Coadsorption of Polylysine and the Cationic Surfactant

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Ind. Eng. Chem. Res. 1996, 35, 1566-1574

MATERIALS AND INTERFACES Coadsorption of Polylysine and the Cationic Surfactant Cetyltrimethylammonium Bromide on Silica Eric M. Furst,† Edward S. Pagac, and Robert D. Tilton* Department of Chemical Engineering and Colloids, Polymers and Surfaces Program, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213

Coadsorption of polymers and surfactants is a poorly understood process that occurs in a variety of complex fluid applications. In single-component solutions, the cationic polyelectrolyte polylysine and the cationic surfactant cetyltrimethylammonium bromide (CTAB) both adsorb to negatively charged silica surfaces. Here we use scanning angle reflectometry to contrast adsorption from single-component solutions with a sequential adsorption process and a coadsorption process. When adsorbed from single-component solutions, polylysine adsorbs irreversibly, whereas CTAB adsorption is reversible. In the sequential adsorption case, CTAB neither displaces nor adsorbs to preadsorbed polylysine layers. When solutions contain both CTAB and polylysine, they coadsorb to form mixed layers. Mixed layer formation is indicated by a dramatic alteration of the kinetics and reversibility of adsorption compared to either singlecomponent case. The amounts of CTAB and polylysine adsorbed in the mixed layers are both similar to the amounts adsorbed from the respective single-component solution. Introduction Ionic surfactants and polyelectrolytes adsorb spontaneously to oppositely charged surfaces. Adsorption of ionic surfactants and polyelectrolytes changes the net interfacial charge density, often leading to surface neutralization or charge reversal. Surface modification by adsorption has significant effects on colloidal forces, wetting, and other surface phenomena. Accordingly, surfactants and polyelectrolytes can be used, for example, to control particle flocculation or fluid penetration and particle deposition in porous media. Many commercially important complex fluids contain both surfactants and surface active polyelectrolytes, including paints, printing inks, foods, laundry detergents, shampoos, cosmetics, and other personal hygiene products. In the environment, the interfacial behavior of surfactants and naturally occurring polyelectrolytes is critical in surfactant-assisted remediation of contaminated soils. Since the behavior of these complex systems depends in large part on the structure and electrostatic characteristics of the adsorbed layers, it is important to understand not only the basic adsorption mechanism for each component individually but also the consequences of polyelectrolyte-surfactant interactions. Molecules may behave quite differently when used in mixtures versus single-component solutions. For example, Somasundaran and Cleverdon (1985) observed that cationic surfactants were less effective in mineral flotation when used in the presence of cationic polyelectrolytes. To explain this, they postulated the formation of a mixed adsorbed layer where the presence of the polyelectrolyte interfered with the tendency of the * Author to whom correspondence should be addressed. E-mail: [email protected]. Fax: (412) 268-7139. † Current address: Department of Chemical Engineering, Stanford University, Stanford, CA 94305.

surfactant to render the mineral surfaces hydrophobic. Ordinarily in such a system where Coulombic repulsion makes the direct binding of surfactants to polyelectrolytes unfavorable, one might expect the surfactant to simply displace the polyelectrolyte from the solid-liquid interface. Here we compare adsorption from single-component solutions of either a cationic surfactant or a cationic polyelectrolyte with sequential adsorption and with coadsorption from binary mixtures. The cationic surfactant is cetyltrimethylammonium bromide (CTAB), and the polyelectrolyte is polylysine. CTAB and polylysine were adsorbed to the negatively charged silicon oxide layer present on thermally oxidized silicon wafers. Experimental Section Materials. Optical grade silicon wafers were obtained from Lattice Materials Corporation. We oxidized them at 1000 °C for 10 min in air to produce ∼20 nm thick oxide layers. Prior to each experiment, we cleaned the oxidized wafers by soaking them in a saturated solution of potassium dichromate in 36 N sulfuric acid for 20 min at room temperature, then thoroughly rinsing with water, soaking for 20 min in 6 M HCl solution, rinsing again with water, soaking in 10 mM NaOH for 10 min, rinsing with water, and finally drying in a nitrogen jet. The base treatment increases the negative surface charge density of the oxide layer and leaves the surface completely wettable by water. We conducted some experiments with silicon wafers rendered moderately hydrophobic by vapor phase silanization with trimethylchlorosilane. Water drops on these silanized surfaces have an advancing contact angle of 54°. All experiments were conducted in 0.01 M KBr solutions, pH 5.5-6, at 25 °C. This pH is well below the pK of polylysine. A.C.S. grade potassium bromide was obtained from Fisher. Crystallized CTAB (>99% purity) and poly-L-lysine hydrobromide, Mw ) 179 500

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(degree of polymerization n ) 859, polydispersity index Mw/Mn ) 1.1), were obtained from Sigma. All water was purified by the MilliQ Plus system (Millipore). We measured surface tensions of CTAB in water and in 0.01 M KBr by the DuNou¨y ring method. We measured the refractive index of CTAB and polylysine solutions using a Brice-Phoenix differential refractometer with a 633 nm laser-line filter in the light path. Reflectometry. We measured adsorption via in situ scanning angle reflectometry (Schaaf et al., 1987; Dijt et al., 1990; Leermakers and Gast, 1991; Charron and Tilton, 1996). This technique enables us to deduce the surface excess concentration from changes in the reflectivity of an interface under laser illumination. Scanning angle reflectometry is based on the sensitivity of the reflectivity of parallel (p) polarized light to small changes in the refractive index profile caused by adsorption at the interface between two optically dissimilar materials. The reflectivity is most sensitive to adsorption when the angle of laser beam incidence (relative to the surface normal) is near the Brewster angle,

θB ) tan-1

() nt ni

(1)

where nt and ni are the refractive indexes of the semiinfinite media containing the transmitted and incident beams, respectively. In the current study, these are silicon and water. Thus, we measure the reflectivity of a p-polarized laser beam,

