Behavior of Cationic Surfactants in Poly (styrene sulfonate) Brushes

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Ind. Eng. Chem. Res. 2008, 47, 6426–6433

Behavior of Cationic Surfactants in Poly(styrene sulfonate) Brushes Akira Ishikubo,†,§ Jimmy Mays,‡ and Matthew Tirrell*,† Department of Chemical Engineering, Materials Research Laboratory, UniVersity of California, Santa Barbara, California 93106, Department of Chemistry, UniVersity of Tennessee, KnoxVille, Tenneseee 37996, Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, and Shiseido Research Center, 2-2-1 Hayabuchi, Tsuzuki-ku, Yokohama-shi 224-8558, Japan

The adsorption behavior of a cationic surfactant (cetyl trimethyl ammonium bromide, CTAB) in a sodium poly(styrene sulfonate) (NaPSS) brush in the presence of 1 mM NaNO3 was investigated by ellipsometry. The interactions between surfactants and brush chains determined from these results are compared with data from other types of experiments on similar systems. Four adsorption regimes were found, including regimes dominated by electrostatic interactions and by hydrophobic interactions. As the concentration of the surfactant increased from zero in the bulk solution surrounding the PSS brush, (1) surfactant monomers replaced Na+ ions reversibly, in a manner analogous to ion exchange, until β (the ratio of the number of bound surfactants to the total number of negatively charged monomers of PSS) reached 0.2-0.3 (1 × 10-6 M CTAB), (2) surfactants in the brushes interacted with each other hydrophobically, making adsorption irreversible and producing contraction of the brush (0.3 < β < 0.4), (3) the rate of adsorption increase was suppressed, by the squeezing out of available volume in the shrinking brush (β ) 0.4), (4) additional uptake of surfactants took place above 1 × 10-4 M CTAB in bulk solution (still 1 order of magnitude lower than the critical micelle concentration (cmc) of CTAB), because of the hydrophobic aggregation of surfactants on surfactant-polyelectrolyte complexes in the brushes. The overall conclusion and insight gained from this work is that surfactants are taken up, even at micromolar concentrations, extensively and in a nonlinear, cooperative fashion, in polyelectrolyte brushes. These data imply that the properties expected from highly hydrated polyelectrolyte brushes (steric protection, lubricity) are strongly modified by the presence of oppositely charged surfactants. Introduction Polyelectrolytes adsorbed on surfaces are used to good effect in personal care products and in many industrial products, e.g., dispersions, emulsions, lubricants, coatings, etc.1 In practical use, oppositely charged surfactants are frequently added to a system to modify surfaces, to add functionality, modify the rheology, or to stabilize the system. Therefore, understanding and quantifying the interactions between surfactants and polyelectrolytes tethered on surfaces should lead to additional applications and a new level of property control of such materials. In particular, data in this paper will show that the introduction of oppositely charged surfactant into aqueous media surrounding polyelectrolyte brush layers produces strong uptake of the surfactant in the brush and effectively modifies the interactions among the segments of the polyelectrolyte chain from repulsive to strongly attractive. From a more fundamental perspective, aggregates of charged surfactants are polyvalent ions, with charged peripheries surrounding hydrophobic cores. In this way, surfactant aggregates are good models for studying controllable, multivalent interactions and, in fact, strongly resemble protein macromolecules in possessing the features mentioned in the preceding sentence. Many, if not most, biological interfaces are soft interfaces, consisting of charged polyelectrolytes highly swollen with water, such as the cellular glycocalyx, the surfaces of lung, eyelid, and cartilage tissues, and many types of fibril and filament structures. Polyvalent electrostatic and hydrophobic interactions * To whom correspondence should be addressed, E-mail: tirrell@ engineering.ucsb.edu. Phone: (805) 893-3141. Fax: (805) 893-8124. † University of California, Santa Barbara. § Shiseido Research Center. ‡ University of Tennessee and Oak Ridge National Laboratory.

