; approximately 70% of the micellized surfactants carry bound counterions; and the radius of a CTAB micelle is 2.9 nm.34 Thus the average molecular weight of a CTAB micelle is 2.72 × 104 g/mol of micelles. The 1.5 mg/m2 surface excess concentration we measured above the cmc would correspond to 83% surface area coverage by adsorbed micelles. So, assuming adsorbed CTAB micelles resemble those in solution, the surface excess concentration measured above the cmc is consistent with a nearly close-packed monolayer of micelles. Typically silica surfaces have approximately 1 charge/20 nm2, although this will depend on surface preparation procedures. Thus, our data above the cmc suggest that 1 micelle adsorbs for every 1.6 charged sites on the surface. This simple calculation is consistent with the recent atomic force microscopy images showing adsorbed spherical aggregates on silica surfaces that were exposed to micellar C14TAB solutions.35 (Spherical aggregates were observed on oppositely charged surfaces, but closely spaced hemicylindrical aggregates of ionic (29) Lindman, B.; Puyal, M. C.; Kamenka, N.; Rymde´n, R.; Stilbs, P. J. Phys. Chem. 1984, 88, 5048. (30) Kale, K. M.; Cussler, E. L.; Evans, E. F. J. Phys. Chem. 1980, 84, 593. (31) Cutler, S. G.; Meares, P.; Hall, D. G. J. Chem. Soc., Faraday Trans. 1 1978, 74, 1758. (32) Gunnarsson, G.; Jo¨nsson, B.; Wennerstro¨m, H. J. Phys. Chem. 1980, 84, 3114. (33) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain; VCH Publishers: New York, 1994. (34) Dorshow, R.; Briggs, J.; Bunton, C. A.; Nicoli, D. F. J. Phys. Chem. 1982, 86, 2388 (and references therein). (35) Manne, S.; Gaub, H. E. Science 1995, 270, 1480.

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surfactants on hydrophobic surfaces have also been imaged.35,36 Thus, there is precedent for the packing of ionic surfactant aggregates in close proximity on surfaces.) If our adsorption isotherms and published AFM images indicate the presence of adsorbed CTAB micelles above the cmc, a question remains as to how they get on the surface. Do micelles adsorb directly from solution (i.e., as pre-assembled aggregates) or do monomers adsorb and then self-assemble on the surface? The kinetic data described below suggest that the adsorption mechanism changes from monomeric surfactant adsorption below the cmc to direct micelle adsorption above the cmc. CTAB Single-Component Adsorption Kinetics. There are very few studies of surfactant adsorption kinetics on solid surfaces.37 A particularly noteworthy feature of the CTAB adsorption kinetics presented in Figure 4 is the extremely long time required for nonmicellar solutions to reach their adsorption plateaus. In the absence of added electrolyte, CTAB required 11 h to attain 90% of its adsorption plateau from a 0.50 mM solution (0.55 × cmc), 25 min for 0.8 mM (0.89 × cmc), and just seconds for 10 mM (11 × cmc) solutions; that is, equilibration times varied by 4 orders of magnitude for different surfactant concentrations that spanned just over 1 order of magnitude. If the kinetics of adsorption are interpreted using a model consisting of convective-diffusion from the bulk to the surface, in series with a surface “reaction”, then these long equilibration times strongly suggest that the process is “reaction-limited” over virtually the entire period of observationseven the first few minutes when the rate is highest. At low surface coverages (early times), the Le´veˆque equation

dΓ γ 1/3 2/3 |0 ) 0.538 D C dt x

()