Rp(θ) ≡

Ip(θ) I0p

(2)

over a small range (4.2°) of incident angles centered on θB ) 71°. Ip and I0p are the intensities of the reflected and incident beams, respectively. The measured reflectivity profiles Rp(θ) are analyzed to determine the optical average thickness and refractive index of the adsorbed layers. A schematic diagram of the reflectometer is presented in Figure 1. The adsorption chamber sits at the center of rotation of a two-armed reflectometer. The incidence arm supports a 10 mW helium-neon laser and a GlanThompson polarizer, and the reflection arm supports a silicon detector. To measure Ip(θ) over a range of incident angles, the arms are rotated simultaneously by equal but opposite angles, with a resolution of 0.1°. The adsorption chamber consists of a rectangular slit flow cell (2.47 cm × 1.27 cm × 0.145 cm), with a fused silica prism and an oxidized silicon wafer providing the upper and lower walls. Molecules adsorb to the silicon oxide surface from solutions in parabolic laminar flow at a wall shear rate of 2.6 s-1 driven by a peristaltic pump. The adsorption chamber is maintained at 25 °C in all experiments by circulating temperature-controlled water through the flow cell housing. The prism is cut so that when the laser beam is perpendicular to the prism face it propagates through the prism and the solution to strike the wafer at the Brewster angle for water/silicon. When changing the angle of incidence, beam refraction at the prism-air and prism-solution interfaces is taken into account by Snell’s Law. Refraction also causes the point of reflection to move slightly on the oxide surface when the incident angle is changed. This position changes by approximately 10% of the beam diameter over the entire range of incident angles

Figure 1. A parallel-polarized beam from the helium-neon laser transmits through the prism and solution to reflect off the oxidized silicon wafer. The reflected beam transmits through the solution and prism to strike a silicon detector. The flow cell depth is 1.45 mm. The direction of the polarization vector is denoted by the arrow superimposed on the laser beam. The incident and reflected light sides of the optical train are supported by reinforced optical rails. The angle of incidence is controlled by simultaneously rotating the two optical rails by equal and opposite angles, using a simple gear mechanism. The flow cell is a rectangular slit sealed by fluoropolymer O-rings. The flow direction is parallel to the z-axis. The prism/flow cell assembly is mounted on an x-y-z translation stage for alignment and for scanning the surface to measure the oxide layer uniformity.

sampled. Since this small beam displacement has no effect on the light detection, and the oxide layer thickness is uniform over several millimeters, the beam displacement has no impact on our results. Optical losses in the laser beam due to reflections at the two prism-air interfaces and the twice-traversed prism-solution interface must be taken into account prior to data analysis. We correct for these optical losses by calculating the Fresnel reflection and transmission coefficients for each instance where the beam traverses an interface. The net effect of the various reflections, other than the reflection of interest at the silicon oxide-water interface, is to decrease the measured intensity Ip(θ) of the reflected beam by 3%. This loss factor is constant to within experimental resolution over the entire range of incident angles sampled in an experiment. In order to calculate Rp(θ) from Ip(θ) using eq 2, we must measure I0p. This is done prior to installing the flow cell by aiming the laser beam directly onto the detector. Because of the small attenuation of the beam due to the various reflections from prism surfaces noted above, a +3% correction is made to Ip(θ) before calculating Rp(θ). One concern in reflectometry is detection of multiple reflections, i.e., detection of the primary reflected beam and the superfluous beam reflected from the prismsolution interface. Because of the fairly large flow cell depth, when the incident beam strikes the silicon oxidesolution interface at 71°, the superfluous beam is displaced by approximately 8 mm from the primary reflected beam. Thus, it is easily prevented from striking the detector. Since the prism is in contact with the flowing polyelectrolyte and/or surfactant solution, the optical consequences of adsorption to the prism-solution interface in principle could introduce some error by altering the uniform 3% optical loss correction applied to Ip(θ). In practice this is not a problem, because the laser beam

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strikes the prism-solution interface at an angle well removed from its Brewster angle. The reflectivity of that interface is therefore insensitive to polyelectrolyte and/or surfactant adsorption. All measured changes in the reflectivity profile during an experiment are dictated solely by adsorption to the silicon oxide-solution interface, and no corrections are needed to account for adsorption to the prism itself. When molecules adsorb to the silicon oxide layer, the reflectivity is described by a two-layer striated interface model. In order to analyze reflectivity profiles obtained after adsorption, we must first determine the thickness of the oxide layer. We do this by measuring the reflectivity profile prior to the start of adsorption. We determine the oxide layer thickness by nonlinear least squares regression of Rp(θ) according to the theoretical reflectivity profile for a single homogeneous layer, calculated by the Abele`s matrix method (Azzam and Bashara, 1977). This is a standard method for calculating the reflectivity of a composite interface. In analyzing the reflectivity profile, we use nox ) 1.46 for the refractive index of the oxide layer and nSi ) 3.882 + 0.019i for the complex refractive index of silicon at the 632.8 nm HeNe laser wavelength. We measure the oxide layer thickness both with the wafer exposed to air and with it fully bathed by 0.01 M KBr solution in the flow cell, and consistency between the two measurements is required before proceeding. The primary optical effect of a thickened oxide layer on the silicon wafer is to increase the overall reflection coefficient and thereby improve the signal-to-noise ratio. After adsorption has reached a plateau, we determine the effective thickness, deff, and refractive index, neff, of the adsorbed layer by nonlinear least squares regression of Rp(θ), using the Abele`s method and the two-layer optical model (bulk silicon + oxide layer of known thickness + homogeneous adsorbed layer of unknown thickness and index + bulk aqueous solution). In the regression routine we generally set an upper limit of 1.5 on neff, but scatter in the data sometimes leads to refractive index estimates that exceed this limit. In such cases we fix neff (at a value of 1.48) and regress the data with deff as the only variable, as is common practice in ellipsometry studies. This provides a physically more realistic estimate of the adsorbed layer thickness. The thickness and refractive index determined in this way are individually subject to considerable experimental error and obviously depend on the particular optical model chosen for the interfacial refractive index profile, i.e., the conformation of the adsorbed molecules. Nevertheless, the surface concentration calculated from these values is model-independent and far more precise (de Feijter et al., 1978; Schaaf et al., 1987).