control the behavior of proteins in contact with these soft biological interfaces. There have been many studies concerning surfactants interacting with oppositely charged polyelectrolytes randomly adsorbed on surfaces. These studies have employed a range of techniques, including neutron reflectivity,2 attenuated total reflection infrared spectroscopy,3 sum-frequency generation spectroscopy,4 ellipsometry,5 atomic force microscopy,6 and the surface forces apparatus,7 to investigate the adsorption properties and structures of these complexes on surfaces. However, many of these experimental systems are relatively complicated as the effects of adsorption and desorption of surfactants and polyelectrolytes have not been separated, and take place competitively and simultaneously. Presence of surfactant can strongly affect the adsorption of polyelectrolyte segments. Our system for creation of polyelectrolyte surface layers separates the polymer adsorption exclusively into the hydrophobic block of the polymer, effectively eliminating the direct adsorption of the polyelectrolyte. The end-tethered brush affords unique opportunities to study polymer physics not possible in randomly adsorbed layers. In brushes, one end of the chain is fixed, whereas the other end is free to respond to environmental influences, unaffected by direct adsorption of the polyeletrolyte segments to the substrate.8 Polyelectrolyte brushes consist of macromolecules endattached to the surface so densely that the chains stretch away from the surface, resulting in a radius of gyration and end-toend distance larger than that for the unperturbed chain (e.g., in the melt).8 The polymer concentration in the brush layer is in the semidilute regime. There have been other studies, both experimental and theoretical, on the structure and physical properties of polyelectrolyte brushes.9 For example, it has been

10.1021/ie800004w CCC: $40.75  2008 American Chemical Society Published on Web 08/09/2008

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found that theoretical expectations and experimental results concerning physical properties, such as brush height or interaction between two opposing brushes, are in good agreement in brush systems immersed in monovalent ionic media.10 It was also found that enhanced adsorption and an attractive force occurs between two opposing brushes when multivalent ions like Ca2+ or Al3+ are added to a bulk solution.11 Because of high polymer density and the resulting large steric hindrance to interpenetration, brushes can be good dispersion stabilizers or lubricators.12 As mentioned above, the brushes coexist with various kinds of surfactants or powders in many practical situations. Although the interaction between polyelectrolyte brushes and oppositely charged surfactants could be especially important because of their strong polyvalent interactions, there have been few studies concerning this interaction. There have been three notable studies that investigated quantitatively the uptake of surfactants in oppositely charged polyelectrolyte brushes.13–15 Pyshkina et al.13 studied the interactions between poly(acrylic acid) brushes and alkyl trimethyl ammonium bromide (CnTAB, n is the number of carbons in the alkyl group; if no n is indicated, n ) 16) by reflectometry. Konradi and Ru¨he14 investigated the interactions between poly(methacrylic acid) brushes and CnTAB by infrared spectroscopy (IR) and ellipsometry. The former authors reported that the uptake of CnTAB was strongly affected by the grafting density and proceeded in a Langmuir-type manner when the grafting density was above a certain value. The maximum uptake of CnTAB was much smaller than that in the case of free polyelectrolyte chains and CnTAB. The latter authors reported that a brush contraction occurred upon addition of a small amount of surfactant. Ballauff and co-workers15 studied the interaction between NaPSS brushes (on polystyrene nanoparticles) and CTAB by light scattering and cryo-TEM. They observed the brush contraction in the presence of a small amount of CTAB and found from cryo-TEM observation that the PSS brush partly collapsed at around β ) 0.6 without salt. However, the interaction between surfactants and oppositely charged polyelectrolyte brushes is far from fully explored experimentally, especially in quantitative terms, and in quenched polyelectrolyte brushes such as NaPSS, where the linear charge density is fixed in the synthesis by the degree of sulfonation, does not respond to pH. In this work, the adsorption of CTAB in NaPSS brushes was investigated using ellipsometry. We use the term “adsorption” here to mean the process of uptake of CTAB in the NaPSS brush. The behavior of surfactants in brushes observed here by ellipsometry will be discussed in conjunction with results obtained using a surface force apparatus (SFA) to study the interaction of two opposing brushes in the presence of CTAB, which is being reported elsewhere.16 Experimental Section Materials. Amphiphilic diblock copolymer, poly(tert-butylstyrene)-b-sodium poly(styrene sulfonate) (PtBS-NaPSS) was synthesized by anionic polymerization as described previously.17 Pertinent characterization information is given in the caption of Figure 1. CTAB was purchased from Aldrich and recrystallized twice from methanol/acetone. NaNO3 (99.99+%) and octadecyltrichlorosilane (95%) (OTS) were obtained from Aldrich and used without further purification. (100)-oriented, double-sided polished, test-grade silicone wafers were purchased from Virginia Semiconductor. Instruments. The adsorption experiments were conducted on a variable-angle Beaglehole picometer ellipsometer, which