(6)

is often used to describe the initial steady-state rate of transport-limited adsorption for convective-diffusion in parabolic slit flow.38 Here γ is the wall shear rate (1.5 s-1), x is the distance from the flow cell inlet to the observation point (1.23 cm), D is the bulk diffusion coefficient, and C is the bulk concentration. If we use this equation to calculate the apparent diffusion coefficient from the initial CTAB adsorption rate for 0.5 mM CTAB, we obtain D ) 3.7 × 10-9 cm2/s. This is 3 orders of magnitude smaller than the published diffusion coefficient of the CTA+ ion,29 DCTA ) 5 × 10-6 cm2/s. Similarly small apparent diffusion coefficients were attained at all concentrations examined. While at face value this is consistent with a reactionlimited adsorption mechanism, it must be noted that the Le´veˆque analysis does not account for electrostatic effects when an ionic surfactant adsorbs to a charged surface. Since CTAB adsorption causes a net surface charge reversal at very low coverage,1 virtually all of the adsorbing CTA+ ions must diffuse against an electrostatic repulsion from the positively charged adsorbed layer. By simultaneously solving the Nernst-Planck and Poisson equations, MacLeod and Radke39 showed that electrostatic repulsion from the interface can retard the rate of diffusional (nonconvective) adsorption by more than an order of magnitude for ionic surfactants (compared to (36) Wanless, E. J.; Davey, T. W.; Ducker, W. A. Langmuir 1997, 13, 4223. (37) Tiberg, F.; Jo¨nsson, B.; Lindman, B. Langmuir 1994, 10, 3714. (38) Lok, B. K.; Cheng, Y. L.; Robertson, C. R. J. Colloid Interface Sci. 1983, 91, 104. (39) MacLeod, C. A.; Radke, C. J. Langmuir 1994, 10, 3555.

Figure 4. Adsorption kinetics for CTAB in water with no added electrolyte at the following bulk surfactant concentrations: (a) 0.50 mM (0.55 × cmc); (b) 0.80 mM (0.89 × cmc); (c) 10 mM (11 × cmc).

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Table 1. Initial Rates of CTAB Adsorption from Dilute Single-Component Solutions [CTAB] (mM)

[KBr] (mM)

[CTAB]/cmc

(dΓ/dt)a0 (mg/m2‚min)

0.10 0.20 0.20 0.45 0.90

10 10 0 0 0

0.50 1.0 0.22 0.50 1.0

0.18 ( 0.002 1.12 ( 0.03 (8.5 ( 0.1) × 10-3 0.12 (est.) 1.2 ( 0.03

adsorption rates for nonionic surfactants or for ionic surfactants in the presence of swamping background electrolyte). The impact of this electrostatic hindrance for cases of convective-diffusion has yet to be quantitatively analyzed, although one would expect comparable trends for our slit-flow situation. Ionic strength has a large effect on CTAB adsorption rates, as can be seen in the partial listing of initial adsorption rates in Table 1. In the absence of added electrolyte, the bulk CTAB concentration determines the total ionic strength, so increasing the CTAB concentration has both mass action and electrostatic screening effects. We found that adding 10 mM background electrolyte, while holding the surfactant concentration constant, dramatically increased the adsorption rate. For example, for 0.2 mM CTAB in the presence of 10 mM KBr (where cmc ) 0.2 mM), the initial adsorption rate was 1.12 ( 0.03 mg/ m2‚min, a factor of 130 times larger than the (8.5 ( 0.1) × 10-3 mg/m2‚min rate measured for 0.2 mM CTAB in pure water (0.22 × cmc). Of course, this difference may be exaggerated by the presence of a small number of micelles in the 10 mM KBr solution, where the CTAB concentration was 1 × cmc. For nonmicellar solutions containing KBr, a CTAB concentration of only 0.10 mM (0.5 × cmc in 10 mM KBr) gave an initial adsorption rate of 0.18 ( 0.002 mg/m2‚min in 10 mM KBr. This was still 21 times larger than the initial adsorption rate for 0.20 mM CTAB in pure water. On the other hand, if we compare adsorption rates for solutions at the respective cmc for either pure water or 10 mM KBr solutions, we find quite similar rates. At 0.90 mM CTAB in pure water (1 × cmc), the initial adsorption rate was 1.2 mg/m2‚min, nearly matching the rate for 0.20 mM CTAB (1 × cmc) in 10 mM KBr. Likewise, at 0.5 × cmc in pure water, the rate was approximately 0.12 mg/ m2‚min (estimated by linear interpolation of our sub-cmc data for pure water). This is within 33% of the rate mentioned above for 0.5 × cmc in 10 mM KBr. Thus, the surfactant activity expressed on a scale relative to the cmc is appropriate for interpreting adsorption kinetics. (Note that the Debye-Hu¨ckel activity coefficients, based on simple electrolytes, are nearly unity for all these solutions.) The initial adsorption rate is plotted in Figure 5 for a broad range of CTAB concentrations with no added electrolyte. The rate increases abruptly at the cmc and continues to increase with the bulk concentration, indicating a distinct change in mechanism at the cmc. In a study of nonionic surfactant adsorption, Klimenko et al.40 suggested that the rate of adsorption should be correlated to the bulk monomer surfactant concentration and thus should remain relatively constant above the cmc, whereas Tiberg et al.37 observed that the adsorption rate of poly(ethylene glycol) alkyl ethers on silica increased with concentration above the cmc. This increase in adsorption rate at the cmc has been predicted for nonionic surfactants (40) Klimenko, N. A.; Permilovsaya, A. A.; Traysorukova, A. A.; Kaganovskii, A. M. Kolloid. Zh. 1975, 37, 972.

