Complexation between Sodium Poly(styrenesulfonate) and

Article pubs.acs.org/JPCB

Complexation between Sodium Poly(styrenesulfonate) and Alkyltrimethylammonium Bromides in the Presence of Dodecyl Maltoside Edit Fegyver† and Róbert Mészáros*,†,‡ †

Laboratory of Interfaces and Nanosized Systems, Institute of Chemistry, Eötvös Loránd University, 1117 Budapest, Pázmány Péter sétány 1/A, Hungary ‡ Department of Chemistry, University J. Selyeho, 945 01 Komárno, Slovakia S Supporting Information *

ABSTRACT: In the present paper, the impact of dodecyl maltoside (C 12 G 2 ) on the association of sodium poly(styrenesulfonate) (PSS) with dodecyl- and hexadecyltrimethylammonium bromides (DTAB and CTAB) was studied. A low amount of nonionic surfactant enhances the binding of the investigated cationic amphiphiles on PSS, reducing the cationic surfactant-to-polyanion ratio needed for charge neutralization and precipitation. This effect is more pronounced for DTAB than for CTAB due to the considerably higher free surfactant concentration of the former cationic amphiphile. The synergistic surfactant binding also affects the nonequilibrium features of PSS/CTAB association via enhancing the kinetically stable concentration range of overcharged polyion/surfactant nanoparticle dispersions. With increasing C12G2 concentration, however, an opposite effect of the uncharged additive dominates. Namely, the CTAB molecules are solubilized excessively into mixed surfactant micelles, which reduces the surface charge of the PSS/CTAB/C12G2 nanoparticles and thus destabilizes their dispersion. At appropriately large nonionic surfactant concentrations, the binding of CTAB is largely reduced, resulting in the redissolution of the precipitate. In contrast, neither the destabilization nor the resolubilization effects of the added dodecyl maltoside were observed for the PSS/DTAB system due to the much lower driving force of DTAB binding compared to CTAB. Our results clearly demonstrate that the alkyl chain length of the ionic amphiphile has a pronounced effect on both the equilibrium and nonequilibrium aspects of polyion/mixed surfactant complexation which might be further exploited in various next generation applications.

1. INTRODUCTION Multicomponent systems, which contain oppositely charged polyelectrolytes (P) and ionic surfactants (S) with nonionic additives, are the focus of many studies because of their potential applications in home and personal care products, cosmetics, drug delivery systems, etc.1−5 Such mixtures have complex phase properties which determine their utilization for practical formulations. Most previous studies dealt with the association of ionic and nonionic surfactants with oppositely charged macromolecules assuming thermodynamic equilibrium. An important finding of these investigations is that at an appropriate excess of the uncharged amphiphile the associative phase separation of the oppositely charged components diminishes.1 Dubin and co-workers established a simple physical model in which the conditions of precipitation were related to the electrostatic interaction between a charged sphere (mixed micelle) and a polyion. According to their approach, above a critical nonionic-to-ionic surfactant ratio (i.e., below a critical charge density of the mixed micelles) the polyelectrolyte/mixed © 2015 American Chemical Society

surfactant (Smix) interaction becomes negligible and thus phase separation does not occur.6−10 Though this model provides an elegant way to estimate the phase properties of polyion/mixed surfactant systems, it is limited to a high excess of the mixed micelles in relation to the polyelectrolyte molecules, where the changes in the chemical potentials of the two surfactants are ignored. An alternative explanation for the suppression of precipitation is given by Corbyn et al. based on their studies on DNA/tetradecyltrimethylammonium bromide (TTAB)/nonionic surfactant mixtures.11 The authors suggested thatvia neglecting the binding of the uncharged amphiphile and the mixed surfactant assemblies onto the DNA moleculesthe diminishing polyion/ionic surfactant association is attributable to the pronounced stripping of TTAB molecules from the DNA/TTAB complexes into the polyelectrolyte-free mixed micelles with increasing nonionic-to-ionic surfactant ratio.11 Received: February 5, 2015 Revised: March 19, 2015 Published: March 25, 2015 5336

DOI: 10.1021/acs.jpcb.5b01206 J. Phys. Chem. B 2015, 119, 5336−5346

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The Journal of Physical Chemistry B

potentiometric, calorimetric, and surface tension methods and fluorescence, dynamic light scattering (DLS), neutron reflectivity, and small angle X-ray scattering (SAXS) measurements.17−33 The scarce studies available on the impact of nonionic surfactant on PSS/CnTAB complexation34 also make the PSS/CnTAB/C12G2 mixtures suitable candidates for exploring the polyanion/mixed surfactant association. In addition to the features of equilibrium complexation, we also aim at studying the impact of nonionic surfactant on the nonequilibrium properties of polyion/mixed surfactant association in order to map the conditions of dispersion stability as a function of the composition of the mixtures. These kinds of investigations can shed new light on the dynamically arrested states in oppositely charged systems, and they can provide novel and simple routes for the preparation of stable polyion/ mixed surfactant nanoparticles with well-defined properties. Since the alkyl chain length of the ionic surfactant also plays an important role in the P/S association,35−37 in this paper we investigate the complexation of PSS with two homologous cationic surfactants, dodecyl- and cetyltrimethylammonium bromides (DTAB and CTAB, respectively), in the presence of dodecyl maltoside using fluorescence, DLS, turbidity, and electrophoretic mobility measurements. We interpret our results by the novel approach introduced in our recent studies15,16 and compare them with the characteristics of PDADMAC/SDS/C12G2 association.

