Surface Adsorption and Phase Separation of Oppositely Charged

Mar 24, 2010 - 11810 East Miami River Road, Cincinnati, Ohio 45253-8707. Received January 24, 2010. Revised Manuscript Received March 15, 2010...
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Surface Adsorption and Phase Separation of Oppositely Charged Polyion-Surfactant Ion Complexes: 3. Effects of Polyion Hydrophobicity Olga Santos,*,† Eric S. Johnson, ‡ Tommy Nylander, † Rajan K. Panandiker,§ Mark R. Sivik,§ and Lennart Piculell† †

Division of Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, POB 124, SE-221 00 Lund, Sweden, ‡P&G Beauty & Grooming, Sharon Woods Technical Center, 11511 Reed Hartman Highway, Cincinnati, Ohio 45241-2422, and §Procter & Gamble Company, Miami Valley Innovation Center, 11810 East Miami River Road, Cincinnati, Ohio 45253-8707 Received January 24, 2010. Revised Manuscript Received March 15, 2010

The adsorption to hydrophilic silica surfaces in relation to the bulk phase behavior was investigated by in situ null ellipsometry and bulk turbidity measurements for four cationic copolymers of varying hydrophobicity in mixtures with anionic surfactant sodium dodecylsulfate (SDS). The purpose was to reveal the effect of polyion hydrophobicity on the association with surfactant at surfaces and in the bulk. All polyelectrolytes contained 20 wt % cationic units and had similar molecular weights. An increase in surfactant concentration by the stepwise addition of SDS to a dilute polyelectrolyte solution led to an increase in both the adsorbed amount and bulk turbidity, starting at a defined SDS concentration, as a result of the formation of insoluble polyion-surfactant ion complexes. At some higher SDS concentration, the formed aggregates started to redissolve gradually in the bulk and desorb from the surface because of the overcharging of the complexes. The SDS concentration at which the maxima in adsorption and turbidity occurred increased with decreasing polyion hydrophobicity; the more hydrophobic polyions bound excess SDS more readily, and the aggregates therefore redissolved at a lower SDS concentration. The adsorption from polyelectrolyte-SDS solutions, premixed at SDS concentrations above the adsorption maximum, which occurred on diluting the solution by “rinsing” the ellipsometer cuvette with 1 mM NaCl, was also investigated. On dilution, both the turbidity and the adsorbed amount increased as the excess surfactant in the polyion-surfactant ion complexes progressively decreased. More efficient deposition was achieved if the initial SDS concentration was close to the adsorption maximum. The latter situation could be achieved either by adjusting the SDS concentration or, at a fixed SDS concentration, by choosing a polyion with the appropriate hydrophobicity.

I. Introduction The interaction of ionic surfactants with oppositely charged polymers typically gives rise to an associative phase separation where a concentrated precipitate (solid) or a coacervate (liquid) of surfactant-polymer complexes separates out from a dilute solution. The details of the phase separation depend on parameters such as the molecular weight, flexibility, charge density, and hydrophobicity of the polyion and on the self-assembly of the surfactant ion.1-3 For many but not all polyion-surfactant ion pairs, a further increase in the surfactant concentration eventually leads to a second cooperative binding step at a second critical association concentration, cac(2), and a concomitant redissolution of the complexes.4 The present study focuses on the consequences of the hydrophobic interaction between the polyion and the surfactant, which is thought to be essential for the second cooperative binding step at cac(2).4 In common applied formulations such as hair-care products, fabric detergents, and pharmaceutical products, associating mix*Corresponding author. Present address: Biomedical Laboratory Science & Biomedical Technology, Faculty of Health and Society, Malm€o University, Malm€o SE- 205 06, Sweden. E-mail: [email protected]. (1) Thalberg, K.; Lindman, B. Polymer-Surfactant Interactions: Recent Developments. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D.,Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 203. (2) Langevin, D. Adv. Colloid Interface Sci. 2009, 147-148, 170–177. (3) Claesson, P. M.; Dedinaite, A.; Meszaros, R.; Varga, I. In Colloid Stability and Application in Pharmacy; Colloids and Interface Science Series; Tadros,T. F., Ed.; Wiley-VCH: Weinheim, Germany, 2007; Vol. 3, pp 337-395. (4) Lynch, I.; Sjostrom, J.; Piculell, L. J. Phys. Chem. B 2005, 109, 4258–4262.