Figure 5. Initial rate of CTAB adsorption (b) and of CTAB coadsorption with 200 ppm polylysine (O) with no added electrolyte. For coadsorption Γ ) Γ CTAB + Γ PL. The dashed line marks an abrupt increase in rate at the cmc.

at the air-water interface by Kabalnov and Weers.41 In their model, micelles were nonadsorbing, but they served as sources of monomeric surfactants that would rapidly replenish the surface sublayer during the adsorption process. Although the abrupt increase we observed for the adsorption rate at the cmc does not contradict this nonadsorbing micelle model, we will present desorption data below that lead us to conclude that micelles do adsorb directly to silica surfaces. In other words, the abrupt change in mechanism at the cmc can be explained as a transition from monomer adsorption to micelle adsorption. CTAB Adsorption Mechanisms. Although the rather small initial CTAB adsorption rates we measured are consistent with the tendency of electrostatic repulsion to hinder the rate of diffusion-limited adsorption, we performed a series of concentration cycling experiments (examples plotted in Figures 6 and 7) that indicate that the adsorption rate is controlled by slow rearrangements of surfactant molecules at the interface. This is revealed by monitoring the response of an adsorbed layer to sudden changes in the bulk concentration. For the experiment presented in Figure 6, we first adsorbed CTAB from a 0.80 mM solution in water (0.89 × cmc) and allowed it to attain its plateau. We then rinsed the adsorbed layer in pure water for a short time, but before the layer could desorb completely, we reintroduced the original CTAB solution to the flow cell. CTAB then “readsorbed” to the same plateau surface concentration as the original adsorption, but at a much greater rate. Whereas 40 min was required for Γ to increase from 1.3 to 2.8 mg/m2 in the first adsorption step, only seconds were required for the “readsorption” that started from 1.3 mg/m2 after the partial desorption step. The rate-controlling properties of the adsorbed layer were much different during the original adsorption compared to what they were during the subsequent readsorption into a partially desorbed, aged layer. Evidently, a significant degree of layer organization survived the partial desorption, and readsorption could occur relatively rapidly by inserting surfactants into the template offered by a preorganized layer. Figure 7 illustrates a similar experiment, only there the bulk concentration started at 0.80 mM CTAB (0.89 × (41) Kabalnov, A.; Weers, J. Langmuir 1996, 12, 3442.

Coadsorption of CTAB and PL

Figure 6. Adsorption during a CTAB concentration cycle from 0.80 mM (0.89 × cmc) to pure water (marked “desorption”) and back to 0.80 mM (marked “readsorption”) with no added electrolyte.