Γ)

(neff - n0)deff dn/dC

(3)

In eq 3, n0 is the refractive index of the bulk solution, and dn/dC is the refractive index increment of the adsorbed material. As expected, we find the calculated value of Γ to be insensitive to the value of neff, whether that parameter is regressed or fixed at different values. In other words, the value of Γ determined by reflectometry is insensitive to the conformation of the adsorbed layer. This also may be demonstrated quite easily by generating theoretical reflectivity profiles Rp(θ) for arbitrary interfacial

refractive index profiles, then regressing that reflectivity profile according to the single homogeneous film optical model, and finally noting the similarity of the surface concentrations calculated from the two different optical models (Charron, 1996). Following Schaaf et al. (1987), we measure adsorption kinetics by monitoring Rp at the Brewster angle as a function of time. The quantity Rp(θB,t)1/2 - Rp(θB,0)1/2, where Rp(θB,0) refers to the reflectivity of the oxide layer prior to adsorption, is proportional to the surface concentration Γ(t). We only measure the full reflectivity profile after the adsorption has reached a plateau. At that point we calculate the surface concentration as described above from the full reflectivity profile and relate it to Rp(θB,t)1/2 - Rp(θB,0)1/2 to determine the proportionality constant. We then use this proportionality constant to convert all prior measurements of Rp(θB,t)1/2 to Γ(t) in the case of single-component solution experiments. The same proportionality constant can also be predicted by the Abele`s method using the known oxide layer thickness. This serves as an experimental consistency check. The approximation of a linear relationship between Rp(θB,t)1/2 - Rp(θB,0)1/2 and Γ(t) is best at low surface concentrations. The deviation from linearity is approximately 2% at the highest surface concentrations we observed (∼3 mg/m2) with the ∼20 nm oxide layer thicknesses employed in this study. In this investigation, we first measured adsorption of CTAB and of polylysine each from single-component solutions. We then compared these results with sequential adsorption of polylysine followed by CTAB. We compared the single-component and sequential adsorption cases with coadsorption from binary mixtures of CTAB and polylysine. We were unable to measure the sequential adsorption of CTAB followed by polylysine due to the reversibility of CTAB adsorption. Instead, we measured polylysine adsorption to moderately hydrophobic silanized surfaces that in some respects mimic the effect of a preadsorbed CTAB layer. Since reflectometry lacks molecular specificity, it is not possible to directly determine the surface concentration of both CTAB and polylysine in mixed adsorbed layers. Therefore, we must make informed assumptions about the adsorbed layer in drawing conclusions regarding layer compositions in the coadsorption experiments. Results and Discussion The surface tensions of CTAB solutions in 0.01 M KBr are plotted in Figure 2. The critical micelle concentration (cmc) indicated by the break in the surface tension plot is 0.18 mM. The lack of a surface tension minimum in Figure 2 indicates that the concentration of any surface active contaminants must be very small. For CTAB in deionized water, we found a cmc of 0.87 mM (data not shown), consistent with the literature value (Mukerjee and Mysels, 1971). Likewise, there was no surface tension minimum for CTAB in deionized water. Using the differential refractometer to measure the concentration dependence of aqueous CTAB solution refractive indexes, we determined dn/dC|CTAB ) 0.15 cm3/g. We found the same value for CTAB in pure water as in 0.01 M KBr solutions. For polylysine in 0.01 M KBr, we determined dn/dC|PLL ) 0.16 cm3/g, a value comparable to refractive index increments for other polypeptides (Brandrup and Immergut, 1989). Adsorption from Single-Component CTAB Solutions. Several reflectivity profiles corresponding to

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Figure 2. Surface tension of CTAB solutions in 0.01 M KBr. The critical micelle concentration indicated by the intersection of the two lines is 0.18 mM.

Figure 3. Reflectivity profiles for CTAB adsorption on silicon oxide from single-component solutions in 0.01 M KBr. Profiles correspond to bulk CTAB concentrations of 0, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30 mM, with the uppermost profiles corresponding to the highest concentrations. Curves are the theoretical regressions.

CTAB layers adsorbed from 0.01 M KBr solutions of varying CTAB concentration are shown in Figure 3. The adsorption isotherm is presented in Figure 4. The plateau CTAB surface concentration, ΓCTAB ) 2.0 mg/ m2, corresponds to 30 Å2/CTAB molecule. This somewhat exceeds the ∼20 Å2 /CTAB that would be expected of a close-packed CTAB monolayer (Garoff, 1987). The mean molecular area per CTAB on silica is somewhat smaller than the 55 Å2/CTAB attained at saturation at the air/0.01 M KBr solution interface that we calculate from the surface tension data via the Gibbs equation. Our results are consistent with the “defective bilayer” formed with pure water that was observed by Rennie et al. (1990) using neutron reflectivity. Our 30 Å2/ CTAB mean molecular area is less than the 55 Å2/CTAB obtained by Wahlgren and Arnebrant (1991) in an ellipsometry study of CTAB adsorbing on silica from phosphate-buffered, 0.15 M NaCl solutions. Here we note that the discrepancy between our results and theirs

Figure 4. Adsorption isotherm for CTAB on silicon oxide in 0.01 M KBr.