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tures of cationic polymers and anionic surfactants are essential ingredients.5-7 The deposition of the complexes formed by such oppositely charged pairs at the solid-liquid interface plays a key role in these applications, and a close correlation between bulk association/phase separation and efficient surface deposition is typically assumed. Indeed, such a correlation is implied by the experimental findings.8,9 However, a number of issues that are important both fundamentally and in applications remain to be resolved regarding the deposition of polyion-surfactant ion complexes at surfaces. These include a molecular understanding of the quantitative aspects of deposition, such as optimizing the amount of deposited material and the mixing range where the material deposits. Another challenging issue concerns the typical observation of history-dependent nonequilibrium deposition processes.10-19 Moreover, the interactions between polyions (5) Marchioretto, S.; Blakely, J. SOFW J. 1997, 123, 811–818. (6) Rodrı´ guez, R.; Alvarez-Lorenzo, C.; Concheiro, A. Eur. J. Pharm. Sci. 2003, 20, 429–438. (7) Bronich, T. K.; Nehls, A.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Colloids Surf., B 1999, 16, 243–251. (8) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson, O. Adv. Colloid Interface Sci. 1996, 63, 1–21. (9) Anthony, O.; Marques, C. M.; Richetti, P. Langmuir 1998, 14, 6086–6095. (10) Dedinaite, A.; Claesson, P. M.; Bergstrom, M. Langmuir 2000, 16(), 5257– 5266. (11) Shubin, V. Langmuir 1994, 10, 1093–1100. (12) Terada, E.; Samoshina, Y.; Nylander, T.; Lindman, B. Langmuir 2004, 20, 1753–1762. (13) Terada, E.; Samoshina, Y.; Nylander, T.; Lindman, B. Langmuir 2004, 20, 6692–6701. (14) Rojas, O. J.; Claesson, P. M.; Berglund, K. D.; Tilton, R. D. Langmuir 2004, 20, 3221–3230.

Published on Web 03/24/2010

DOI: 10.1021/la1003353

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and surfactant ions at interfaces are controlled not only by the conditions of the bulk solution (pH, ionic strength, temperature, etc.) but also by the properties of the surface such as its surface charge density and hydrophobicity.10,20-22 A polyion-surfactant ion formulation as used in hair care and fabric care normally has an initial free surfactant concentration that is much higher than cac(2) so that the polyion-surfactant complexes contain a sufficient excess of surfactant to be soluble. Phase separation is then achieved by the dilution inherent in the different steps in the washing process. The dilution leads to a decrease in the free surfactant concentration and a concomitant decrease in the surfactant ion content in the complex, which eventually leads to a loss of solubility and, typically, an increase in surface deposition. In a series of joint studies in our laboratories, we systematically investigated surface deposition from aqueous mixtures of oppositely charged polymers and surfactants. In the first of these studies,23 Svensson et al. investigated the adsorption and phase behavior of two cationically modified polysaccharides that are widely used in applications, namely, cationic guar (cat-guar) and hydroxyethyl cellulose (cat-HEC), when mixed with anionic surfactants. The study demonstrated a strong correlation between bulk phase separation (turbidity increase) and the maxima in the adsorbed amount on hydrophilic silica and showed that cat-guar produced phase separation over a wider range of surfactant concentration and was less prone to bind excess anionic surfactant compared to cat-HEC. Further studies showed that this difference in excess surfactant binding also had important quantitative consequences for surface deposition accomplished by dilution.24 The proposed molecular explanation of the observed differences was that cat-guar is less hydrophobic than cat-HEC. These findings inspired us to conduct a more systematic study on the effect of polyion hydrophobicity on adsorption in relation to the phase behavior of the polyion-anionic surfactant system. For this purpose, here we use a series of synthetic cationic polyions with similar charge and molecular weight but with varying hydrophobicity. In situ ellipsometry allows us to follow the adsorption/deposition as well as the effect of diluting the solution. The obtained results are correlated with the bulk phase behavior, as obtained by simple turbidity measurements. The focus of this study is to provide a mechanistic understanding of the effect of dilution on deposition. In spite of its practical importance, this phenomena is not very well understood.