Janiak, Piculell, and their co-workers proposed another interpretation for the suppression of associative phase separation in P/Smix systems.12−14 The authors investigated the true ternary phase diagrams of stoichiometric polyelectrolyte/ionic surfactant complex salts with CnEm type nonionic surfactants and water. They found that above a certain nonionic-to-ionic amphiphile ratio the insoluble complex salt can be resolubilized due to the largely increased hydrophilicity of the formed P/Smix complexes.12−14 In contrast to the previously mentioned studies, we have shown recently that the addition of n-dodecyl-β-D-maltoside (C12G2) can both enhance and suppress the association between poly(diallyldimethylammonium chloride) (PDADMAC) and sodium dodecyl sulfate (SDS).15,16 Based on these observations, a novel approach was proposed according to which the addition of an uncharged cosurfactant has two distinct impacts on the P/S association.16 At low nonionic-toionic surfactant ratios the synergistic binding of the two surfactants on the polycation is the major effect which results in a more extended precipitation composition range of PDADMAC and SDS. At higher nonionic-to-ionic surfactant ratios, however, another impact of the additive becomes dominant. Namely, the ionic surfactants are progressively solubilized within the polyion-free mixed micelles, which reduces considerably their activity and therefore their binding to the polyelectrolyte chains. Furthermore, in this latter composition range, the formed P/Smix assemblies are enriched in the uncharged amphiphile which enhances further their hydrophilicity and results in the redissolution of the concentrated polyion/mixed surfactant phase.16 This effect becomes more pronounced by increasing ionic strength due to the reduced critical micelle concentrations (cmc’s) of the ionic surfactants at large electrolyte concentrations. It has also been demonstrated that the nonionic cosurfactant considerably affects the nonequilibrium characteristics of polycation/SDS systems at low ionic strengths.15,16 In the two-phase composition range of these latter systems, charge stabilized dispersion of overcharged polycation/SDS nanophases can be prepared via the utilization of a rapid mixing procedure.15 The synergistic binding of the ionic and nonionic surfactant increases the kinetically stable composition range of polycation/SDS mixtures at low concentration of the uncharged amphiphiles.15 At higher nonionic-to-ionic surfactant ratios, however, an opposite effect is observed; i.e., the stripping of SDS from the PDADMAC/SDS assemblies into the mixed micelles leads to decreasing charge density of the polycation/ SDS nanoparticles, destabilizing their dispersion.16 Furthermore, the addition of a moderate amount of supporting electrolyte could also destabilize the PDADMAC/SDS/nonionic additive dispersions formed at high anionic surfactant-topolycation ratios.16 One of the major objectives of the present work was to explore whether the impact of nonionic surfactant addition on the equilibrium properties of polyanion/cationic surfactant systems can be explained similarly to the previously investigated polycation/SDS/nonionic surfactant systems. In order to be consistent with our previous studies, dodecyl maltoside was used as a nonionic cosurfactant, which is a biodegradable sugar surfactant with huge potential in various future applications. As model polyanion/cationic surfactant systems the aqueous mixtures of sodium poly(styrenesulfonate) (PSS) and alkyltrimethylammonium bromides (CnTAB) were chosen since the PSS/CnTAB association is well-studied among others with

2. EXPERIMENTAL SECTION 2.1. Materials. High molecular mass (1000 kDa) sodium poly(styrenesulfonate) (PSS) was purchased in powder form from Sigma-Aldrich. A 2 wt % stock solution was purified by a Sartorius Stedim Vivaflow 50 (regenerated cellulose based) filter membrane with a cutoff molecular weight of 100 kDa. The hexadecyltrimethylammonium bromide (CTAB, Sigma-Aldrich, ≥98.0%) and dodecyltrimethylammonium bromide samples (DTAB, Sigma-Aldrich, ∼99.0%) were recrystallized from a 1:1 benzene−acetone mixture. The cmc’s of DTAB and CTAB in water were found to be 15.20 and 0.92 mM at 25.0 ± 0.1 °C from conductivity measurements. n-Dodecyl-β-D-maltoside (C12G2, Amresco, 99.0%) was used without further purification, and its cmc was found to be 0.15 mM from surface tension measurements. For the steady-state-fluorescence measurements a pyrene probe (Sigma-Aldrich, ≥98.0%) was applied. Ultrapure water (Milli-Q) was used for the preparation of the solutions. 2.2. Solution Preparation. The final concentration of PSS was 100 mg·dm−3 in each case. 2.2.1. Stopped-Flow Mixing.38 Equal volumes of cationic surfactant and polyelectrolyte solutions (with double their intended final concentrations) were mixed by means of an Applied Photophysics (Model RX 1000) stopped-flow apparatus. This mixing method is highly efficient since the solutions are mixed within 10 ms. In the case of added C12G2 the initial PSS and CnTAB solutions contained the nonionic surfactant at the same concentration and were mixed by the stopped-flow-mixing apparatus. 2.2.2. Slow Mixing.39 A CnTAB/C12G2 solution was added drop by drop to a PSS/C12G2 solution in equal volumes under continuous stirring. Specifically, a drop of cationic surfactant (or CnTAB/C12G2) solution was added into the polyelectrolyte (or PSS/C12G2) solution, and stirred at 2000 rpm. After 1 or 2 s of stirring the following drop was added and this procedure was continued until the last drop of CnTAB/C12G2 solution was 5337