Figure 1. Structure of PtBS-PSSNa (poly(tert-butylstyrene)-b-sodium poly(styrene sulfonate)). For the sample used in this work, 〈n〉 ) 15, 〈m〉 ) 612. The PSS block was 84% sulfonated. The brush adsorption density was approximately 1 × 1016 chains/m2.

uses a HeNe laser light source (λ ) 632.8 nm), has an angular resolution of 0.01°, and is based on phase modulation.18 The ellipsometer measures directly the real and imaginary components of the ellipsometric ratio F ) rp/rs ) tan Ψei∆ where Re(F) ) tan Ψ cos ∆ and Im(F) ) tan Ψ sin ∆. rp and rs are the complex overall reflection coefficients of p- and s-polarizations, respectively. The angles Ψ and ∆ correspond to the respective ratios of attenuation and the phase change between the p- and s-polarizations. Experimental Procedure. OTS modification, crucial to high brush density and reproducible brush assembly,10 was carried out as follows. First, pieces of silicon wafer (approximately 1 cm2) were dipped in a freshly made 70:30 (v/v) sulfuric acid/ hydrogen peroxide solution for 30 min followed by rinsing with Milli-Q water for 3-5 min. The average thickness of the SiO2 (n ) 1.46) layer on the cleaned wafer was about 15 Å. After being rinsed with ethanol, acetone, and toluene, the wafer was immersed in 1 × 10-3 M solutions of OTS in toluene for 1-2 h. The film-covered substrate was then removed from the solution and baked at 110 °C for 2 h to remove any excess water and allow complete hydrolysis of the OTS layer. Ellipsometry showed that the final film thickness was approximately 20-22 Å (the contour length of a single molecule was 26 Å). Typical contact angles of water droplets were about 110°. The preparation of PSS brushes on the OTS-modified silicon wafer has been described previously.19 In brief, an OTSmodified silicon wafer was placed in the center of a specially built cylindrical glass cell with a total volume of 17 mL. The cell was then filled with pure Milli-Q water. At this stage, the Brewster angle was found to be 71 ( 1° and Im(F) ) 0.005. When Im(F) remained constant for 15 min, a 100 ppm polymer solution in 1 M NaNO3 was introduced through the cell inlet, and the cell was left for over 15 h while monitoring the change in Im(F) (Figure 2). A steady increase in this value is seen, because of hydrophobic attachment of the hydrophobic PtBS block to the OTS layer. The polymer solution was then replaced with a 1 mM NaNO3 solution. At this point, ∆Im(F) () Im(F) - Im(F)baseline) increased steeply because of the difference in refractive index, followed by a slow decrease, indicating some slight polymer detachment (Figure 2). However, this detachment ceased after 2 h and no further desorption was observed even after replacing the solution with fresh 1 mM NaNO3.19b CTAB solutions in the presence of 1 mM NaNO3 were injected into the cell stepwise from 1 × 10-8 to 1 × 10-3 M, diluted from 1 × 10-3 to 1 × 10-8 M, and ellipsometry data were obtained at each concentration. The temperature of the solutions was controlled at 30 ( 1 °C. Im(λ) was measured every 2 s for 1 to 24 h depending upon the adsorption behavior. Each experiment was performed multiple times to ensure reproducibility. Data Analysis. The change in Im(λ) is directly proportional to the zeroth moment λ0 of the refractive index profile of the adsorbed layer;20 that is, ∆Im(F) ) 2π(nsubstrate2 + nsolvent2)0.5nsolvent(Γ0/λ)

(1)

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Figure 2. Time dependence of ∆Im(λ) corresponding to the adsorption and desorption of PtBS-PSS. The arrow indicates the point of injection of 1 mM NaNO3 solution.

