Effect of Linear Nonionic Polymer Additives on the Kinetic Stability of

Article pubs.acs.org/Langmuir

Effect of Linear Nonionic Polymer Additives on the Kinetic Stability of Dispersions of Poly(diallyldimethylammonium chloride)/Sodium Dodecylsulfate Nanoparticles Katalin Pojják, 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 S Supporting Information *

ABSTRACT: In this article, the impact of different neutral polymers on the kinetic stability of charge-stabilized poly(diallyldimethylammonium chloride) (PDADMAC)/sodium dodecylsulfate (SDS) colloidal dispersions is analyzed using dynamic light scattering, electrophoretic mobility, turbidity, and coagulation kinetics measurements. Poly(ethyleneoxide) (PEO), poly(vinylpyrrolidone) (PVP), and dextran of comparable molecular masses as well as a higher-molecular-weight dextran sample were tested as nonionic additives. The light scattering and mobility data indicate that the PEO and PVP molecules may adsorb on the surface of the PDADMAC/SDS nanoparticles formed in the presence of excess surfactant. The primary effect of these additives is manifested in enhanced coagulation of the PDADMAC/SDS nanoparticles due to bridging at lower polymer concentrations and depletion flocculation at higher polymer concentrations. These findings are in sharp contrast to the earlier published effect of the same nonionic polymers on the poly(ethyleneimine) (PEI)/SDS colloidal dispersions, which can be sterically stabilized at appropriate PEO or PVP concentrations. However, the adsorption of the investigated dextran samples is negligible on the PDADMAC/SDS nanoparticles. Therefore, dextran molecules may cause only depletion flocculation in the PDADMAC/SDS system in the vicinity of the critical overlap concentration. lyte12,13 but might increase by the addition of nonionic amphiphiles.17−19 Polymers are known to affect the stability of colloidal dispersions considerably. Depending on their hydrophobicity and structural and electric characteristics, macromolecules are proven to be versatile tools for tuning the aggregation of colloidal particles. Recently, the impact of different neutral homopolymers has been studied on the dispersions of PEI/SDS nanoparticles formed in excess surfactant.20,21 It was found that high-molecular-weight poly(ethyleneoxide) (PEO) and poly(vinylpyrrolidone) (PVP) may adsorb on the surface of the PEI/SDS nanoparticles in the composition range where the uncharged polymers do not associate with the anionic surfactant. In the presence of a small amount of adsorbed PEO or PVP, bridging flocculation among the PEI/SDS nanoparticles was observed. However, at high surface coverages of PEO and PVP, thick adsorbed polymer layers were formed on the nanoparticles, resulting in sterically stabilized dispersions, which could maintain their kinetic stability even at high ionic strength.20,21 In contrast, the investigated highmolecular-weight polysaccharide dextran could not adsorb considerably on the PEI/SDS nanoparticles and therefore did

1. INTRODUCTION Mixtures of oppositely charged polymers and surfactants have been the focus of numerous papers in the past few decades because of their extensive utilization in several industrial, biological, and personal care applications.1−8 Many studies discuss the features of such systems by assuming equilibrium association between the charged macromolecules and surfactants.1,3 However, polyelectrolyte (PE)/surfactant (S) systems might be trapped in nonequilibrium states under certain circumstances.9−16 For example, the formation of electrostatically stabilized PE/S colloidal dispersions was confirmed at high surfactant-to-polyelectrolyte ratios of mixtures of sodium dodecylsulfate (SDS) with various cationic polyelectrolytes (e.g., poly(ethyleneimine) (PEI),11,12 poly(vinylamine) (PVAm),9,13 and poly(diallyldimethylammonium chloride) (PDADMAC)13). Furthermore, charge-stabilized dispersions can also be prepared for the polyanion/cationic surfactant system of sodium poly(styrenesulfonate) and hexadecyltrimethylammonium bromide.14 The stability of these dispersions was ascribed to the electrostatic repulsion provided by the adsorption of excess surfactant ions on the surface of the PE/S nanoparticles. The kinetically stable composition range of these systems was found to depend largely on the efficacy of the applied mixing protocols.11,14 In subsequent studies, it has been shown that the stability of polyelectrolyte/surfactant dispersions diminishes in the presence of supporting electro© 2013 American Chemical Society