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The Journal of Physical Chemistry B added to the PSS/C12G2 solution. The nonionic surfactant concentration was the same for both solutions. 2.3. Methods. 2.3.1. Electrophoretic Mobility Measurements. The mean electrophoretic mobility (uζ) of the P/S complexes in the presence and absence of nonionic amphiphiles was determined at 25.0 ± 0.1 °C immediately after solution preparation using a Malvern Zetasizer Nano Z instrument. The apparatus uses the M3-PALS technique, which is a combination of laser Doppler velocimetry and phase analysis light scattering. In the case of concentrated surfactant solutions the detected uζ values are weighted averages of the mean mobility of the polyion/mixed surfactant complexes and that of the mixed bulk micelles. However, from laser Doppler electrophoresis experiments only these averaged mobility values can be unequivocally determined from the mean frequency shift between the scattered light and that of the reference beam. Therefore, in this concentration range only the sign of the mobility data is relevant in the discussion of the results.16 2.3.2. Turbidity Measurements. The turbidity was determined at 25.0 ± 0.1 °C from the transmittance (T) of the mixtures which was measured at 400 nm by a UV/vis (PerkinElmer Lambda 2) spectrophotometer. The turbidity is given as 100 − T%, and the measurements were carried out immediately as well as 1 week after solution preparation. 2.3.3. Steady-State-Fluorescence Measurements. Pyrene fluorescence spectra were measured on a Cary Eclipse spectrofluorometer. The excitation wavelength was 320 nm, and emissions were recorded in the 360−420 nm wavelength range. The pyrene concentration was 5 × 10−7 M in each sample to minimize the probe’s effect on the polyelectrolyte/ surfactant interaction. The samples were prepared as reported in ref 15. The characteristic I1/I3 ratio of the fluorescence intensities belonging to the first (372 nm) and third (385 nm) vibronic bands of the pyrene spectrum was used to monitor the micropolarity of the formed polyelectrolyte/surfactant complexes and/or surfactant micelles. Because of the PSS−pyrene interaction, the emission spectra of samples which contain PSS were corrected as follows: the spectra of undoped samples were subtracted from the spectra of pyrene-doped samples. 2.3.4. Dynamic Light Scattering (DLS) Measurements. The light scattering measurements were performed at θ = 90° scattering angle and at 25.0 ± 0.1 °C, immediately after the preparation of the systems, by means of Brookhaven equipment consisting of a BI-200SM goniometer system, a BI-9000AT digital correlator, and an argon ion laser operating at 488 nm wavelength. The experiments were carried out in the dilute surfactant concentration range, where the contribution of the mixed micelles is negligible. This was indicated by the unimodal size distribution of the PSS molecules and that of the PSS/ surfactant complexes derived on the basis of the CONTIN analysis of the normalized autocorrelation functions. The apparent mean hydrodynamic diameter (dH) of the polyion/ mixed surfactant complexes was determined on the basis of the second order cumulant expansion method and the Einstein− Stokes relation. Prior to the measurements, the solutions were filtered through 0.45 μm pore size membrane filters.

Figure 1. Fluorescence I1/I3 ratios of the (a) PSS/DTAB system and (b) PSS/CTAB system plotted against the appropriate ionic surfactant concentration in the absence and presence of C12G2 (0.1 mM), cPSS = 100 mg·dm−3.

100 mg·dm−3 PSS as well as in the absence and presence of 0.1 mM C12G2. In the absence of the nonionic additive, the I1/I3 ratio is roughly constant up to a critical aggregation concentration (cac) of the cationic amphiphiles (denoted by T1 in Figure 1) and then it starts to decrease due to the appearance of hydrophobic polyelectrolyte/surfactant nanoassemblies. With a further increase of the cationic surfactant concentration, I1/I3 levels off at values (≈1.34) close to the ones measured above the cmc of the pure surfactants (see Figure S1 of the Supporting Information) in the absence of PSS. The comparison of Figure 1 with Figure S1 in the Supporting Information reveals that the intermediate concentration range corresponding to the transition between the aqueous and hydrophobic environments of the pyrene probe is not as sharp as in the case of the cmc of pure surfactant solutions. However, the T1 data determined from Figure 1 (≈0.0100 and 0.0025 mM for DTAB and CTAB, respectively) are in good agreement with the reported cac values of earlier potentiometric and fluorescence studies on PSS/CnTAB systems.19−22 Some studies revealed that the PSS/CnTAB complexation is driven not only by the electrostatic and hydrophobic interactions related to the headgroup and tails of the cationic amphiphiles, but also by the specific interactions that originated from the penetration of the aromatic groups of the PSS into the polyanion/cationic surfactant complexes.17,30 The orders of magnitude lower critical aggregation concentration of the cationic surfactants on PSS compared to their cmc was also rationalized with this structure.23 In the presence of 0.1 mM dodecyl maltoside, the micropolarity of the pyrene probe changes similarly with the

3. RESULTS AND DISCUSSION 3.1. Binding Affinity of Cationic Surfactants in the Absence and Presence of C12G2. The interaction between PSS and the investigated surfactants is characterized in Figure 1 via plotting the I1/I3 ratio of the pyrene fluorescence spectrum against the concentrations of DTAB and CTAB, respectively, at 5338


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The Journal of Physical Chemistry B cationic surfactant concentration as in the case of the nonionic amphiphile-free mixtures. However, the I1/I3 ratios start to decrease from lower concentration of the cationic surfactants (T1 ≈ 0.0020 and 0.0012 mM for DTAB and CTAB, respectively) than in the absence of the sugar surfactant. According to additional fluorescence measurements, presented in Figure S1 in the Supporting Information, C12G2 does not form mixed micelles with DTAB or CTAB in the very low cationic surfactant concentration range. Furthermore, the interaction between PSS and C12G2 is also negligible (see Figure S2 of the Supporting Information). This means that, in the presence of 0.1 mM dodecyl maltoside, the detected T1 values signal the onset of the appearance of PSS/CnTAB/C12G2 mixed nanoassemblies. The reduction of the T1 values in the presence of dodecyl maltoside is primarily attributed to the enhanced binding affinity of both the ionic and nonionic surfactants to the PSS molecules. A similar impact of the nonionic surfactant was also observed recently in the polycation/anionic surfactant system of PDADMAC/SDS.15,16 The synergistic binding of the uncharged and charged amphiphiles can be explained analogously to the enhanced binding of the same types of surfactants on solid surfaces.40,41 Specifically, it was shown that the addition of the otherwise nonadsorbing dodecyl maltoside increases the adsorption of SDS on oppositely charged alumina surfaces. The synergistic binding or adsorption of the amphiphiles is driven by the hydrophobic interactions as well as by the attenuated repulsive interactions between the headgroups of the bound ionic surfactant.40,41 3.2. Association and Phase Properties in the Dilute Surfactant Concentration Range. 3.2.1. PSS/CTAB/C12G2 System. In Figure 2 the results of the DLS, electrophoretic mobility, and turbidity measurements of PSS/CTAB mixtures, prepared by the stopped-flow-mixing protocol at cPSS = 100 mg· dm−3, are shown as a function of CTAB concentration in the absence and presence of dodecyl maltoside. The inset of Figure 2a is the enlargement of the graph around the charge neutralization concentration range. Without added nonionic surfactant transparent mixtures can be prepared at low CTAB-to-PSS ratios. With increasing CTAB concentration the charges of the polyion are compensated by the bound surfactant ions, resulting in decreasing net charge and size of the PSS/CTAB assemblies and increasing turbidity of the mixtures. At a certain CTAB concentration electroneutral complexes are formed which aggregate and precipitation occurs. With a further increase of the ionic surfactant concentration, transparent mixtures of positively charged complexes with reduced mean size are formed by the applied ultrafast mixing procedure. In this composition range the apparent mean size of the particles is not dependent on the CTAB concentration. The addition of C12G2 does not affect considerably the shape of the mobility vs CTAB concentration curve. However, a small reduction in the value of the charge neutralization CTAB concentration can be observed (see inset of Figure 2a); i.e., the phase separation is observable from smaller CTAB concentrations. These results can be rationalized by taking into account the enhanced binding of CTAB to the PSS molecules with added C12G2 in accordance with the results of Figure 1b. The observed CTAB concentrations at zero mobility of the P/Smix complexes in the absence and presence of different amounts of C12G2 are shown in Table 1. As indicated by Table 1, the difference between the highest and lowest CTAB concentrations is only around 0.1 mM. This observation is