II. Experimental Section Materials. Surface Preparation. The substrates for the adsorption studies were silicon wafers (p-type, boron-doped, resistivity 1-20 Ω cm) that had been oxidized in an oxygen atmosphere at 920 °C for about 1 h (giving an oxide layer (15) Berglund, K. D.; Przybycien, T. M.; Tilton, R. D. Langmuir 2003, 19, 2705– 2713. (16) Braem, A. D.; Biggs, S.; Prieve, D. C.; Tilton, R. D. Langmuir 2003, 19, 2736–2744. (17) Penfold, J.; Tucker, I.; Staples, E.; Thomas, R. K. Langmuir 2004, 20, 7177– 7182. (18) Penfold, J.; Tucker, I.; Thomas, R. K. Langmuir 2005, 21, 11757–11764. (19) Tonigold, K.; Varga, I.; Nylander, T.; Campbell, R. A. Langmuir 2009, 25, 4036–4046. (20) Dedinaite, A.; Claesson, P. M. Langmuir 2000, 16, 1951–1959. (21) J€onsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; Wiley & Sons Ltd: Chichester, U.K., 1998. (22) Samoshina, Y.; Nylander, T.; Shubin, V.; Bauer, R.; Eskilsson, K. Langmuir 2005, 21, 5872–5881. (23) Svensson, A. V.; Huang, L.; Johnson, E. S.; Nylander, T.; Piculell, L. ACS Appl. Mater. Interfaces 2009, 1, 2431–2442. (24) Svensson, A. V.; Johnson, E. S.; Nylander, T.; Piculell, L. ACS Appl. Mater. Interfaces 2010, 2, 143–156.

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Table 1. Polyelectrolyte Characteristicsa polymers

Mw (g/mol)

nonionic/cationic molar ratio

polydispersity Mw/Mn

AA/MAPTAC 699 000 92:8 8.8 HEA/MAPTAC 935 000 88:12 5.9 AMP/MAPTAC 625 000 86:14 3.2 HPA/DMAM 525 000 83:17 2.2 a Data obtained at Procter & Gamble. Molecular weight averages were obtained by gel permeation chromatography using poly(ethylene oxide) standards for calibration.

thickness of about 300 A˚, annealed and cooled under a flow of argon and cut into slides with dimensions of 30 mm 12 mm; Bo Thuner, Department of Chemistry, IFM, Link€ oping University, Sweden). These slides were cleaned in 25% NH3 (pro analysi, Merck), 30% H2O2 (pro analysi, Merck), and H2O (1:1:5 by volume) at 80 °C for 5 min, followed by a mixture of 32% HCl (pro analysi, Merck), 30% H2O2 (pro analysi, Merck), and H2O (1:1:5 by volume) at 80 °C for 10 min as described earlier.25 The cleaned surfaces were then stored in ethanol (99.5%). Prior to the experiments, the surfaces were dried under a flow of nitrogen and cleaned with rf plasma for 5 min (Harrick Scientific Corp., model PDC-3XG) in residual air at 0.03 mbar at a power of 30 W. The hydrophobized silica surfaces were prepared by gas-phase silanization. The cleaned and plasma-treated wafers were exposed to dimethyloctylchlorosilane (purity >95%, Sigma-Aldrich) and allowed to react overnight under vacuum at room temperature. The hydrophobized silica surfaces were then sonicated for 20 min first in tetrahydrofuran and then in ethanol for a total of three cycles and finally stored in ethanol. These surfaces exhibited a water contact angle of 96 ( 2° (measured by the sessile drop method with a goniometer from Kruss). Chemicals. Acrylamide/methacrylamidopropyl trimethylammonium chloride (AA/MAPTAC), hydroxyethyl acrylate/methacrylamidopropyl trimethylammonium chloride (HEA/MAPTAC), acrylomorpholine/methacrylamidopropyl trimethylammonium chloride (AMP/MAPTAC), and hydroxypropyl acrylate/dimethylaminoethyl methacrylate (HPA/DMAM) copolymers were synthesized using the following procedure. A flask was charged with argon and equipped with an overhead stirrer, a heating mantle, and a thermometer. Water (600 mL, 50 °C) was added to the flask, followed by the addition of the cationic monomer (e.g., DMAM), 2 N HCl (38.5 mL), and neutral monomer (e.g.,HPA). 2,20 -Azobis(2-methylpropionamidine) dihydrochloride (0.30 g, 0.001 mol) as a 10 wt %/volume solution (3 mL) was added to the reaction mixture. The contents of the flask were kept warm and were allowed to mix for 18 h. The cooled reaction mixtures yielded the desired polymer solution. The monomer weight ratio at synthesis was 80:20 (neutral/ cation) for all copolymers. NMR measurements did not detect any residual unreacted monomers after synthesis. This means that the mass/charge ratio was the same for all MAPTAC-containing polyions (925 g/equiv) and slightly lower for HPA/DMAM (786 g/equiv). However, as a consequence of the different molar masses of the various neutral units, the molar ratio of neutral/ cationic units and therefore also the average distance between charges along the polymer chain varied for the copolymers (Table 1). Results from molecular weight characterizations by gel permeation chromatography are also given in Table 1, and the chemical structures of the polyions are given in Figure 1. The polyions were obtained as stock solutions at a concentration of around 6.5% and were used without purification. The polyions were dissolved in 1 mM NaCl to a concentration of 1000 ppm. The surfactant used was SDS from BDH (critical micelle concentration in water at room temperature of 8.3 mM).