may arise not only from the different solution compositions but also from the calculations used to relate the adsorbed layer optical properties to the surface excess concentration. Wahlgren and Arnebrant (1991) used the partial specific volume of CTAB and the molecular weight to molar refractivity ratio calculated for CTAB by Arnebrant et al. (1989) to calculate Γ via the Cuypers equations (Cuypers et al., 1983). Using the LorentzLorenz equation (Cuypers et al., 1983) we calculated the refractive index increment that corresponds to these same values for the partial specific volume and the molecular weight to molar refractivity ratio. Thus, we predicted the refractive index increment for CTAB to be 0.23 cm3/g. That is 1.5 times larger than our measured value of dn/dC|CTAB. Applying this predicted value rather than the measured refractive index increment to our reflectometry results would yield a 1.5 times lower CTAB surface concentration (larger mean molecular area). Next we turn our attention to the nature of the adsorbed CTAB layer. Although it will certainly depend on the particular silica preparation procedure, CTAB can be expected to neutralize the silica surface at approximately 2000 Å2/CTAB (Zorin et al., 1992). In addition, surface force measurements conducted between silica surfaces in CTAB solutions (Parker et al., 1993) indicate that surface neutralization and then charge reversal occur at very low bulk CTAB concentrations. We therefore expect that charge reversal has occurred at all CTAB concentrations examined in this investigation. The CTAB adsorption isotherm in Figure 4 displays a maximum at the cmc. Surfactant adsorption maxima have been observed previously, most commonly in systems having small surface area/volume ratios, as in this study. Maxima are caused by trace surface active impurities that adsorb at low surfactant concentrations but are preferentially solubilized in micelles above the cmc (Trogus et al., 1979; Arnebrant et al., 1989). Although the lack of a discernible surface tension minimum would suggest that our CTAB preparation contained no significant amount of surface active impurities, the adsorption isotherm is evidently the more sensitive indicator of trace impurities. We note that the CTAB adsorption behavior was not altered by either of two additional purification procedures, recrystallization

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Figure 5. CTAB reaches its plateau surface concentration on silica in 0.01 M KBr within approximately 10-20 min of adsorption. Desorption is rapid upon flushing the flow cell with 0.01 M KBr solution, but approximately 15% remains adsorbed. In this experiment, the bulk concentration of CTAB was 0.20 mM.

from cold aqueous solution or ether-induced precipitation from methanol. CTAB adsorption and desorption kinetics are presented in Figure 5 for the case when the CTAB concentration is near the cmc. CTAB attained its plateau within 10-20 min of the start of adsorption. Upon rinsing the flow cell with 0.01 M KBr solution, most of the CTAB desorbed, but there was a persistent fraction, on the order of 0.37 mg/m2, that remained adsorbed after rinsing. We also noted that the surface was not as wettable after CTAB adsorption and rinsing as it was prior to the experiment. This suggests that the surface charge was diminished and a significant number of nonpolar species remained on the surface. At concentrations well in excess of the cmc, CTAB adsorption reached a plateau within 2-3 min of adsorption. Data for 0.8 mM CTAB are presented in Figure 6. Upon rinsing with 0.01 M KBr, desorption was rapid and, unlike the case for CTAB solutions near the cmc, complete. This suggests that the material that remained adsorbed after rinsing in the near-cmc experiments was the same trace surface active species that produced the maximum in the adsorption isotherm. Above the cmc, this species is fully sequestered in micelles and does not adsorb. Adsorption from Single-Component Polylysine Solutions. Figure 7 shows reflectivity profiles before and after polylysine adsorption from a 200 µg/cm3 solution in 0.01 M KBr. Its surface concentration was quite small, ΓPLL ) 0.45 ( 0.1 mg/m2, so the adsorption produced a rather small shift in the reflectivity profile. Polylysine adsorption reached its full extent within approximately 5 min of the start of adsorption. The adsorption was irreversible upon rinsing with 0.01 M KBr solution for 2-3 h. The polylysine surface concentration measured here is consistent with the value of ΓPLL ) 0.5 mg/m2 reported for mica surfaces (AfsharRad et al., 1988). At this low coverage, the polylysine chains adopt a flat conformation on the negatively charged surface. We did not vary the concentration of polylysine since polyelectrolyte adsorption to oppositely charged surfaces is generally insensitive to the bulk polyelectrolyte concentration (Papenhuijsen et al., 1985).

Figure 6. At a CTAB concentration of 0.8 mM in 0.01 M KBr, well above the cmc, adsorption is rapid, and desorption upon rinsing with 0.01 M KBr is complete.

Figure 7. Polylysine adsorption in 0.01 M KBr causes a slight upward shift of the reflectivity profile. The lowest points (b) correspond to the bare surface. Data points corresponding to the polylysine adsorption, rinsing with 0.01 M KBr, and subsequent exposure to CTAB solutions of 0.05, 0.10, 0.15, 0.25, and 1.0 mM concentrations all overlap each other.

Sequential Adsorption. After adsorbing polylysine at 200 µg/cm3, we first rinsed the flow cell with a polylysine-free, 0.01 M KBr solution and subsequently rinsed it with 0.01 M KBr solutions containing CTAB in a series of increasing concentrations. At no time in the sequential adsorption process was there ever a mixture of polylysine and CTAB in solution. The sequential adsorption reflectivity profiles are also presented in Figure 7. It is evident that polylysine did not desorb when rinsed with pure 0.01 M KBr, nor was it displaced by CTAB at any concentration. No desorption was detectable with polyelectrolyte-free CTAB solutions at concentrations as large as 1 mM (5 × cmc). Note that CTAB neither decreased nor increased the reflectivity profile at any concentration. Thus CTAB not only failed to displace preadsorbed polylysine but also failed to adsorb to the preadsorbed polylysine layer, in spite of its high surface affinity (compare with Figures 3 and

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Figure 8. These reflectivity profiles correspond to coadsorption experiments with 200 µg/cm3 of polylysine plus CTAB at varying concentrations in 0.01 M KBr. The lowest data points (b) represent the bare surface. The profiles shift upward slightly for CTAB concentrations of 0.05 (9) and 0.10 mM (2). The uppermost data (1) correspond to 0.15 mM CTAB. The data in the middle of the range ([) correspond to the layer remaining adsorbed after flushing the flow cell with 0.01 M KBr following polylysine coadsorption with 0.15 mM CTAB.