Figure 2. (a) Apparent mean hydrodynamic diameter (dH) and mean electrophoretic mobility (uζ) of the PSS/CTAB/C12G2 complexes and (b) turbidity (100 − T%) of the systems against the CTAB concentration at 100 mg·dm−3 PSS concentration. The mixtures were prepared via the stopped-flow-mixing protocol. The different symbols denote various fixed C12G2 concentrations: 0 (orange ■), 0.1 (green ○), and 0.3 mM (blue ●). The orange and blue striped areas indicate the composition range of precipitated systems without added nonionic surfactant as well as in the presence of 0.3 mM C12G2, respectively.

Table 1. Analytical CTAB Concentrations at Zero Electrophoretic Mobility of the PSS/Surfactant Complexes at Various Analytical C12G2 Concentrations 100 mg·dm−3 PSS C12G2 concn (mM) 0.00 0.10 0.30 0.60 1.00

CTAB concn at uζ = 0 (mM)a 0.54 0.50 0.46 0.44 0.45

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

a

Determined from third order polynomial regression of the mobility vs CTAB concentration data in the absence and presence of the nonionic surfactant.

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constant C12G2 concentrations (at cPSS = 100 mg·dm−3) and in the absence of the sugar surfactant.

similar to the impact of C12G2 on the PDADMAC/SDS association,15 where the maximum reduction of the charge neutralization SDS concentration was found to be ≈0.2 mM. According to Figure 2, another important effect of the added sugar surfactant is manifested in the increasing transparent CTAB concentration range of overcharged polyelectrolyte/ surfactant complexes with reduced apparent mean size. In the presence of CTAB excess, the formed P/Smix particles have mean diameters similar to those of the nonionic surfactant-free ones. As reported in ref 33, in that concentration range the application of the stopped-flow mixing leads to the formation of a colloidal dispersion of PSS/CTAB nanophases stabilized via the adsorption of CTAB on their surface. The addition of a small amount of C12G2 enhances the binding of CTAB onto PSS and thus the surface charge of the overcharged polyion/ surfactant nanoparticles at a given PSS/CTAB composition. This also means that the reduced precipitation concentration range in the presence of dodecyl maltoside is not related to the equilibrium phase properties but it is the consequence of the enhanced kinetically stable concentration range of PSS/CTAB/ C12G2 dispersions. This latter interpretation is also supported by the comparison of the effect of slow and stopped-flow mixing at the same compositions of the PSS/CTAB and PSS/CTAB/C12G2 systems. As shown in Figure 3 and Figure S3 in the Supporting

Figure 4. (a) Mean electrophoretic mobility (uζ) of the PSS/DTAB/ C12G2 complexes and (b) turbidity (100 − T%) of the systems against the DTAB concentration at 100 mg·dm−3 PSS concentration. The mixtures were prepared via the stopped-flow-mixing protocol. The different symbols denote various fixed C12G2 concentrations: 0.0 (green ■), 0.3 (orange ●), and 1.0 mM (blue ▲). The green, orange, and blue boxes indicate the composition ranges of precipitated systems without added nonionic surfactant as well as in the presence of 0.3 and 1.0 mM C12G2, respectively.

The variation of the mobility with DTAB concentration follows trends similar to those in the PSS/CTAB systems. In the presence of C12G2, the electrophoretic mobility increases sharply and reaches zero value at remarkably lower DTAB concentrations than in the nonionic surfactant-free mixtures. Consequently, the onset of precipitation is also shifted to significantly smaller DTAB concentrations. These results are also attributed to the synergistic binding of the two types of surfactants to the PSS molecules similarly to the PSS/CTAB/ C12G2 system. The charge neutralization DTAB concentrations at various dodecyl maltoside concentrations are shown in Table 2. As indicated by the data, in the presence of 1.0 mM C12G2, the charge neutralization of the PSS/mixed surfactant complexes occurs at ∼3.70 mM lower DTAB concentration than without the nonionic additive. This concentration shift is an order of magnitude higher than that for the PSS/CTAB system in the presence of the same concentration of dodecyl maltoside (see Table 1). These findings can be rationalized by taking into account the significantly lower hydrophobic driving force of DTAB binding to the PSS molecules as compared to CTAB. The upper limit of the reduction of the charge neutralization concentration of DTAB or CTABdue to the added nonionic amphiphileis

Figure 3. Turbidity of PSS/CTAB/C12G2 systems against the CTAB concentration at cC12G2 = 0.3 mM and at 100 mg·dm−3 PSS obtained via application of the stopped-flow-mixing (blue ▲) and slow-mixing (blue ○) protocols. The striped and filled boxes signal concentration ranges where precipitation occurs in the case of stopped-flow mixing and slow mixing, respectively.