(25) Tiberg, F. J. Chem. Soc., Faraday Trans. 1996, 92, 531–538.

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Figure 1. Structures of the investigated polyions. (a) AA/MAPTAC, (b) HEA/MAPTAC, (c) AMP/MAPTAC, and (d) HPA/DMAM.

Experimental Setup. Turbidity Measurements. The turbidity of the bulk solutions was determined by absorbance measurements in the visible-light region (λ = 500 nm). Polystyrene cuvettes with a path length of 1 cm were used. The conditions in the cuvette during the turbidity measurements were similar to those in the ellipsometry measurements, with the subsequent addition of surfactant to a polymer solution and stirring between measurements. The concentration of NaCl was 1 mM. The absorbance was measured 5 min after the surfactant addition and repeated after 10, 20, and 30 min of stirring. The temperature during the measurements was 25-30 °C. Representative experiments from duplicate measurements are shown. The deviations from the mean were less than 3%. Ellipsometry. The adsorbed amount and adsorbed layer thickness of the surfactants onto the hydrophobic silica surfaces were measured in situ by null ellipsometry. Ellipsometry is an optical technique based on the fact that polarized light changes its state of polarization when reflected against a surface. In null ellipsometry, the polarizer and analyzer positions corresponding to the minimum transmission of light to the detector are measured and the corresponding values Δ and Ψ are the relative phase shift and amplitude change, respectively, that characterize the change in state of polarization of the reflected light. Because measurements are made every 3 s, the method allows dynamic studies of film growth/removal with a time resolution that is relevant for many processes. A modified, automated Rudolph thin film ellipsometer, type 43 603-200E (Rudolph Research, Fairfield, N.J.) was used, with polarized xenon light of wavelength 4015 A˚, incident at an angle of 68.23° to a plane normal to the surface. A detailed description of the theory, instrumentation, and applications of ellipsometry is given by Azzam and Bashara.26 The relative errors in refractive index and thickness are high for small adsorbed amounts (1 mg/m2. The relative error in the adsorbed amount is much smaller, 15% at 0.1 mg/m2 and less than 1% for 1 mg/m2.25 The ellipsometer angles, Δ and Ψ, were measured in two different media, air and solvent, as described by Tiberg and Landgren.27 In this way, the refractive index of the silicon wafer and the thickness and refractive index of the silicon oxide layer can be determined. After adding the polyelectrolytesurfactant solution, the refractive index and thickness of the adsorbed film can be calculated from the ellipsometer angles if one assumes an optical model with four homogeneous layers with planar interfaces, namely, the solvent, the film, the silicon oxide layer, and the silicon wafer. In practice, the thickness obtained from the ellipsometer measurements represents an average thickness and not the full extension of, say, a layer of expanded polymer molecules adsorbed on the surface. The mass per surface (26) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; NorthHolland: Amsterdam, 1977. (27) Tiberg, F.; Landgren, M. Langmuir 1993, 9, 927–932.