4). This is perhaps due to the way in which polylysine neutralizes the silica surface. The tendency of polyelectrolytes to adsorb to the point of surface neutralization has been predicted theoretically (Papenhuijsen et al., 1985) and observed experimentally with the surface force apparatus (Afshar-Rad et al., 1988; Dahlgren and Claesson, 1993). Whereas hydrophobic alkane tails are exposed to water when CTAB neutralizes the silica surface, polylysine presents no hydrophobic residues when it adsorbs. When CTAB adsorbs from singlecomponent solutions, the hydrophobic surfactant tails drive cooperative adsorption of more surfactants, thereby causing charge reversal. In contrast, there is little driving force for surfactant adsorption after the surface has been neutralized by the irreversibly bound, preadsorbed polylysine chains in the sequential adsorption experiments. Gebhardt and Fuerstenau (1984) studied a sequential adsorption process that was effectively the opposite of ours (i.e., anionic surfactant, anionic polyelectrolyte, and a positively charged surface). Similar to the current results, they found no adsorption of sodium dodecylsulfonate to a preadsorbed poly(acrylic acid) layer on hematite. Coadsorption. While CTAB did not adsorb to a preadsorbed polylysine layer, the system behaved much differently when CTAB and polylysine were allowed to coadsorb from a binary mixture. Reflectivity profiles before and after adsorption from solutions containing polylysine at 200 µg/cm3 and CTAB at concentrations ranging between 0.05 and 0.15 mM are plotted in Figure 8. Also shown in Figure 8 is the reflectivity profile obtained after rinsing (with 0.01 M KBr solution) an adsorbed layer formed by coadsorbing polylysine and 0.15 mM CTAB. Upon first inspection, the adsorption behavior illustrated in Figure 8 appears similar to that of the single-component CTAB solutions. The reflectivity profiles shifted upward with increasing CTAB concen-

Figure 9. Unlike adsorption from either single-component solution, coadsorption kinetics are distinguished by a slow approach to adsorption saturation. The effective optical thickness is directly proportional to Rp(θB,t)1/2 - Rp(θB,0)1/2. Upon flushing the flow cell with 0.01 M KBr, approximately 35% of the material, as quantified by the effective optical thickness, remains adsorbed. In this experiment, the binary mixture contained 200 µg/cm3 of polylysine and 0.20 mM CTAB.

tration, as in Figure 3. While the reflectivity profiles for single-component CTAB solutions and the CTAB/ polylysine mixtures appeared similar, there were indeed pronounced differences in the adsorption behavior. The differences were in the degree of reversibility upon rinsing with 0.01 M KBr and in the adsorption kinetics. Since reflectometry lacks molecular specificity, one cannot directly measure the composition of a mixed adsorbed layer. In such a case the best way to represent the total surface concentration of adsorbed material is by the effective optical thickness of the layer, deff(neff n0), determined by regression of the reflectivity profiles. This is similar to the “total optical effect” of an inhomogeneous layer discussed by McCrackin and Colson (1964). After rinsing in the binary mixture experiments conducted near the CTAB cmc, the amount remaining adsorbed corresponded to an effective optical thickness of deff(neff - n0) ) 0.15 ( 0.02 nm, whereas the value at the adsorption plateau before rinsing was deff(neff - n0) ) 0.42 ( 0.06 nm (see Figure 9). More than 35% of the original effective optical thickness persisted after rinsing. Recall that in the corresponding single-component experiments polylysine adsorption was entirely irreversible, and less than 18% of the adsorbed amount, or effective optical thickness, remained after rinsing in the case of CTAB adsorption near its cmc. (We refer to this as the amount of CTAB remaining adsorbed, although this material was most likely the same trace surface active contaminant that produced the maximum in the CTAB adsorption isotherm. Whatever the nature of the remnant, optically it was the equivalent of a 0.37 mg/m2 layer of CTAB.) The effective optical thickness of the remnant layer in the binary mixture experiments is well described by assuming the amounts of polylysine and CTAB remaining adsorbed are equal to the amounts that remained adsorbed after rinsing in each singlecomponent experiment, i.e., ΓPLL ) 0.45 ( 0.1 mg/m2 and ΓCTAB ) 0.37 ( 0.005 mg/m2. This is based on calculations of the effective optical thickness of a multicomponent adsorbed layer, discussed below.

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Since the optical average refractive index and optical average thickness of an adsorbed layer are related to the refractive index profile n(z) by (McCrackin and Colson, 1964; de Feijter et al., 1978)

∫0∞n(z)[n(z) - n0] dz neff ) ∫0∞[n(z) - n0] dz

(4)

and

deff )

∫0∞[n(z) - n0] dz

(5)

neff - n0

and the Gibbs surface excess concentration of any component i in a multicomponent adsorbed layer is given by

Γi )

∫0∞∆Ci(z) dz

(6)

where ∆Ci(z) is the difference between the local concentration of component i and its concentration in the bulk solution; it is readily shown that the effective optical thickness of an N-component adsorbed layer is

Γi ∑ | dC i i)1 N

deff(neff - n0) )

dn

(7)

Equation 7 assumes ideal behavior in the mixed layer, i.e., that the refractive index increment of any component in the layer is not affected by the presence of the others, so that

∆Ci(z) ∑ | dC i i)1 N

n(z) ) n0 +

dn

(8)

Thus, the effective optical thickness of the mixed adsorbed layer of CTAB and polylysine is determined from its composition by

deff(neff - n0) ) ΓCTAB

|

|

dn dn + ΓPLL (9) dC CTAB dC PLL

Inserting the polylysine and CTAB remnant surface concentrations (ΓPLL ) 0.45 ( 0.1 mg/m2 and ΓCTAB ) 0.37 ( 0.005 mg/m2) into eq 9 yields an effective optical thickness of 0.13 ( 0.02 nm, agreeing within experimental error with the 0.15 ( 0.02 nm value determined by scanning angle reflectometry after rinsing in the binary experiments. Thus, the mixed layers most likely contained the same amount of polylysine as would adsorb from a single-component polylysine solution. To estimate the amount of CTAB adsorbed at the plateau from the effective optical thickness measured prior to rinsing, we again apply eq 9 and the assumption that the mixed adsorbed layer still contained 0.45 ( 0.1 mg polylysine/m2 at the plateau. From this we estimate that the mixed layer contained 2.2 ( 0.2 mg of CTAB/ m2 at the adsorption plateau. This is similar to the amount of CTAB adsorbed from the single-component solutions. In binary coadsorption experiments conducted well above the cmc, there was again a significant amount of material that remained adsorbed after rinsing. At the adsorption plateau, the effective optical thickness was 0.31 nm. After rinsing, this decreased to 0.083 nm.