Figure 7. Adsorption during a CTAB concentration cycle from 0.80 mM (0.89 × cmc), to 3.6 mM (4 × cmc) and back to 0.80 mM. Switching from 4 × cmc to 0.89×cmc initially causes a partial desorption, followed by readsorption.

cmc) for the first 140 min, increased to 4×cmc, and finally decreased back to 0.89 × cmc. When the concentration was increased to 4 × cmc, the surface concentration decreased from 3.0 mg/m2 to a new steady-state value of approximately 1.6 mg/m2, consistent with the value reported in the isotherm (Figure 3). Then, replacing the 4 × cmc solution with the 0.89 × cmc solution first caused a sharp drop in surface concentration, followed by a gradual rise to the same plateau surface concentration reached originally at 0.89 × cmc. This readsorption was somewhat faster than the original adsorption at 0.89 × cmc, but it was not nearly as rapid as the readsorption in Figure 6 for the same solution composition and nearly the same starting surface concentration. While not proving that CTAB forms a defective bilayer below the cmc and a monolayer of micelles above it, these concentration cycling experiments do indicate that the structures of CTAB layers formed above the cmc are much

Langmuir, Vol. 14, No. 9, 1998 2339

Figure 8. Initial rate of desorption for single-component CTAB layers (b) and for coadsorbed layers of CTAB and polylysine (O) with no added electrolyte. CCTAB is the CTAB concentration present during adsorption. Coadsorption experiments contained 200 ppm polylysine. Desorption takes place into pure water.

different from those formed below it, even when the instantaneous surface concentrations are equal. The brief period of desorption that preceded the readsorption step at 0.89 × cmc in Figure 7 may have resulted from a competition between micelle desorption and monomer adsorption. Note that the ability to reproducibly cycle between plateau surface concentrations in these experiments confirms that the CTAB adsorption isotherm in Figure 3 is a reversible, equilibrium isotherm. We measured desorption rates for CTAB layers formed at a variety of bulk concentrations by concluding each experiment with an extended period of rinsing in pure water. Desorption was not commenced until the CTAB surface concentration had clearly achieved its equilibrium plateau value. Furthermore, to ensure that we were quantifying the desorption behavior of truly equilibrated layers, we varied the amount of time that an adsorbed layer was allowed to age after attaining its adsorption plateau before initiating the desorption. We varied these “plateau aging times” between 2 and over 24 h, but the aging time had no effect on the desorption rates. This supports the assertion that the layers had indeed equilibrated before desorbing. The initial desorption rate, -(dΓ/dt)d0, is plotted against the bulk concentration of the CTAB solution from which the adsorbed layer was formed in Figure 8. There was no added electrolyte in either the adsorption or desorption step. We are aware of only one other study reporting continuous measurements of surfactant desorption from a solid-liquid interface. In that study, Tiberg et al.37 found that nonionic surfactants could only desorb from silica when the bulk surfactant concentration dropped below some critical desorption concentration. Furthermore, the desorption rate was independent of the bulk concentration from which the adsorption was accomplished. Here we found that CTAB desorption rates increased linearly with the bulk surfactant concentration present during the adsorption, even above the cmc. Unlike the initial adsorption rate, there was no abrupt change in the desorption rate at the cmc, despite the sharp decrease in the equilibrium surface concentration. The initial desorption rate is dictated by the difference in activity between the adsorbed species in the bulk (zero)

2340 Langmuir, Vol. 14, No. 9, 1998

Figure 9. Adsorption kinetics for 200 ppm polylysine with no added electrolyte.