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area (Γ) was then calculated from the following equation: Γðmg=m2 Þ ¼

df ðnf - n0 Þ dn=dc

ð1Þ

where df is the thickness of the film (A˚), nf is the refractive index of the film, n0 is the refractive index of the solvent in the film, and dn/ dc is the refractive index increment as a function of the bulk concentration. A dn/dc value of 0.15 has been used in this study for the polymers and the surfactant.11,12 The hydrophilic silica surfaces are negatively charged in water because of the deprotonation of the Si-OH groups. The surface charge density of silica at pH 6.5 in 1 mM electrolyte solution (KCl) has been determined to be ca. -0.3 μC/cm2.22 This corresponds to a molar quantity of anionic charges of ca. 3  10-9 mequiv/cm2 or a surface area of 5000 A˚2/charge. When hydrophobic silica surfaces were used, ethanol was pumped through the cuvette before the aqueous solution was added to reduce air/gas trapped on the surfaces. After characterization of the silica surface in air and aqueous solution, the adsorption experiments were started by adding 0.5 mL of a 1000 ppm polymer solution to the cuvette filled with 4.5 mL of 1 mM NaCl, resulting in a polymer concentration in the cuvette of 100 ppm. The solution in the cuvette was agitated with a magnetic stirrer at a stirring speed of 700 rpm. The increase in SDS concentration was achieved by the stepwise addition of a small volume of a 10, 100, or 500 mM (as appropriate) SDS stock solution. In the experiments where the effect of the dilution of the polymer/surfactant mixture was studied, 0.5 mL of a premixed polymer-surfactant solution containing 1000 ppm polymer and varying concentrations of surfactant was added to a cuvette filled with 4.5 mL of 1 mM NaCl. Dilution was achieved by “rinsing” with 1 mM NaCl, using a circuit with tubes in and out of the cuvette. The pumping rate during rinsing was approximately 5 mL/min. Representative experiments from duplicate measurements are shown. Deviations from the mean were less than 10% for the adsorbed amount and less than 3% for the adsorbed layer thickness.

III. Results Polyion Adsorption at Hydrophobized Silica. A convenient way to assess the relative hydrophobicities in a set of polycations is to compare their adsorption to that of hydrophobized silica.23 Because the mass per unit charge is the same for the polyions used in the present work except for HPA/DMAM, one may assume that it is mainly the difference in hydrophobicity that controls possible differences in the extent of adsorption. The adsorbed amount at the plateau level for the different polyions is shown in Figure 2, indicating that the hydrophobicity of the polyions follows the order HPA/ DMAM>AMP/MAPTAC=HEA/MAPTAC > AA/MAPTAC. DOI: 10.1021/la1003353

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Figure 2. Adsorbed amount (a) and thickness of the adsorbed layer (b) at hydrophobic silica for HPA/DMAM (), AMP/MAPTAC (0), HEA/MAPTAC (O), and AA/MAPTAC (4) as a function of time. The polyelectrolyte concentration was 100 ppm.

This conclusion is supported by the fact that the adsorption of the various polyions at nonmodified hydrophilic silica follows the reverse trend. (See the next section.) The deduced order of decreasing hydrophobicity would also be inferred from the chemical structures of the neutral co-monomers (Figure 1), and in the following text, we will simply refer to this order as the order of decreasing hydrophobicity of the polyions. The thickness data (Figure 2b) indicate that polyions HPA/ DMAM, AMP/MAPTAC, and HEA/MAPTAC adsorb flatly on the surface. The low adsorbed amount of AA/MAPTAC makes it difficult to resolve the thickness and refractive index of the film independently, and a large amount of scatter in the thickness data was observed (not shown), which makes it hazardous to draw any conclusion regarding the layer structure of this polyion. Sequential Addition of Surfactant Ions to Polyions. A convenient, efficient protocol for studying either the phase behavior or the surface adsorption of polyion-surfactant ion mixtures at different ratios of surfactant ion to polyion is the stepwise addition of surfactant to a dilute polyion solution in a “titration experiment”. The results of such titration experiments are shown in Figure 3. For some polyions, the solutions appeared to be slightly turbid at an SDS concentration of 0.01 mM (Figure 3a), indicating some polyion-surfactant association. At an SDS concentration of around 0.1 mM, the turbidity of the HPA/DMAM solution increased sharply, indicating massive aggregation of the polyion-DS complexes. For the less hydrophobic polyions, the initial increase in turbidity was more gradual and the sharp increase came at higher SDS concentrations. The polyions gave turbidity maxima at increasing SDS concentrations in the order HPA/DMAM (0.2 mM) < AMP/MAPTAC = HEA/MAPTAC (0.4 mM) < AA/MAPTAC (2 mM). At even higher SDS concentrations, the turbidity decreased because of the dissolution of the aggregates, and at surfactant concentrations well above the maximum, the turbidity leveled off to a value that was similar for all of the polyions but higher than that of the surfactant-free polyelectrolyte solution. Turbidity measurements were performed after 5, 10, 20, and 30 min, and we noted that the turbidity decreased with time at conditions corresponding to the recorded maximum in turbidity (Figure 3a). This decrease could 9360 DOI: 10.1021/la1003353