Recall that adsorption was entirely reversible when CTAB adsorbed from single-component solutions well above the cmc. If we again assume that the irreversibly bound material in this coadsorption case is the same as what remained adsorbed in either single-component case, i.e., ΓCTAB ) 0 and ΓPLL ) 0.45 ( 0.1 mg/m2, we calculate an effective optical thickness of 0.072 ( 0.02 nm. This is again within experimental error of the measured effective optical thickness after rinsing. Applying the same analysis as above, we estimate that the mixed layer contained 1.5 mg of CTAB/m2 and 0.45 mg of polylysine/m2 at the adsorption plateau prior to rinsing. Just as was the case near the cmc, the amount of CTAB adsorbed from the binary mixture is similar to the amount adsorbed from the single-component solution. Whether the CTAB concentration is near or well above the cmc, it is evident that CTAB and polylysine coadsorb to form a mixed layer. The formation of a mixed layer is also clearly demonstrated by the difference in the adsorption kinetics for the binary solutions and the single-component solutions. Typical binary coadsorption kinetics for a CTAB concentration near the cmc are presented in Figure 9. The most important feature in the kinetic data is the slow approach to adsorption saturation. Whereas in the one-component experiments CTAB and polylysine adsorption both were complete within 5-20 min, the mixture had not quite reached a plateau even after 1 h. The mixed adsorbed layer accommodated additional molecules by undergoing slow structural changes that did not occur in either single-component layer. Coadsorption Driving Force. Although polylysine adsorption to negatively charged silica surfaces is electrostatically driven in the case of a single-component solution, we did consider the possibility that the moderate surface hydrophobization caused by adsorbing CTAB may have played a role in the polylysine coadsorption mechanism. To mimic the effect of surface hydrophobization, we attempted to adsorb polylysine to the moderately hydrophobic surface produced by vapor phase silanization of silica with trimethylchlorosilane water (θadv ) 54°). Polylysine adsorbed sparingly to this surface, only reaching ΓPLL ) 0.16 ( 0.05 mg/m2. Thus, surface hydrophobization was probably not important in the coadsorption mechanism. Since ΓCTAB and ΓPLL in the coadsorbed layers are similar to the corresponding values attained in either single-component system, the mixed adsorbed layers would contain a higher density of ionizable groups than either single-component layer if the chain conformations in the coadsorbed layer were the same as those in the single-component layers. Increased intralayer electrostatic repulsions would tend to resist coadsorption (although the degree of ionization in the mixed layers is probably less than in the single-component layers). To counter the electrostatic penalty for coadsorption, there may be an enhanced entropic driving force not present in the single-component cases. The entropic driving force includes mixing entropy and configurational entropy contributions. The entropy of mixing surfactants and polymer segments at the interface favors coadsorption (Fowkes, 1964). We may speculate as to the role of configurational entropy effects. When polyelectrolytes adsorb from singlecomponent solutions to oppositely charged surfaces, they tend to neutralize the surface charge. Increasing the charge density of the polyelectrolyte (by increasing the

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Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996 1573

number of ionized groups along the polyelectrolyte backbone) leads to a more rodlike conformation and a flatter adsorbed layer (Dahlgren and Claesson, 1993). The situation is much different in the mixed CTAB/ polylysine layers, where hydrophobic cooperativity leads to charge reversal rather than neutralization. As a result, intralayer electrostatic repulsion may cause the polyelectrolyte to adopt a more extended conformation normal to the surface in order to thicken the composite layer and thereby decrease the charge density. Since polylysine is not rodlike at 0.01 M ionic strength (Nemoto et al., 1984), it is sufficiently flexible to form the loops and tails required for it to adopt a more extended conformation and gain configurational entropy in the process. Adsorption Mechanism. Examination of the adsorption kinetics for the single-component and the coadsorption experiments indicates that adsorption is interface-limited, rather than transport-limited, throughout the measurement. Initial steady-state adsorption rates calculated from the Le´veˆque solution for convective diffusion in parabolic slit flow (Lok et al., 1983) far

|

1 dΓ γ 1/3 ) DC0 dt 0 Γ(4/3)91/3 Dl

( )

(10)

exceed the initial adsorption rates we measured in all cases. In eq 10 γ is the wall shear rate, l is the distance from the flow cell inlet to the point of observation, D is the diffusion coefficient, Γ(4/3) is the Gamma function, and Co is the bulk concentration. Fully developed laminar flow is of course a requirement for application of the Le´veˆque solution. Our group (Robeson, 1995) previously has verified that the parabolic flow profile is fully developed at the point of reflection in this flow cell, as demonstrated by the quantitative agreement between eq 10 and initial adsorption rates for the protein lysozyme. Using the literature value (Daniel and Alexandrowicz, 1963) for the intrinsic (lower limit) diffusion coefficient for polylysine (degree of polymerization 900), DPLL ) 1.5 × 10-7 cm2/s, we would predict a transport-limited initial adsorption rate of 0.04 mg m-2 s-1. At that rate the adsorbed layer would saturate in 10 s, compared to the ∼5 min required in our single-component experiments. While the adsorption may have been transportlimited for a very short time, less than our time resolution of 1 s, the great majority of the polylysine adsorbed via an interface-limited process. For CTAB at its cmc, 0.2 mM, where the free surfactant concentration is approximately equal to the cmc, the measured initial adsorption rate was 0.02 mg m-2 s-1 in the single-component experiments. That would correspond to an apparent diffusion coefficient of 2.5 × 10-7 cm2/s, or a factor of 20 less than the reported diffusion coefficient of the CTA+ surfactant ion, DCTA ) 5 × 10-6 cm2/s (Lindman et al., 1984). For CTAB at 0.8 mM, i.e. 4 × cmc, the measured initial steady-state adsorption rate was 0.022 mg m-2 s-1. If we first assume that only free CTA+ ions adsorb while micelles do not adsorb, the appropriate concentration to use in eq 10 is the free surfactant concentration. It is known that free surfactant concentrations do decrease somewhat above the cmc. Lindman et al. (1984) observed that when the total CTAB concentration was 4 × cmc in water (but with no added electrolyte), the free surfactant concentration did not decrease by more than a factor of 2 relative to the cmc. Thus, if Co was between 0.2 and 0.1 mM in our experiments at 4 × cmc, this