Information, contrary to the ultrafast mixing procedure, via the application of the slow-mixing protocol precipitation is observable over a wide concentration range of CTAB excess in both the absence (Supporting Information, Figure S3) and the presence of the sugar surfactant. During the slow-mixing procedure, the irreversible coagulation of the PSS/mixed surfactant nanophases occurs before enough CTAB can adsorb on their surface and eventually an equilibrium macroscopic two-phase system is formed.33 The time dependent features of the systems made by the two kinds of mixing protocols are illustrated by the photos of Figure S4 of the Supporting Information. 3.2.2. PSS/DTAB/C12G2 System. The mean electrophoretic mobility of the PSS/DTAB/C12G2 complexes and the turbidity of the systems, made by stopped-flow mixing, are plotted against the DTAB concentration in Figure 4 at different 5340


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The Journal of Physical Chemistry B Table 2. Analytical DTAB Concentrations at Zero Electrophoretic Mobility of the PSS/Surfactant Complexes at Various Analytical C12G2 Concentrations 100 mg·dm−3 PSS C12G2 concn (mM) 0.00 0.15 0.30 1.00

DTAB concn at uζ = 0 (mM)a 4.59 3.26 2.19 0.88

± ± ± ±

0.25 0.25 0.25 0.15

a

Determined from second order polynomial regression of the mobility vs DTAB concentration data in the absence and presence of the nonionic surfactant.

Figure 5. Turbidity of the PSS/DTAB/C12G2 system against the DTAB concentration at cC12G2= 0.3 mM and at 100 mg·dm−3 PSS via application of the stopped-flow-mixing (blue ●) and slow-mixing (red ○) protocols. The blue and red striped boxes indicate the composition ranges of precipitated systems for stopped-flow mixing and slow mixing, respectively.

roughly equal to the equilibrium concentration of the free ionic surfactant, measured at zero mobility in the absence of the ζ=0 ζ=0 uncharged additive (cue,DTAB and c ue,CTAB ).15 According to the binding isotherms of DTAB and CTAB on PSS without added C12G2, shown in Figure S5 in the Supporting Information, the compensation of the PSS charges occurs at much lower free surfactant concentration for CTAB compared to DTAB, i.e. ζ=0 ζ=0 cue,CTAB ≅ 0.10 mM and cue,DTAB ≅ 3.90 mM, respectively. (The determination of the binding isotherms is based on the electrophoretic method of Mezei et al.,42 the details of which are summarized in the Supporting Information.) These concentration values are in good agreement with the observed maximum shift in the analytical DTAB and CTAB concentrations belonging to zero mobility of the PSS/mixed surfactant complexes. The results are also consistent with our previous study on PDADMAC/SDS/nonionic surfactant systems,15 where the maximum reduction in the analytical charge neutralization SDS concentration was also found to be commensurate with the free SDS concentration measured at zero mobility of the PDADMAC/SDS complexes (at the same polyelectrolyte concentration as well as in the absence of nonionic additives). Another interesting feature of Figure 4 is thatcontrary to the PSS/CTAB/C12G2 systemsthe electrophoretic mobility levels off at relatively low values. Furthermore, transparent mixtures of overcharged polyion/mixed surfactant assemblies cannot be prepared at DTAB excess even by the utilization of the stopped-flow-mixing method. Similarly, precipitation occurs in the same DTAB concentration range via application of the slow-mixing protocol as illustrated in Figure 5, where the turbidity vs DTAB concentration data are compared for the two mixing methods in the presence of 0.3 mM C12G2. Similar results were found at all investigated dodecyl maltoside concentrations as well as in the absence of C12G2. These observations are also related to the difference in the alkyl chain length of the cationic surfactants. The lower hydrophobic driving force of DTAB results in a smaller adsorbed amount of the cationic amphiphile on the surface of the formed neutral PSS/DTAB nanophases as compared to the PSS/CTAB system. This explains the less pronounced charge reversal in the case of PSS/DTAB mixtures even at high DTAB-to-PSS ratios. Thus, the adsorption of DTAB cannot charge up appropriately the polyelectrolyte/surfactant nanophases in order to electrostatically stabilize their dispersion; therefore, they coagulate irreversibly even in the case of the ultrafast mixing procedure. Furthermore, the synergistic binding of the nonionic and ionic surfactants cannot increase further the surface charge of the PSS/DTAB nanoparticles; therefore the aggregation of polyion/mixed surfactant nano-

phases (i.e., the precipitation of PSS/DTAB/C12G2 system) also cannot be prevented even at large excess of DTAB. 3.3. Impact of Nonionic Surfactant Excess. In order to study the impact of a larger amount of dodecyl maltoside, measurements were also carried out at various fixed CnTAB concentrations but over a much larger concentration range of C12G2 than in the previous sections. As it is shown, the results are significantly dependent on the cationic surfactant-to-PSS ratios, i.e., whether the polyion or the cationic amphiphile is in excess. 3.3.1. Mixtures at Low Cationic Surfactant-to-Polyanion Ratios. In this section the focus is on mixtures with those bulk concentrations of CnTAB where negatively charged PSS/ cationic surfactant nanoassemblies are formed in the absence of dodecyl maltoside. In Figure 6, the mobility and turbidity data of PSS/DTAB (Figure 6a,b) and PSS/CTAB mixtures (Figure 6c,d) prepared by the stopped-flow-mixing protocol are plotted against the C12G2 concentration. The turbidity values of the systems prepared by the slow-mixing protocol are also shown in Figure 6b,d. In both cases the PSS/CnTAB assemblies have almost the same mobility values (i.e., roughly the same net negative charge) in the absence of the nonionic surfactant. Upon addition of a small amount of maltoside, the charges of the PSS molecules are further compensated and then a charge inversion from negatively to positively charged complexes occurs in a very narrow C12G2 concentration range with a simultaneous sharp increase in the turbidity of the systems followed by precipitation. In the intermediate sugar surfactant concentration range, after a maximum the mobility starts to decrease again but precipitation is still observable. With a further increase of the C12G2 concentration, a second charge reversal can be detected (from positively to negatively charged polyion/ mixed surfactant complexes) and then the mobility values level off and transparent mixtures with very low turbidity can be prepared above 60 and 4 mM C12G2 for the investigated PSS/ DTAB and PSS/CTAB mixtures, respectively. As shown in Figure 6b,d, the turbidity data are not dependent on the applied preparation protocols. In section 3.1 it was shown that the presence of the nonionic surfactant can enhance the ionic surfactant binding to the polyelectrolyte chain. This effect is indicated by the sharp decrease of mobility with dodecyl maltoside addition and the 5341