where nsubstrate and nsolvent refer to the refractive indices of silicon and salt solution, respectively. Γ0 can be expressed as Γ0 )

∫

∞

0

n dz

(2)

where n refers to the refractive index of the adsorbed materials. The adsorbed amount λ (mass/area) is evaluated from λ0 as follows: Γ(mass/area) ) Γ0(dn/dc)-1

(3)

where dn/dc is the refractive index increment of the adsorbed material. In this study, the adsorbed materials were PtBS-PSS and CTAB. Therefore, Γ ) ΓCTAB + ΓPtBS-PSS

(4)

and nsolvent ) nwater+cCTAB(dn/dc)CTAB + cPtBS-PSS(dn/dc)PtBS-PSS (5) From eqs 1–5, the adsorption amount of CTAB, ΓCTAB, can be expressed as ΓCTAB ) ∆Im(F)(λ/2π)nsolvent/(nsubstrate2 + nsolvent2)0.5(dn/dc)CTAB-1 - ΓPtBS-PSS(dn/dc)PtBS-PSS(dn/dc)CTAB-1 (6) If the number of tethered polymer chains on the surface is constant during the experiments, which is true in our work (after the slight polymer detachment is complete), ΓCTAB is directly proportional to ∆Im(F). From ΓCTAB, the ratio of the number of bound surfactants to the known total number of negatively charged monomers of PSS, β, is calculated. Control Experiments. Whether the addition of surfactant to a preassembled brush layer of the type we have used causes desorption of the brush, presumably by displacing the adsorbing hydrophobic block, is an important matter to understand in these experiments. We have examined this carefully and the results are discussed with reference to Figure 3. No evidence of polymer detachment from the surface on introduction of surfactant was found below the cmc of CTAB. Figure 3a is an adsorption profile of CTAB, in a fully assembled PSS brushes formed as described above, when a 1

Figure 3. Time dependence of ∆Im(λ) corresponding to when the CTAB concentration in the bulk solution was (A) increased or (B) decreased. CTAB concentration in the bulk solution changed (a) from 1 × 10-6 to 3 × 10-6 M, (b) from 3 × 10-4 to 3 × 10-3 M, and (c) from 1 × 10-5 to 1 × 10-6 M.

× 10-6 M solution of CTAB was replaced by 3 × 10-6 M solution of CTAB, indicating a typical adsorption profile below the cmc (1 × 10-3 M). ∆Im(F) increased in the beginning slowly or steeply, depending on the concentration jump, and then saturated. On the other hand, panel b is an adsorption profile when a 3 × 10-4 M solution of CTAB was replaced by a 3 × 10-3 M solution of CTAB, indicating the typical profile when the concentration of the added solution was above the cmc. ∆Im(F) increased in the beginning followed by a slow, slight decrease. This decrease in ∆Im(F) is attributable to polymer detachment; ∆Im(F) decreased even though extra CTAB was fed into the system. This slow detachment is observed in Figure 2 as well, when the polymer was detached from the surface by replacing the polymer solution with 1 mM NaNO3. Because no decrease in ∆Im(F) was observed when the concentration of CTAB was below the cmc (Figure 3a), no detachment took place under these conditions. Figure 3c is a desorption profile of CTAB when a 1 × 10-5 M solution of CTAB was replaced by a 1 × 10-6 M solution of CTAB, indicating a typical profile below cmc. ∆Im(F) decreased steeply immediately after the injection of dilute surfactant solution, and it rapidly became constant. Because we observed no signs of a slow decrease in ∆Im(F), the polymer detachment is believed to be insignificant during the dilution process as long as the concentration was below the cmc.

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Figure 5. Adsorption isotherms for CTAB on PSS brushes (filled symbols) and for C12TAB on dilute free PSS (open symbols) from ref 23. The solid lines are the results of fitting to eq 7. The arrow indicates the onset of irreversibility of CTAB adsorption within the brush.