initial adsorption rate would correspond to an apparent diffusion coefficient ranging from 3 × 10-7 to 8 × 10-7 cm2/s, according to eq 10. This is a factor of 6-16 times smaller than the free CTA+ diffusion coefficient. If on the other hand we assume that micelles adsorb, the appropriate concentration in eq 10 is the concentration of surfactants in micelles (not the concentration of micelles), i.e., approximately 0.6-0.7 mM. Given that assumption, the initial adsorption rate would correspond to a diffusion coefficient between 4 × 10-8 and 5 × 10-8 cm2/s. This is 16-20 times smaller than the diffusion coefficient of CTAB micelles, Dmicelle ) 8 × 10-7 cm2/s (Dorshow et al., 1982). We conclude that CTAB adsorption was not transport-limited at any experimentally accessible time. Since all coadsorption rates were considerably slower than the corresponding single-component polylysine or CTAB adsorption rates, we conclude that coadsorption was interface-limited as well. One possible exception would be if CTAB and polylysine were to associate in the bulk, in spite of their electrostatic repulsion, and form large aggregates with small diffusion coefficients. The absence of any visually observable turbidity suggested that this unlikely aggregation did not occur. An activation barrier for CTAB and polylysine adsorption evidently existed in both the single-component and coadsorption cases. One probable contributor to the activation barrier is the surface charge reversal caused by adsorption at very low surface concentrations. Because of the charge reversal, adsorption proceeds against an electrostatic repulsion from the surface. Again, it is of course possible that the earliest adsorption prior to the point of charge reversal may have been transportlimited, but we did not resolve events occurring in less than 1 s. It is evident that the great majority of the total adsorbed mass was introduced to the mixed layer under interfacial control. If not for the evidence that adsorption was interfacelimited, it might be tempting to think of the coadsorption mechanism as a type of sequential adsorption. Based on the relative sizes (diffusion coefficients) of CTAB and polylysine, one would have expected the adsorbed layer to be dominated by CTAB at early times. The large polylysine chains could then join the adsorbed layer by displacing some of the “preadsorbed” CTAB, and perhaps this displacement would be the ratelimiting step. This explanation is not appealing, because the coadsorption process was in fact not under diffusional control, so suppositions based on the relative transport rates of CTAB and polylysine should not be considered. While our results are open to other interpretations, we prefer an adsorption model wherein the slow binary coadsorption kinetics and the long-time evolution of the mixed layer are controlled by the slow re-distribution of polylysine segments between loops, trains, and tails, as the adsorbed layer expands to reduce the density of ionizable groups. It is interesting that we observed no change whatsoever when preadsorbed polylysine layers were exposed to CTAB in the sequential adsorption experiments. In keeping with the above coadsorption model, perhaps the very large fraction of preadsorbed polylysine segments in trains presents too large an energy barrier for the layer restructuring that is required for CTAB to mix with the polylysine. Conclusions At 0.01 M ionic strength polylysine adsorbs to hydrophilic silica surfaces at modest surface concentrations,

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1574 Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996

but its adsorption is irreversible with respect to dilution of the bulk solution. Although CTAB has a high affinity for the silica surface, it neither adsorbs to a preadsorbed polylysine layer nor displaces it, even at bulk concentrations 5 times in excess of the cmc. While CTAB will not adsorb in the presence of preadsorbed polylysine, it does adsorb from a binary mixture. Coadsorption proceeds via a complex, interface-limited mechanism and produces a mixed layer of intermediate adsorption reversibility. Most notably, the mixed layers contain approximately the same amounts of polylysine and CTAB as those layers that form by adsorption from either single-component solution. Upon rinsing the mixed layers, it appears that most or all of the CTAB desorbs while all of the polylysine remains adsorbed. This study supports previous claims (Somasundaran and Cleverdon, 1985) that although it may not be intuitively expected, similarly charged surfactants and polyelectrolytes can coexist in mixed adsorbed layers. The formation of such composite adsorbed layers draws attention to the risks of interpreting or predicting multicomponent complex fluid behavior on the basis of single-component solution behavior for any of the components. Acknowledgment This work is based on material supported in part by the National Science Foundation under Grant No. CTS9308569 and by E. I. DuPont de Nemours & Co., Inc. We gratefully acknowledge the American Chemical Society POLYED Undergraduate Summer Scholarship Program for Research in Polymer Science for supporting E.M.F. and the Keck Foundation for supporting E.S.P. We also thank Prasad N.S.B. for major contributions to the design of the scanning angle reflectometer. Literature Cited Afshar-Rad, T.; Bailey, A. I.; Luckham, P. F.; MacNaughton, W.; Chapman, D. Forces between Model Polypeptides and Proteins Adsorbed on Mica Surfaces. Colloids Surf. 1988, 31, 125. Arnebrant, T.; Ba¨ckstro¨m, K.; Jo¨nsson, B.; Nylander, T. An Ellipsometry Study of Ionic Surfactant Adsorption on Chromium Surfaces. J. Colloid Interface Sci. 1989, 128, 303. Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: Amsterdam, 1977. Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd Ed.; Wiley: New York, 1989. Charron, J. R. Block Copolymer Adsorption to Insoluble Lipid Monolayers. Ph.D. Dissertation, Carnegie Mellon University, PA, in preparation, 1996. Charron, J. R.; Tilton, R. D. A Scanning Angle Reflectometry Investigation of Block Copolymer Adsorption to Insoluble Lipid Monolayers at the Air-Water Interface. J. Phys. Chem. 1996, 100, 3179. Cuypers, P. A.; Corsel, J. W.; Janssen, M. P.; Kop, J. M. M.; Hermens, W. T.; Hemker, H. C. The Adsorption of Prothrombin to Phosphatidylserine Multilayers Quantitated by Ellipsometry. J. Biol. Chem. 1983, 258, 2426. Daniel, E.; Alexandrowicz, Z. Sedimentation and Diffusion of Polyelectrolytes. Part II. Experimental Studies with Poly-LLysine Hydrohalides. Biopolymers 1963, 1, 473. Dahlgren, M. A. G.; Claesson, P. M. Interaction and Adsorption of Polyelectrolytes on Mica. Nord. Pulp Pap. Res. J. 1993, 8, 62. de Feijter, J. A.; Benjamins, J.; Veer, F. A. Ellipsometry as a Tool to Study the Adsorption Behavior of Synthetic and Biopolymers at the Air-Water Interface. Biopolymers 1978, 17, 1759.