not be attributed to a redissolution of the complexes. Rather, a gradual coarsening of the phase-separated particles was observed so that the system changed from a uniformly turbid dispersion to a suspension of macroscopic flocs in a transparent medium. The interfacial behavior of the polyions upon sequential SDS addition is presented in Figure 3b,c. All of the polyions adsorbed readily to the hydrophilic silica (Figure 3b), but the variation in the adsorbed amount among the polyions was much less pronounced than for the adsorption at hydrophobized silica (Figure 2). The largest adsorbed amount was obtained for AA/ MAPTAC (1.4 mg/m2) and AMP/MAPTAC (1.3 mg/m2), followed by HEA/MAPTAC (1 mg/m2) and finally HPA/DMAM (0.85 mg/m2) (Figure 3b). As pointed out above, this trend is the reverse of that observed in Figure 2a, that is, the adsorbed amount on hydrophilic silica decreases with increasing polyion hydrophobicity. With the addition of SDS, a very sharp increase in the adsorbed amount was observed at a surfactant concentration that increased with decreasing polyion hydrophobicity. Further addition of SDS eventually led to a decrease in the adsorbed amount (maximum in adsorbed amount versus SDS concentration). For each polyion, this maximum was reached at SDS concentrations slightly higher than the corresponding turbidity maximum, except for AA/MAPTAC where the maxima in turbidity and the adsorbed amount coincided. Except for AA/MAPTAC, the thickness of the adsorbed layer of polyions in the absence of SDS was very small, indicating that the polyions adsorbed in a flat configuration (Figure 3c). Again, with the exception of AA/MAPTAC, the thickness of the adsorbed layer showed a local maximum at an SDS concentration corresponding to the adsorption maximum (Figure 3c). A further increase in the SDS concentration caused an initial decrease in the thickness. Above ca. 1 mM SDS, there was a sudden increase in thickness, followed by a leveling off at higher SDS concentrations. As for AA/MAPTAC in the present investigation, no thickness maximum was observed in similar titration experiments when SDS was added to a solution of cat-guar or cat-HEC in contact with hydrophilic silica.23 A comparison with the data on adsorbed amounts for the various systems (Figure 3b) immediately suggests that the maximum in thickness appears only when the peak in the adsorbed amount is very pronounced (high and narrow). Langmuir 2010, 26(12), 9357–9367

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Figure 3. Turbidity in the bulk (a), adsorbed amount at hydrophilic silica (b), and thickness of the adsorbed layer (c) for AMP/MAPTAC (0), HEA/MAPTAC (O), HPA/DMAM (), and AA/MAPTAC (4) as functions of the SDS concentration. The polyelectrolyte concentration was 100 ppm, and SDS was added in steps.

Effect of Dilution on Association in Solution and at Surfaces. Premixed mixtures of polyion and SDS at different SDS concentrations were prepared to study dilution-induced phase separation and surface deposition, as described in the Introduction. The dilution of the mixture by a gradual rinsing of the ellipsometer cuvette with 1 mM NaCl was followed by ellipsometry (Experimental Section). Similar dilution experiments were performed while monitoring the turbidity. Turbidity Measurements. Turbidity measurements (Supporting Information) confirmed that a phase separation of an initially monophasic mixture at high surfactant concentration could be induced by dilution. Generally, the turbidity varied in a nonmonotonic fashion on dilution, with an increase followed by a decrease as the surfactant concentration decreased. Naturally, the maximum turbidity was lower if the initial mixture had a higher surfactant concentration because more NaCl solution then had to be added before the region of phase separation was reached. Again, a decrease in the turbidity with time was observed especially for compositions in the vicinity of the turbidity maximum. When solutions of different polyions, for instance, AA/ MAPTAC and HEA/MAPTAC, were diluted from the same Langmuir 2010, 26(12), 9357–9367