Figure 4. (A) Adsorption of CTAB on the OTS modified silicon wafer (open circles) and on NaPSS brushes on the OTS modified surface (filled circles). (B) Time dependence of ∆Im(λ) response when 1 × 10-7 M CTAB was introduced to (a) OTS-modified silicon wafer and (b) NaPSS brushes on the OTS-modified surface.

The following values of constants were used in this study: λ ) 632.8 nm, nwater ) 1.33, (dn/dc)NaNO3 ) 0.115 mL/g,23 (dn/ dc)CTAB ) 0.15 mL/g,21 (dn/dc)PtBS-PSS ) 0.168 mL/g.19 Results and Discussion The adsorption isotherm of CTAB on PSS brushes as a function of CTAB concentration is shown in Figure 4A. The isotherm of CTAB on the OTS surface (before polymer brush assembly) is also shown in order to verify the CTAB adsorption to brushes. Adsorption begins below 1 × 10-7 M CTAB in the case of the OTS surface, whereas no adsorption was observed in the case of PSS brushes. This observation is also apparent in Figure 4B, which shows the adsorption profiles at 1 × 10-7 M CTAB. If bare, exposed OTS surface area remained within the PSS brush surface, the surfactant would be adsorbed directly to the substrate under these conditions. Seeing none, it was concluded that surfactants only adsorb within PSS brushes with negligible direct adsorption on the OTS surface. Figure 4A shows that surfactant adsorption proceeds in two steps. It begins between 1 × 10-7 and 1 × 10-6 M, slows

around 1 × 10-5 to 1 × 10-4 M, followed by a second uptake above 1 × 10-4 M. This result resembles previously reported results on the adsorption of cationic surfactants on poly(acrylic acid) brushes13 and that of cationic surfactants on poly(methacrylic acid) brushes.14 Both groups reported that surfactant adsorption involved two steps: a saturation curve, followed by a second uptake regime. This two-step interaction regime has not been reported in systems consisting of surfactants in dilute polyelectrolyte solutions (see, for example, reference 1). There have been many studies on the interaction of CnTAB with PSS in dilute solution systems.2,23–25 The adsorption isotherms of CnTAB on free PSS chains in dilute aqueous solution have been reported using ion selective electrodes.23,24 In Figure 5, the isotherm of C12TAB on PSS from a previous report24 is shown together with our data. The vertical-axis, β, represents the ratio of the number of bound surfactants to the total number of negatively charged monomers of PSS. In the case of C16TAB (CTAB) instead of C12TAB in Figure 5, the isotherm moves, as expected, to a lower concentration because of the stronger hydrophobic interaction among surfactants without a large difference in the shape of the profile. In the case of a dilute PSS solution, the onset of detectable complexation of the surfactant with the polymer is about 3 orders of magnitude lower than the cmc of the surfactant (ca. 1 × 10-2 M). The β value increases steeply beause of the cooperative interaction and saturates at around 0.5. It is known that the β value approaches 1.0 if the polyelectrolytes are sufficiently hydrophilic, such as for dextran. On the other hand, the β value saturates below 1.0 in the case where the polyelectrolytes are somewhat hydrophobic, because the interaction between the hydrophobic parts of polyelectrolytes and surfactant aggregations disturbs the participation of all of the negative charges of the polymer with the surfactant-polymer complex, because of steric obstacles. This is a possible reason why the β value begins to saturate at below 0.5 in these polyelectrolyte brush experiments. The β value began to saturate at 0.3-0.4, in the case of CTAB adsorption on PSS brushes as shown in Figure 5, indicating

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that the β value is less than that for the dilute solution system. The previous report13 showed that incipient saturation took place at β ) 0.3-0.8 depending on the grafting density, even though the polyelectrolyte used was hydrophilic PAA.13 That report concluded that this is attributable to the density of polymer segments. Another difference between the PSS brushes and the dilute PSS solution in Figure 5 is found in the slope of the β value at the incipient saturation stage. In the case of C12TAB adsorption on PSS in dilute solution, β increases steeply in the incipient stage because of cooperative binding. If the alkyl chain length is 16 (as in CTAB), instead of 12, the slope should be steeper because of its stronger hydrophobic interaction. In fact, the slope is much lower in the case of adsorption on PSS brushes, indicating that the cooperativity of binding is relatively low. To estimate the cooperativity of binding, a modified Zimm-Bragg theory was used.25 In this work, β is defined as 2β - 1 ) (y - 1)/{(1 - y)2+4yu-1}1/2