Received: June 8, 2013 Revised: July 16, 2013 Published: July 19, 2013 10077

dx.doi.org/10.1021/la4021542 | Langmuir 2013, 29, 10077−10086

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preparation methods (two-step-mixing-I and two-step-mixing-II protocols) were also applied where the final concentration of the given components was attained in two steps. The specifics of these mixing protocols will be discussed in section 3.3. 2.2.2. Mixing Method in the Presence of NaCl. In the case of added electrolyte, the final composition of the mixtures was attained in two steps. First, the PDADMAC/SDS/polymer additive mixtures were prepared by stopped-flow mixing. After 24 h, these solutions were mixed with 0.2 M NaCl in equal volumes with the stopped-flow method. (An example of how the final composition of the systems was attained can be found in a scheme in Figure S1 of the Supporting Information.) 2.2.3. Electrophoretic Mobility Measurements. To study the changes in the charged nature of the nanoparticles due to polymer addition, the mean electrophoretic mobility (uζ) of the PDADMAC/ SDS complexes in the absence and presence of uncharged polymer additives and inert electrolyte was determined at 25.0 ± 0.1 °C by means of a Malvern Zetasizer NanoZ instrument, which uses a combination of laser Doppler velocimetry and phase analysis light scattering in the technique called M3-PALS. The measurements were performed 24 h after mixing for the salt-free systems and immediately after mixing in the presence of inert electrolyte. The relative standard error of the uζ values was 5−10%. 2.2.4. Dynamic Light Scattering Measurements (DLS). The apparent mean hydrodynamic diameter (dH) of the PDADMAC/ SDS complexes in the presence and absence of polymer additives was determined with Brookhaven equipment consisting of a BI-200SM goniometer system and a BI-9000AT digital correlator. The measurements were carried out 24 h after solution preparation at a θ = 90° scattering angle and at 25.0 ± 0.1 °C. The light source was an argon ion laser (Omnichrome, model 543AP) operating at 488 nm wavelength and emitting vertically polarized light. The intensity− intensity time-correlation functions were measured (homodyne method) and then converted to the normalized electric field autocorrelation functions by means of the Siegert relation. The autocorrelation functions were analyzed by the second-order cumulant expansion and CONTIN methods. The investigated PDADMAC sample was found to be polydisperse, having a wide unimodal distribution. The addition of SDS did not significantly change the character of the size distribution as was indicated by the CONTIN analysis. Therefore, the apparent diffusion coefficient (Dapp) of the PDADMAC/SDS complexes in the presence and absence of uncharged additives was derived from the mean relaxation rate (Γ̅ (q), first cumulant)

not affect the kinetic stability of PEI/SDS dispersions significantly in the investigated composition range.21 The purpose of the present work is to investigate whether the above-described effects of neutral polymer addition on the PEI/SDS dispersion can be extended to other PE/S dispersions as well. For this aim, in this Article aqueous mixtures of poly(diallyldimethylammonium chloride) and sodium dodecylsulfate are investigated in the presence of poly(ethyleneoxide), poly(vinylpyrrolidone), and dextran. The focus is on the PDADMAC/SDS composition range where charge-stabilized colloidal dispersions can be prepared via the rapid mixing of the solution components. The changes in the bulk properties of the PDADMAC/SDS systems upon addition of uncharged polymer additives are characterized by means of electrophoretic mobility, dynamic light scattering, and turbidity measurements. The aggregation kinetics of the formed PDADMAC/SDS nanoparticles in the presence of salt is monitored as a function of the neutral polymer concentration by coagulation kinetics experiments. Finally, the observations are compared to the earlier reported results on PEI/SDS/neutral polymer mixtures.

2. MATERIALS AND METHODS 2.1. Materials. Poly(diallyldimethylammonium chloride) (PDADMAC, Mw = 400−500 kDa, Sigma-Aldrich) was purchased in the form of a 20 wt % aqueous solution. The stock solution used for solution preparation was purified via dialysis by making use of Amicon Ultracel regenerated cellulose filters with a cutoff molecular weight of 30 kDa. The sodium dodecylsulfate sample (SDS, Sigma-Aldrich) was recrystallized from a 1:1 benzene−ethanol mixture. According to electrical conductivity measurements, the critical micellization concentration (cmc) of SDS after purification was 8.2 mmol/dm3 at 25.0 ± 0.1 °C. The investigated neutral polymer additives were poly(ethyleneoxide) (PEO, Mw = 100 kDa, Sigma-Aldrich), poly(vinylpyrrolidone) (PVP, Mw = 160 kDa, Sigma-Aldrich), and dextran of different molecular masses (Mw1 = 100−200 kDa, Mw2 = 400 kDa, Sigma-Aldrich). The bulk radius data of the uncharged polymers and their critical overlap concentrations, determined on the basis of refs 22 and 23 from viscosity measurements, are summarized in Table 1. All

Table 1. Radius of Gyration (Rg) and Critical Overlap Concentration (c*) of the Applied Neutral Polymers Determined on the Basis of References 22 and 23 from the Intrinsic Viscosity of the Polymer Solutionsa PEO 100 kDa PVP 160 kDa dextran 100−200 kDa dextran 400 kDa

Rg (±1 nm)

c* (±2 g/dm3)

12 11 9 15

10 16 35 20

Dapp(q) =

Γ̅ (q) q2

(1)

where q is the scattering vector (q = (4πn/λo) sin(θ/2), n is the refractive index of the solution, and λo is the wavelength of the incident light). The apparent mean hydrodynamic diameter (dH) of the complexes was calculated from Dapp from the Einstein−Stokes equation

a

Details of the calculations can be found in the Supporting Information.