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Figure 6. (a) Mean electrophoretic mobility of the PSS/mixed surfactant complexes (uζ) (blue ■) and (b) turbidity (100 − T%) of these systems made by stopped-flow mixing (blue ■) and slow mixing (orange ▲) against the C12G2 concentration at cPSS = 100 mg·dm−3 and cDTAB = 2.00 mM. (c) Mean electrophoretic mobility (uζ) (blue ■) of the PSS/mixed surfactant nanoassemblies and (d) turbidity (100 − T%) of the systems prepared by stopped-flow mixing (blue ■) and slow mixing (orange ▲) against the C12G2 concentration at cPSS = 100 mg·dm−3 and cCTAB = 0.46 mM. The orange boxes and blue striped areas indicate the precipitated composition ranges for slow mixing and stopped-flow mixing, respectively. The mobility data measured for slow mixing are not shown in (a) and (c) since they are approximately the same within experimental error as in the case of stopped-flow mixing.

observations on the PDADMAC/SDS/C12G2 mixtures, where similar mobility and turbidity vs nonionic surfactant concentration curves to the ones in Figure 6 were observed.16 Although the general behavior of the PSS/CTAB and PSS/ DTAB systems with added dodecyl maltoside is similar, there is a significant difference in the extension of the phase separation concentration range. This phenomenon can be connected to the different equilibrium concentrations of the free cationic surfactants with different alkyl chain lengths at the given PSS and CnTAB analytical concentrations in the absence of C12G2. The previous comparison of the binding isotherms of the cationic amphiphiles on PSS in Figure S5 in the Supporting Information revealed that, at zero mobility of the PSS/CnTAB complexes, the free cationic surfactant concentration is 1 order of magnitude higher for DTAB than for CTAB. This also means that at the investigated PSS and CnTAB compositions a considerably higher mixed micelle concentration (i.e., nonionicto-ionic surfactant ratio) is necessary to reduce largely enough the free DTAB concentration in order to redissolve the precipitate as compared to the PSS/CTAB mixtures. 3.3.2. Mixtures at High Cationic Surfactant-to-Polyanion Ratios. In this section, those cationic surfactant concentration ranges are investigated where positively charged PSS/cationic surfactant complexes are formed at the given polyelectrolyte concentration in the absence of dodecyl maltoside. In Figure 7 the mean electrophoretic mobility of the PSS/CTAB/C12G2 complexes and the turbidity of the mixtures made by stoppedflow mixing are shown as a function of the nonionic surfactant concentration at 0.6 and 2.0 mM CTAB, respectively. The

charge reversal of the assemblies. At higher C12G2-to-CnTAB ratios, however, an opposite impact of the nonionic amphiphile becomes dominant, which is manifested in the maximum of the mobility vs nonionic surfactant concentration curves and the second charge reversal of the polyion/mixed surfactant nanoassemblies. This latter observation is related to the increasing amount of free mixed micelles present in the system. The distribution of the different amphiphiles between the polyion bound mixed assemblies, the free mixed micelles, and the surfactant monomers is determined by the variation of the chemical potentials of the ionic and nonionic surfactants with the system composition. Due to the two-component pseudophase nature of the mixed micelles, with increasing nonionic-to-ionic surfactant ratio the chemical potential of the nonionic surfactant must increase whereas the chemical potential of the ionic surfactant decreases (either in the absence or in the presence of the polyelectrolyte). On one hand, this leads to the pronounced solubilization of the CnTAB molecules within the mixed micelles which results in a significant reduction of the free cationic surfactant concentration and its bound amount onto the PSS molecules. On the other hand, the C12G2-toCnTAB molar ratio of the polyion/mixed surfactant complexes and their hydrophilicity is also enhanced by increasing excess of the uncharged amphiphile. Thus, above a certain dodecyl maltoside concentration thermodynamically stable solutions of the PSS/CnTAB/C12G2 complexes are formed regardless of the experimental route of their preparation. These results are in line with recent 5342


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Figure 7. Mean electrophoretic mobility (uζ) (orange ●) of the PSS/CTAB/C12G2 complexes and turbidity (100 − T%) (green ▲) of the systems made by stopped-flow mixing against the concentration of dodecyl maltoside at cPSS = 100 mg·dm−3 and at (a) 0.6 and (b) 2.0 mM CTAB. The right column contains the schematic illustration of the variation of the bound amount of CTAB (β) on the PSS molecules as a function of the free CTAB concentration (ce,CTAB) in the absence and presence of C12G2. Those β values where the charges of the PSS molecules are neutralized (i.e., where uζ = 0) are denoted by dashed blue lines. The ce,CTAB values, belonging to the given PSS/CTAB composition in the absence and presence of the nonionic additive, are indicated by brown and red dashed lines, respectively. Finally, the red vertical and the red curved arrows show the changes in the values of β at low (+C12G2) and high (++C12G2) nonionic surfactant concentrations, respectively. The horizontal red arrows designate the reduction of ce,CTAB induced by the stripping into mixed micelles at high amount of the nonionic amphiphile (++C12G2).