and at the interface (fixed by equilibration with the bulk solution present during adsorption). Thus, the desorption data indicate that the activity of the adsorbing species continues to increase with increasing surfactant concentration above the cmc. For the nonionic surfactants mentioned above, the existence of a critical desorption concentration and the insensitivity of the desorption rate to the bulk concentration used during adsorption indicate that only monomers, whose activity is constant above the cmc, adsorb to silica. For ionic surfactants, the equilibrium monomeric surfactant ion activity decreases above the cmc, yet the desorption rate increases with increasing total surfactant concentration. Only the activity of micelles increases above the cmc (for either ionic or nonionic surfactants). The desorption rate data in Figure 8 thus argue in favor of direct CTAB micelle adsorption (and desorption) on silica. To briefly summarize CTAB adsorption: the adsorption isotherm, adsorption kinetics, and desorption kinetics reveal that the adsorption mechanism and the adsorbed layer structure both change abruptly at the cmc. Monomers adsorb below the cmc, and micelles adsorb above it. Layer rearrangements are apparently rate-determining. Through its influence on the surfactant activity, increasing ionic strength dramatically accelerates CTAB adsorption. Polylysine Adsorption. In our previous study we showed that polylysine adsorbs irreversibly but to a relatively low surface concentration (0.45 mg/m2) on silica in 10 mM KBr solutions.13 Compared to CTAB, polylysine adsorbed rapidly, reaching its full extent within 5 min of the start of adsorption from 200 ppm solutions. In this study, polylysine adsorption to silica from 200 ppm solutions with no added electrolyte was also rapid and irreversible. The plateau surface concentration, Γ ) 0.20-0.25 mg/m2, was somewhat lower than in 10 mM KBr, as expected.42 Polylysine adsorption kinetics are plotted in Figure 9. The initial adsorption rate for polylysine was approximately 0.3 mg/m2‚min at this concentration. For the sake of interpreting coadsorption results in the next section, the reader should note here that the polylysine adsorption rate is similar in magnitude (42) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993.

Pagac et al.

to the highest CTAB adsorption rates measured for the range of surfactant concentrations examined, despite polylysine’s small diffusion coefficient and low bulk concentration. We did not vary the polylysine concentration in our experiments, since polyelectrolyte adsorption to oppositely charged surfaces is typically independent of its bulk concentration.43 By monitoring the electrophoretic mobility of colloidal silica particles before and after adsorption, we observed that polylysine reversed the sign of the net interfacial charge from negative to positive. Coadsorption: Surface Concentrations. The coadsorption isotherm presented in Figure 3 clearly shows that the presence of polylysine resulted in a lower total surface concentration for all bulk CTAB concentrations in the absence of added electrolyte. The difference in total surface concentration between the CTAB-only and CTAB/ polylysine mixture was larger at lower CTAB bulk concentrations. The difference was nearly 1.0 mg/m2 at 0.20 mM CTAB, decreased to approximately 0.3 mg/m2 at the cmc, and remained fairly constant for all higher CTAB concentrations. Recall that this total surface concentration is the sum of ΓCTAB + ΓPL, so CTAB adsorption is diminished significantly by the presence of polylysine. We confirmed the presence of polylysine in the coadsorbed layer by desorption experiments, where an irreversibly adsorbed amount of approximately 0.2-0.25 mg/m2 persisted after rinsing (similar to the amount of polylysine that would adsorb from its single-component solution). This is similar to observations we reported in ref 13. As we reported previously,13 in the presence of 10 mM KBr polylysine had little or no effect on the extent of CTAB adsorption from mixtures. In those 10 mM KBr experiments, the Debye length was 3 nm, and electrostatic repulsions within the adsorbed layer were weaker than they were in the current experiments with no added electrolyte. We therefore view the negative effect of polylysine on the extent of CTAB adsorption under low ionic strength conditions as primarily an electrostatic exclusion of CTA+ from the vicinity of the adsorbed polylysine molecules. Effectively, there is less surface area available for CTAB adsorption. In the absence of background electrolyte, the surfactant concentration dictates the solution ionic strength. Enhanced screening of intralayer electrostatic repulsions above the cmc may explain why polylysine has a greater effect on CTAB adsorption below the cmc than it does above it, although it is difficult to separate the effects of altered electrostatic interactions and altered layer structure above the cmc. Coadsorption Kinetics. Adsorption kinetics for 0.5 mM CTAB (0.55 × cmc) solutions are compared to coadsorption kinetics in Figure 10a. The coadsorption kinetics were significantly slower at all times, and a lower total surface concentration was attained by the mixtures. Coadsorption with polylysine extended the time required to reach the adsorption plateau. Although CTAB adsorbed to a greater final extent from a single-component solution than it did in the presence of polylysine, CTAB by itself required “only” 11 h to reach 90% of the adsorption plateau, whereas the mixture required 13.5 h to reach 90% of its plateau. The presence of strongly adsorbed polylysine must interfere with the layer restructuring that controls CTAB adsorption kinetics. Figure 10b compares adsorption and coadsorption kinetics for concentrated 10.0 mM CTAB (11 × cmc) solutions. These were quite similar, attaining their (43) Papenhuijsen, J.; van der Schee, H. A.; Fleer, G. J. J. Colloid Interface Sci. 1985, 104, 540.