(7)

y ) KuCs

(8)

-1

where K (L mol ) is the binding constant between the surfactant and a negatively charged polymer segment of the polyelectrolyte, and u is a cooperativity parameter. Cs is the concentration of surfactant in bulk solution. K and u were estimated by fitting to experimental data, and the results of the fittings are shown as solid lines in Figure 5. The following values were obtained as a result of the fitting: K ) 160 and u ) 100 for the dilute system,23 and K ) 170 000 and u ) 1 for the PSS brushes. K for the PSS brushes is much higher, probably because of longer alkyl chains and higher local concentration of polymer segments. The u value for the PSS brushes is much smaller and close to zero, indicating that binding proceeds without significant cooperativity and in a Langmuir manner. Osada et al. demonstrated similar results concerning the adsorption of N-dodecylpyridinium chloride on poly[2(acrylamido)-2-methylpropanesulfonic acid] (PAMPS) gels.27 They reported that u for cross-linked PAMPS gel is much smaller than that for uncrosslinked PAMPS, and demonstrated that u decreased as the degree of cross-linking increased. This cooperativity of the binding will be discussed in the last part of this section. The reversibility of the adsorption of CTAB was investigated. Various concentrations of CTAB in the presence of 1 mM NaNO3 were studied and the data are shown in Figure 5 to 1 × 10-8 M. The adsorption was found to become irreversible above around 1 × 10-6 M, as indicated by an arrow in Figure 5. The change in ∆Im by dilution below and above the critical concentration (the arrow in Figure 5) is shown in Figure 6A below (1 × 10-6 M) and (B) above (3 × 10-5). The b and d lines in Figure 6 mean ∆Im at 1 × 10-8 M. Figure 6 shows that ∆Im decreased after dilution in (A) and became constant in (B). From these results, the adsorption was found to proceed uncooperatively and reversibly in the region below 1 × 10-6 M, indicating that surfactants simply replaced Na ions. In Figure 5, the adsorption begins to be suppressed above 1 × 10-6 M and began to saturate at about 1 × 10-5 to 1 × 10-4 M and β ) 0.3-0.4, followed by the second uptake regime above 1 × 10-4 M CTAB. In previous experiments using the SFA,16 the PSS brushes were found to begin to contract at 1 × 10-6 M CTAB. The heights of the PSS brushes16 and adsorption isotherms are shown in Figure 7. The brushes contract from 100 to 250 nm when the CTAB concentration increases from 1 × 10-6 to 1 × 10-4 M. The driving force for this contraction is presumably attributable to the formation of hydrophobic

Figure 6. ∆Im(λ) response as a function of time when the CTAB concentration was decreased (A(a)) from 1 × 10-6 to 1 × 10-8 M, and (B(c)) from 3 × 10-6 to 1 × 10-8 M. (b, d) ∆Im(λ) response when a 1 × 10-8 M solution of CTAB was added just after preparation of the brushes (no adsorption was observed). Arrows indicates points of concentration change.

aggregates among surfactants, and the concomitant poly valent binding to the negative charges of the PSS brushes. This is a reasonable explanation for the irreversibility of CTAB above about 1 × 10-6 M due to this attraction produced by newly formed aggregates. For a better understanding of what occurs in this regime (1 × 10-6 to 1 × 10-4 M), the reason why β begins to saturate at 0.3-0.4 should be considered further. In case of dilute PSS in solution, β saturates at around 0.5,23 which has been attributed to a participation of some hydrophobic part of PSS (either the main chain or unsulfonated residues, in our case) in the hydrophobic domains of aggregating surfactants. The same phenomenon may occur the brush system (which is in the semidilute regime), but the difference is that β ) 0.4 is smaller than that in the case of dilute solution system. A possible reason for this is increased excluded volume resistance to surfactant adsorption in the denser polymer brush. This agrees phenomenologically with the reported result that β at the first incipient saturation increased as the grafting density of the polymer brush decreased.13 Furthermore, in the brush, the charge neutralization of sulfonates by counterion condensation is higher by about

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Figure 7. CTAB concentration dependence on the brush height (ref ) (filled symbols) and the adsorption isotherm (open circles). Two data sets are included for brush height to show the reproducibility. The dashed line indicates the original brush height. Solid lines are guides for the eye.