expected variation of the free cationic surfactant concentration and the bound amount of CTAB on PSS in the absence and presence of C12G2 is also illustrated schematically in Figure 7. At the lower CTAB-to-PSS ratio in Figure 7a, only slightly overcharged polyanion/cationic surfactant complexes with low mobility are formed in the absence of the sugar surfactant. The addition of the nonionic surfactant has an interesting effect on the mobility; i.e., uζ first increases and then decreases as a function of C12G2 concentration. The turbidity changes in an opposite way in the same composition range; i.e., it decreases and then increases with increasing dodecyl maltoside concentration. With a further increase of the C 12 G 2 concentration, the mobility decreases further and charge reversal of the polyion/mixed surfactant complexes occurs simultaneously with the appearance of precipitation. Then the mobility values become more negative, and above a critical dodecyl maltoside concentration the turbidity falls to very low values within a narrow C12G2 concentration range. In Figure 7b, the initial mobility of the overcharged PSS/ CTAB complexes in the absence of the nonionic surfactant is significantly higher than in Figure 7a. The variation of the mobility and turbidity values with the C12G2 concentration is also smoother and more gradual. Specifically, the turbidity of the mixtures increases and the mobility of the PSS/CTAB/

C12G2 nanoassemblies reduces with increasing sugar surfactant concentration, and above a given C12G2 concentration precipitation occurs. At even higher dodecyl maltoside concentrations, charge inversion of the polyion/mixed surfactant complexes takes place, and above a critical C12G2 concentration the turbidity sharply decreases which is followed by the formation of transparent systems of negatively charged polyion/mixed surfactant complexes of low turbidity. These findings can be rationalized through the variation of the mixed surfactant binding onto the polyanion with the concentration of the different system components. According to Figure 2, at 0.6 mM CTAB and 100 mg·dm−3 PSS an unstable colloidal dispersion of slightly overcharged PSS/ CTAB nanoparticles is formed even via the application of the ultrafast stopped-flow-mixing procedure. With the addition of dodecyl maltoside, the positive charge of the PSS/CTAB/ C12G2 nanophases increases due to the synergistic binding of the two types of surfactant on the surface of the particles. In this concentration range, the variation of the free CTAB concentration is not significant. Thus, a PSS/CTAB/C12G2 dispersion with enhanced kinetic stability is formed by the ultrafast mixing procedure and consequently the turbidity of the mixtures decreases at low sugar surfactant concentrations. However, at larger C12G2 concentrations the stripping of the 5343


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The Journal of Physical Chemistry B cationic surfactant into the mixed micelles becomes dominant, which decreases considerably the free CTAB concentration. This leads to a reduced adsorbed amount of CTAB and thus decreasing charge of the PSS/CTAB/C12G2 nanoparticles, destabilizing their colloidal dispersion. At even higher dodecyl maltoside concentrations, the concentration of free CTAB monomers further reduces whereas the concentration of free nonionic surfactant molecules only moderately increases. These simultaneous changes in the chemical potential of the two amphiphiles will reduce the bound amount of the cationic surfactant on PSS below charge neutralization (as illustrated by the schematic binding isotherm belonging to Figure 7a) and also increase the molar ratio of the bound nonionic and ionic surfactants on the polyion. Therefore, above a critical dodecyl maltoside concentration the precipitates are resolubilized because of the increased hydrophilicity of the PSS/CTAB/ C12G2 complexes. In the case of the higher CTAB-to-PSS ratio in Figure 7b, a charge stabilized colloidal dispersion of polyanion/cationic surfactant nanoparticles is formed in the absence of dodecyl maltoside via the application of stopped-flow mixing. Since at 2.0 mM CTAB and 100 mg·dm−3 PSS concentration the PSS/ CTAB nanophases are considerably overcharged, the addition of C12G2 cannot increase further the charge of the polyion/ mixed surfactant nanoparticles as is demonstrated in the schematic CTAB binding isotherm in Figure 7b. Therefore, the turbidity and mobility data are not affected considerably at low dodecyl maltoside concentrations. With increasing C12G2 concentration, however, the stripping of CTAB from the polyion/mixed surfactant assemblies into the mixed micelles becomes more pronounced, whereas only modest changes in the chemical potential of dodecyl maltoside are expected. In the intermediate C12G2 concentration range, the amount of cationic surfactant adsorbed on the PSS/CTAB/C12G2 nanoparticles and thus their excess charge decreases, which leads to the coagulation of the particles. At higher dodecyl maltoside concentrations, the further reduction of the bound amount of CTAB on PSS and the enhanced nonionic-to-ionic surfactant ratio of the PSS/CTAB/C12G2 nanoassemblies lead to their redissolution. The thermodynamic stability of the PSS/CTAB/C12G2 mixtures was also tested via the utilization of the slow-mixing procedure in the same composition range as in the case of Figure 7. As shown in Figure S6 in the Supporting Information, the turbidity of the mixtures is not dependent on the solution preparation protocols at large dodecyl maltoside concentrations, where thermodynamically stable solutions are formed. However, below a critical C12G2 concentration the mixing methods affect largely the phase behavior since the application of slow mixing leads to precipitation over a very wide concentration range of C12G2, while in the same composition range the mixtures are trapped in the charge stabilized colloidal dispersion state if the stopped-flow-mixing procedure is applied. The time scale of precipitation (which was varied from a couple of seconds to days) is dependent on the mixing methods and the composition range of the mixtures. This finding is attributable to a number of different factors such as the extent and evolution of the charge of the polyion/mixed surfactant nanoparticles and that of the local inhomogeneities during mixing, but it could also be related to the exchange kinetics of the two kinds of surfactants between the different mixed surfactant assemblies present in the system.16