Coadsorption of CTAB and PL

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Figure 11. Order-of-addition effects with no added electrolyte: (a) 0.50 mM CTAB (0.55 × cmc) for t g 0; (b) 0.50 mM CTAB (0.55 × cmc) and 200 ppm polylysine for t g 0; (c) 0.50 mM CTAB (0.55 × cmc) for 0 e t e 5 min, followed by 0.50 mM CTAB (0.55 × cmc) and 200 ppm polylysine for t g 5 min; (d) 0.50 mM CTAB (0.55 × cmc) and 200 ppm polylysine for 0 e t e 5 min, followed by 0.50 mM CTAB (0.55 × cmc) for t g 5 min. Note that curves a and b here are the same as the curves plotted in Figure 10a.

Figure 10. Comparison of CTAB adsorption (1) and CTAB coadsorption with 200 ppm polylysine (2), with no added electrolyte, for the following bulk CTAB concentrations: (a) 0.50 mM (0.55 × cmc); (b) 10 mM (11 × cmc).

respective plateaus in a matter of seconds. The plateau surface concentrations were much more similar than in the 0.50 mM CTAB case mentioned above. At 11 × cmc, the mixture attained a slightly lower total surface concentration (1.21 mg/m2) compared to the CTAB-only case (1.53 mg/m2), reflecting the overall behavior shown in the isotherm. Initial coadsorption rate data are compared to singlecomponent CTAB adsorption data in Figure 5. While initial coadsorption rates are for the most part lower than the corresponding single-component CTAB initial rates, the differences are much less obvious than the long-term coadsorption rate depression seen in Figure 10a. Comparison of initial rates is somewhat awkward here, because the rapid initial adsorption of polylysine is what is ultimately responsible for the hindrance of CTAB adsorption later in the coadsorption process. Note that the discontinuous increase in initial adsorption rate at the cmc is preserved when CTAB coadsorbs with polylysine. Similarly, the important features of the initial desorption rate data for CTAB shown in Figure 8 are also preserved in the presence of polylysine. Thus, it

appears that, although CTAB adsorption is diminished overall by polylysine, the essential mechanistic features of CTAB adsorption above and below the cmc are not altered. Micelles still adsorb above the cmc in the presence of polylysine. Nonequilibrium Effects Dominate the Coadsorption Mechanism. Not only are the extent and kinetics of CTAB adsorption hindered by the presence of polylysine, but the order in which the components are allowed to adsorb profoundly changes the process, as shown in Figure 11. Curves a and b show adsorption from 0.50 mM CTAB solutions in water (0.55 × cmc) in the absence and presence of 200 ppm polylysine, respectively. They are intended to serve as references for comparison to sequential adsorption experiments. Curves c and d illustrate orderof-addition effects. In curve c, the first 5 min corresponds to adsorption from a flowing, single-component solution of 0.50 mM CTAB. After 5 min, we switched to a mixture of 0.5 mM CTAB and 200 ppm polylysine. The surface concentration at the time the switch occurred was