25%10 than in solution, possibly adding some electrostatic resistance to further accumulation of cationic CTAB. The second uptake of CTAB occurred above 1 × 10-4 M as shown in Figure 5. This is believed to be due to the aggregation, and progressive accumulation of surfactant monomers into micelles within the brushes. This idea is supported by other experimental results,16 which demonstrate that an adhesive force develops between two opposing PSS brushes when they are in contact in bulk solution with >1 × 10-4 M CTAB. This adhesive force is attributable to multivalent ionic species in the brush that enable negative charges of the PSS brush to be bridged by attractive interactions arising from multivalent, cationic aggregates when compressed.11 This idea is also supported by the fact that the adsorption of CTAB was reversible when the concentration of CTAB was above 1 × 10-4 M. This result is shown in Figure 8 together with the adsorption isotherm. The β value decreased until it reached 0.4; however, no further detachment of surfactants occurred. That is, the adsorption of surfactant above β ) 0.4 proceeded reversibly, whereas that of the surfactants bound to the negative charges of PSS brushes proceeded irreversibly up to β ) 0.4. This is probably because the driving force of the adsorption at β > 0.4 mainly reflects hydrophobic interaction rather than strong electrostatic interaction. The second uptake took place at 1 × 10-4 M, which is still 1 order of magnitude lower than the cmc of CTAB. Hanson et al. investigated the cooperativity of the interaction between surfactants and polyelectrolytes, both experimentally and theoretically.28 In their experiments, the adsorption of surfactants on dilute polyelectrolytes in solution and in polyelectrolyte gels was studied and the onset of cooperative binding (critical association concentration, cac) was found to be dependent on salt concentration, the aggregation numbers of micelles, and the concentration of monomer segments. It was also reported that if the salt concentration and the aggregation number were constant, cac would be in proportion to the concentration of the monomer segments. In our work, it is difficult to determine the cac directly. Evidence of micelle-like aggregations formed in the brushes at 1 × 10-6 M is present, whereas surfactant monomers in the bulk solution form aggregates at about 1 ×

Figure 8. CTAB adsorption and desorption isotherms. β is the molar ratio of CTAB to the total number of negatively charged monomer units of PSS. Open circles show the adsorption process and closed circles show the desorption process. The dashed line is a guide for the eye.

Figure 9. Schematic illustration of the binding of oppositely charged surfactants to PSS brushes. Numbers indicate the different phases of the surfactant uptake process.

10-4 M. If the latter value is assumed to be the cac, the idea of Hanson et al.28 would be consistent with the results of this study. In the case of the dilute solution system, the cac of CTAB to PSS is below 1 × 10-6 M, and higher cac of CTAB in PSS brushes (1 × 10-4 M) could be explained by a difference in the concentration of monomer segments in the polyelectrolyte, which is also consistent with the experimental results of adsorption of CnTAB on poly(methacrylic acid) brushes by Konradi and Ru¨he.14 In the case of polyelectrolyte brushes, the adsorption of surfactant proceeds in two steps, in contrast to systems consisting of dilute polyelectrolyte solutions or polyelectrolyte gels. This is probably due to the higher concentration of monomer segments in the brush. We summarize the adsorption behavior of CTAB on the PSS brushes in Figure 9, from the results of this study obtained using ellipsometry and from results of SFA. In Figure 9, (1) surfactant monomers simply replace the Na ions (in an ion exchange process) until β reaches 0.2-0.3 (∼1 × 10-6 M CTAB in bulk solution), (2) surfactant monomers are bound in the brush aggregates through hydrophobic interaction forming micellelike complexes at β ) 0.3-0.4 (1 × 10-6 to 1 × 10-4 M CTAB), which generates cooperativity in the sense that hydrophobic interaction creates more aggregates, which in turn attract more surfactant, (3) uptake approaches saturation at about β ) 0.4, thus balancing the compression force due to hydrophobic interaction among surfactants and the excluded volume effect of the counterions and polymer segments, (4) surfactant monomers in the bulk solution begin to interact hydrophobically