It is interesting to compare the impact of C 12 G 2 concentration on the PSS/CTAB and PDADMAC/SDS association in the presence of ionic surfactant excess. In that composition range the destabilization of PDADMAC/SDS dispersions, prepared by rapid mixing, was also observed via increasing amount of dodecyl maltoside due to the decreasing free SDS concentration and charge density of the PDADMAC/ SDS/C12G2 nanoparticles.16 It was also shown that by increasing the PDADMAC concentration the effect of mixing becomes more pronounced and the kinetically stable concentration range is reduced due to the largely increased aggregation rate of the polyion/mixed surfactant nanoparticles. In addition, it was found that the kinetically stable concentration range of PSS/CTAB dispersions decreases with increasing PSS concentration or (at a fixed monomer concentration) with decreasing molecular weight of the polyanion.33 Thus, it is likely that the concentration and/or the molecular weight of PSS affects similarly the stability of PSS/CTAB/C12G2 dispersions. In contrast to the PSS/CTAB/C12G2 system, resolubilization (i.e., the formation of an equilibrium one-phase system) was not observed for PDADMAC/SDS/C12G2 mixtures in ref 16, even at very large dodecyl maltoside concentrations (up to 300 mM).16 This result is attributable to the much higher concentration of free SDS molecules in the nonionic surfactant-free mixtures of ref 16 (formed at high SDS-toPDADMAC ratios) as compared to the free cationic surfactant concentration of the PSS/CTAB mixtures in Figure 7. Namely, in the former case a very large excess of C12G2 would be needed to reduce the bound amount of SDS on the PDADMAC molecules below charge neutralization. Finally, we note thatas shown in section 3.2.2in the case of the PSS/DTAB mixtures kinetically stable nanoparticle dispersions cannot be prepared at high DTAB-to-PSS ratios even by the application of the stopped-flow-mixing protocol and/or in the presence of dodecyl maltoside. Therefore, in this composition range one can only investigate the impact of C12G2 addition on the equilibrium phase properties of the system. In a separate series of experiments, some samples were prepared by the stopped-flow-mixing procedure containing 100 mg·dm−3 PSS and 10 mM DTAB at different constant concentrations of the nonionic additive. No redissolution was observed up to 200 mM C12G2. In the investigated composition range, the primary effect of the nonionic surfactant is also related to the stripping of DTAB into the mixed micelles. However, due to the high free DTAB concentration (without added dodecyl maltoside), an extremely large nonionic-to-ionic surfactant ratio would be needed for a significant reduction of the chemical potential and bound amount of DTAB, similarly to the PDADMAC/SDS/ C12G2 mixtures at high SDS-to-PDADMAC ratios.16 At these compositions resolubilization can be expected if an appropriate amount of electrolyte is added to the polyion/mixed surfactant mixtures due to the reduced cmc of the ionic amphiphiles at high ionic strengths.11,16 For instance, the precipitate, formed at cPSS = 100 mg·dm−3, 10 mM DTAB, and 100 mM C12G2 without added salt, was redissolved in 0.3 M NaCl. However, the systematic investigation of the effect of ionic strength was out of the scope of the present work.

4. CONCLUSIONS In the present work the impact of dodecyl maltoside on the PSS/CTAB and PSS/DTAB association was studied. Both of these polyion/mixed surfactant systems share some similar 5344



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features; however, there are also some marked differences between them. At low C12G2 concentrations the synergistic binding of the cationic and nonionic surfactants to the PSS molecules occurs, which will have two major consequences. First, it reduces the C n TAB-to-PSS ratio which is needed for the charge neutralization of the polyanion. Therefore, the equilibrium two-phase composition range becomes more extended, which is clearly observable via the application of the slow-mixing procedure. This effect is most pronounced for the PSS/ DTAB system, where the equilibrium free DTAB concentrationbelonging to the charge neutralization composition is an order of magnitude higher compared to the PSS/CTAB mixtures. Second, the enhanced mixed surfactant binding significantly affects the kinetic stability of the colloidal dispersion of overcharged PSS/CTAB nanophases prepared via the stopped-flow-mixing procedure. Specifically, the addition of a low amount of C12G2 enhances further the charge density of the PSS/CTAB nanoparticles, and thus increases the kinetically stable PSS/CTAB composition range, similarly to the earlier investigated PDADMAC/SDS mixtures. This latter effect of the added sugar surfactant is absent in the case of PSS/DTAB mixtures where charge stabilized dispersions cannot be prepared even at large DTAB excess and via the application of the stopped-flow-mixing procedure. This finding is due to the much lower driving force of DTAB adsorption on the surface of the PSS/DTAB nanophases compared to the PSS/CTAB system, resulting in a modest charge of the PSS/DTAB nanoparticles. At large excess of dodecyl maltoside another impact of the nonionic surfactant additive dominates, especially for the PSS/ CTAB system. Namely, the stripping of the ionic amphiphile from the P/Smix complexes into the mixed micelles reduces the bound amount of the cationic surfactant onto the PSS molecules. Therefore, the increasing C12G2 concentration destabilizes the colloidal dispersion of overcharged PSS/ CTAB nanoparticles due to their decreasing charge, similarly to the PDADMAC/SDS system.16 At even higher concentrations of the sugar surfactant, the precipitates can be resolubilized because of the largely reduced bound amount of CTAB and the enhanced nonionic-to-ionic surfactant molar ratio of the formed PSS/CTAB/C12G2 complexes. In contrast, the stripping of DTAB into mixed micelles is less significant compared to CTAB due to the much larger free DTAB concentration at high ionic surfactant-to-polyion ratios. Consequently, the formation and destabilization of PSS/ DTAB/C12G2 dispersions as well as the redissolution of the precipitates was not observed even up to very high concentrations of dodecyl maltoside. The above-presented results clearly demonstrate that the nonequilibrium character of oppositely charged polyelectrolyte/surfactant association significantly depends on the alkyl chain length of the ionic amphiphile, which can crucially affect the commercial applications of these systems. We have also shown that the addition of nonionic amphiphilesin combination with the application of well-defined solution preparation protocolsprovides a powerful way to manipulate the equilibrium and nonequilibrium characteristics of P/S mixtures.

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ASSOCIATED CONTENT

* Supporting Information S

Additional experimental details and graphs as discussed in the manuscript text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +36-1-372-2500/1906. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Hungarian Scientific Research Fund OTKA K 108646 as well as by the European Commission COST Action CM1101, which is gratefully acknowledged.

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REFERENCES

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