initial SDS concentration, the solution with the more hydrophobic polyion (HEA/MAPTAC) had to be diluted the most before phase separation occurred and the turbidity increased. This is consistent with the results in Figure 3a. Adsorption at a Hydrophilic Surface. Adsorption from the polyelectrolyte-SDS mixtures on rinsing was followed by ellipsometry; see the Experimental Section. A premixed polyelectrolyte-SDS solution containing excess SDS was injected into the cuvette, and after equilibration was achieved, rinsing with 1 mM NaCl was started (at time = 0). This gave rise to a dilution of the mixture in the cuvette. From the titration experiments in Figure 3, four different initial SDS concentrations, that is, 0.5, 2, 5, and 50 mM, were selected. Thus, the effects of rinsing could be monitored for different initial conditions ranging from samples close to the conditions of maximum turbidity or adsorption to samples with surfactant concentrations far above the redissolution boundary to the two-phase region. Three polyions were selected for these rather time-consuming experiments, including the most (HPA/DMAM) and the least (AA/MAPTAC) hydrophobic polyions and one intermediate (HEA/MAPTAC) polyion. For AA/MAPTAC, where the adsorption and turbidity maxima DOI: 10.1021/la1003353

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Figure 4. Adsorbed amount (O) and adsorbed layer thickness () as a function of time during rinsing experiments (see the text) for HEA/ MAPTAC (a) and HPA/DMAM (b). The initial concentrations were 100 ppm polyelectrolyte and 0.5 mM SDS. Rinsing with 1 mM NaCl started at t = 0.

Figure 5. The same as for Figure 4 but with an initial SDS concentration of 2 mM.

occurred at relatively high surfactant concentrations, only the two highest initial surfactant concentrations were used. In 0.5 mM SDS, both HEA/MAPTAC and HPA/DMAM complexes were adsorbed directly after injection in very high amounts (>3 mg/m2) (Figure 4). The adsorbed layer of HEA/ MAPTAC-DS was thicker, 181 A˚ compared to 66 A˚ for the HPA/ DMAM-DS complex. When the bulk solution was diluted by rinsing, the adsorbed amount ultimately decreased to the same value for both mixtures (0.73 mg/m2). However, the adsorbed layers rapidly swelled to a more expanded conformation, which then contracted very slowly during the rinsing process. The largest changes recorded by ellipsometry occurred during a transition interval that lasted for less than 10 min. The changes recorded during this transition interval were monotonic for HEA/MAPTAC, but a small transient maximum was observed in both the adsorbed amount and the layer thickness for HPA/DMAM. At a higher initial SDS concentration further away from the adsorption maxima (2 mM SDS), the initially adsorbed amount from the mixture with HEA/MAPTAC was much lower 9362 DOI: 10.1021/la1003353

(1 mg/m2, Figure 5a). The adsorbed amount after rinsing was almost the same as before rinsing, but a significant adsorption maximum occurred in the transition region. The layer expanded monotonically on rinsing, from 15 to 240 A˚. No initial adsorption to the silica surface was observed from the 2 mM SDS mixture with HPA/DMAM, but rinsing gave rise to an adsorption of 0.8 mg/m2 and the measured thickness of ca. 10 A˚ suggested a thin layer with a flat polyion conformation on the surface. It is noteworthy that although the mixtures of HPA/DMAM with 0.5 and 2 mM SDS gave the same adsorbed amount after rinsing, the thickness of the adsorbed layer was 1 order of magnitude larger for the mixture with 0.5 mM SDS (Figures 4 and 5b). The mixtures with 5 mM SDS are shown in Figure 6 for the three polyions. Here, the mixture with AA/MAPTAC was the only one that adsorbed at the silica surface before dilution. Rinsing induced a slight decrease in the adsorbed amount and an increase in the layer thickness after a pronounced peak in the adsorbed amount just after the start of the rinsing (Figure 6a). The mixtures with HEA/MAPTAC and HPA/DMAM gave Langmuir 2010, 26(12), 9357–9367

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Figure 6. Adsorbed amount (O) and adsorbed layer thickness () as a function of time during rinsing experiments (see the text) for HEA/ MAPTAC (a), HPA/DMAM (b), and HPA/DMAM (c). The initial concentrations were 100 ppm polyelectrolyte and 5 mM SDS. Rinsing with 1 mM NaCl started at t = 0.

surface depositions only after rinsing, with HEA/MAPTAC giving a slightly higher adsorbed amount (Figure 6b,c). In Figure 6c, the layer thickness obtained for the HPA/DMAM mixture is not shown because the low adsorbed amount (