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with complexes in the brushes above 1 × 10-4 M CTAB, forming micelles at a concentration that is 1 order of magnitude lower than the cmc of CTAB in polymer-free solution. Conclusion We investigated the adsorption behavior of CTAB on PSS brushes using ellipsometry and SFA. Four steps were shown to be involved in the process: (1) a simple ion exchange process (β < 0.2-0.3, [CTAB] < 1 × 10-6 M), (2) aggregate formation by hydrophobic interaction of surfactants via irreversible binding and accompanying brush contraction (β ) 0.3-0.4, 1 × 10-6 M < [CTAB] < 1 × 10-4) (3) adsorption saturation due to a balance of aggregation (leading to concentration) formation and excluded volume repulsion (β ) 0.4, 1 × 10-5 M < [CTAB] < 1 × 10-4 M), and (4) a reversible binding of surfactants in the bulk solution to micelle-like, surfactant-polyelectrolyte complexes in the brushes by hydrophobic interactions (β > 0.4, [CTAB] > 1 × 10-4 M). Polyelectrolyte brushes are important materials to control adhesive and lubrication forces at soft aqueous interfaces in technology and biology. SFA and other experiments have shown14,16 that the brush heights and interactions among brushes, which are important factors for dispersion or lubrication, are also affected by the presence of oppositely charged surfactants and, by inference, other polyvalent ions, such as proteins and even simple multivalent metal ions.11,29 To understand and control such systems, it is crucial to understand the behavior of polyvalent ions in polyelectrolyte brushes. In our view, studying the lubrication properties of this system would be an attractive future project. Acknowledgment This work was supported by the NIRT and MRSEC Programs of the National Science Foundation under Awards CTS0103516, DMR-0080034, and DMR-0520415, and by the Shiseido Corporation Research Center in Yokohama, Japan. Discussions with Professor Matthias Ballauff and Christian Schneider under the auspices of the Materials World Network Grant from NSF DMR-0710521 have been valuable in writing this article. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy. Literature Cited ¨ otz, J.; Philipp, B.; Seidel, C.; (1) (a) Dautzenberg, H.; Jaeger, W.; K Stscherbina, D. Polyelectrolytes: Formation, Characterization, and Application; Carl Hanser Verlag: Munich, Germany, 1994. (b) Reich, C. Hair Cleansers, 2nd ed.; Reich, C., Ed.; Marcel Dekker: New York, 1997; Vol. 68. (c) Dobrynin, A. V.; Rubinstein, M. Theory of Polyelectrolytes in Solutions and at Surfaces. Prog. Polym. Sci. 2005, 30, 1049–1118. (d) Tripathy, S. K.; Kumar, J.; Nalwa, H. S., Ed. Applications of Polyelectrolytes and Theoretical Models; American Scientific Publishers: Valencia, CA, 2002; Vol. 3. (2) Taylor, D. J. F.; Thomas, R. K.; Li, P. X. Adsorption of Oppositely Charged Polyelectrolyte/Surfactant Mixtures. Neutron Reflection from Alkyl Trimethylammonium Bromides and Sodium Poly(styrenesulfonate) at the Air/Water Interface: The Effect of Surfactant Chain Length. Langmuir 2003, 19, 3712–3719. (3) Neivandt, D. J.; Gee, M. L.; Tripp, C. P.; Hair, M. L. Coadsorption of Poly(styrenesulfonate) and Cetyltrimethylammonium Bromide on Silica Investigated by Attenuated Total Reflection Techniques. Langmuir 1997, 13, 2519–2526. (4) Duffy, D. C.; Davies, P. B.; Creeth, A. M. Polymer-Surfactant Aggregates at a Hydrophobic Surface Studied Using Sum-Frequency Vibrational Spectroscopy. Langmuir 1995, 11, 2931–2937.

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ReceiVed for reView January 1, 2008 ReVised manuscript receiVed June 18, 2008 Accepted June 26, 2008 IE800004W