Dijt, J. C.; Cohen Stuart, M. A.; Hofman, J. E.; Fleer, G. J. Kinetics of Polymer Adsorption in Stagnation Point Flow. Colloids Surf. 1990, 51, 141. Dorshow, R.; Briggs, J.; Bunton, C. A.; Nicoli, D. F. Dynamic Light Scattering from Cetyltrimethylammonium Bromide Micelles: Intermicellar Interactions at Low Ionic Strengths. J. Phys. Chem. 1982, 86, 2388. Fowkes, F. M. Ideal Two-Dimensional Solutions. IV. Penetration of Monolayers of Polymers. J. Phys. Chem. 1964, 68, 3515. Garoff, S. Molecular Structure and Interfacial Properties of Surfactant-Coated Surfaces. Thin Solid Films 1987, 152, 49. Gebhardt, J. E.; Fuerstenau, D. W. The Effect of Preadsorbed Polymers on Adsorption of Sodium Dodecylsulfonate on Hematite. In Structure/Performance Relationships in Surfactants; Rosen, M. J., Ed.; ACS Symposium Series 253; American Chemical Society: Washington, DC, 1984; p 291. Leermakers, F. A. M.; Gast, A. P. Block Copolymer Adsorption Studied by Dynamic Scanning Angle Reflectometry. Macromolecules 1991, 24, 718. Lindman, B.; Puyal, M.-C.; Kamenka, N.; Rymde´n, R.; Stilbs, P. Micelle Formation of Anionic and Cationic Surfactants from Fourier Transform Hydrogen-1 and Lithium-7 Nuclear Magnetic Resonance and Tracer Self-Diffusion Studies. J. Phys. Chem. 1984, 88, 5048. Lok, B. K; Cheng, Y. L.; Robertson, C. R. Protein Adsorption on Crosslinked Polydimethylsiloxane using Total Internal Reflection Fluorescence. J. Colloid Interface Sci. 1983, 91, 104. McCrackin, F. L.; Colson, J. P. Computational Techniques for the Use of the Exact Drude Equations in Reflection Problems. Ellipsometry in the Measurement of Surfaces and Thin Films; National Bureau of Standards Publication: Washington, DC, 1964; Vol. 256, p 61. Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems NSRDS-NBS 36; U.S. Government Printing Office: Washington, DC, 1971. Nemoto, N.; Matsuda, H.; Tsunashima, Y.; Kurata, M. Dynamic Light Scattering of Poly(L-lysine) Hydrobromide in Aqueous Solutions. Macromolecules 1984, 17, 1731. Papenhuijsen, J.; van der Schee, H. A.; Fleer, G. J. Polyelectrolyte Adsorption. I. A New Lattice Theory. J. Colloid Interface Sci. 1985, 104, 540. Parker, J. L.; Yaminsky, V. V.; Claesson, P. M. Surface Forces between Glass Surfaces in Cetyltrimethylammonium Bromide Solutions. J. Phys. Chem. 1993, 97, 7706. Rennie, A. R.; Lee, E. M.; Simister, E. A.; Thomas, R. K. Structure of a Cationic Surfactant Layer at the Silica-Water Interface. Langmuir 1990, 6, 1031. Robeson, J. L. Application of Total Internal Reflection Fluorescence to Probe Surface Diffusion and Orientation of Adsorbed Proteins. Ph.D. Dissertation, Carnegie Mellon University, PA, 1995. Schaaf, P.; De´jardin, P.; Schmitt, A. Reflectometry as a Technique to Study the Adsorption of Human Fibrinogen at the Silica/ Solution Interface. Langmuir 1987, 3, 1131. Somasundaran, P.; Cleverdon, J. A Study of Polymer/Surfactant Interaction at the Mineral/Solution Interface. Colloids Surf. 1985, 13, 73. Trogus, F. J.; Schechter, R. S.; Wade, W. H. A New Interpretation of Adsorption Maxima and Minima. J. Colloid Interface Sci. 1979, 70, 293. Wahlgren, M. C.; Arnebrant, T. Interaction of Cetyltrimethylammonium Bromide and Sodium Dodecylsulphate with β-Lactoglobulin and Lysozyme at Solid Surfaces. J. Colloid Interface Sci. 1991, 142, 503. Zorin, Z. M.; Churaev, N. V.; Esipova, N. E.; Sergeeva, I. P.; Sobolev, V. D.; Gasanov, E. K. Influence of Cationic Surfactant on the Surface Charge of Silica and on the Stability of Aqueous Wetting Films. J. Colloid Interface Sci. 1992, 152, 170.

Received for review October 30, 1995 Accepted March 4, 1996X IE9506577 X Abstract published in Advance ACS Abstracts, April 15, 1996.