Dapp =

kBT 3πηdH

(2)

where kB is the Boltzmann constant, T is the absolute temperature, and η is the viscosity of the medium. Prior to the measurements, the mixtures were filtered through 0.45 μm pore-size membrane filters. The relative standard error of the dH data was 5%. 2.2.5. Coagulation Kinetics Measurements. The initial rate of coagulation of PDADMAC/SDS complexes as a function of neutral polymer concentration in 0.1 M NaCl was followed at 25.0 ± 0.1 °C via a Perkin-Elmer (Lambda 2) UV/vis spectrophotometer coupled to a stopped-flow mixing unit (Applied Photophysics Ltd., model RX 1000). NaCl (0.2 M) was added after 1 day in equal volumes to the PDADMAC/SDS/neutral polymer mixtures prepared by the stoppedflow method (Figure S1 of the Supporting Information), and the absorbance (Ab) versus time (t) curves were monitored at 480 nm immediately after the addition of salt.

uncharged polymer samples were used without further purification. The effect of salt on the kinetic stability of PDADMAC/SDS and PDADMAC/SDS/neutral polymer systems was studied in the presence of sodium chloride (NaCl, Sigma-Aldrich). During the experiments ultraclean Milli-Q water (Milli-Q Integral 3 system) was used for solution preparation. 2.2. Methods. 2.2.1. Mixing Methods in the Absence of Salt. A majority of the systems were prepared via the stopped-flow mixing method by mixing equal volumes of PDADMAC and SDS solutions using a stopped-flow apparatus (Applied Photophysics Ltd., model RX 1000). This method ensures the mixing of the components within 10 ms.11,14 In the case of added neutral polymer, both the PDADMAC and SDS solutions contained the nonionic polymer additive in equal amounts. At some compositions, two other types of solution 10078


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The initial rate of coagulation can be approximated by second-order kinetics in the case of monodisperse particles24 −

dN = kN 2 dt

(3)

where k is the absolute coagulation rate constant and N is the number of particles per unit volume. If the size of the particles is small compared to the wavelength of the light beam, then the experimental coagulation rate constant (kx) can be determined from the initial slope of the absorbance versus time curves via the Oster equation

⎡ dAb ⎤ = k x = 2lA′N0 2V0 2k ⎣⎢ dt ⎦⎥t ⇒ 0

(4)

where l is the optical path length and N0 and V0 denote the concentration and volume of the primary particles at the beginning of the coagulation process, respectively. A′ is a constant that depends on the wavelength of the incident beam and the refractive indices of the particles and the medium. The relative standard error of the kx values was around 10%. 2.2.6. Turbidimetry. The turbidity (100 − T %) of the PDADMAC/SDS systems in the presence and absence of uncharged additives was determined at 25.0 ± 0.1 °C from the transmittance (T) of the mixtures at 480 nm by making use of a Perkin-Elmer (Lambda 2) UV/vis spectrophotometer. (The small turbidity values of the polyelectrolyte-free nonionic polymer/SDS mixtures were subtracted from the turbidity data of the multicomponent PDADMAC/SDS/ neutral polymer mixtures at a given uncharged polymer and SDS concentration.) The measurements were performed 24 h as well as 8 days after solution preparation. The relative standard error of the turbidity measurements was 2%. 2.2.7. Conductivity Measurements. The interaction between SDS and the applied uncharged polymer additives was monitored by conductivity measurements. The electrical conductivity (G) of SDS and SDS/neutral polymer additive solutions was determined at 25.0 ± 0.1 °C. The results of these measurements can be found in Figure S3 of the Supporting Information. The relative standard error of the electrical conductivity values was 2%.

Figure 1. (a) Electrophoretic mobility (uζ) and (b) apparent mean hydrodynamic diameter (dH) of the PDADMAC/SDS complexes as a function of total SDS concentration in the absence (□) and presence of 2 g/dm3 dextran (▲), PEO (■), and PVP (○) of comparable molecular masses. The sparse area denotes the composition range where precipitated or highly turbid mixtures are formed both in the presence and absence of uncharged polymers. cPDADMAC = 500 mg/ dm3.

3. RESULTS AND DISCUSSION 3.1. PDADMAC/SDS Complexation in the Presence and Absence of Neutral Polymers. We begin our investigation with the characterization of the bulk properties of PDADMAC/SDS systems with and without uncharged additives. Figure 1a,b shows the electrophoretic mobility (uζ) and the apparent mean hydrodynamic diameter (dH) of PDADMAC/SDS nanoassemblies, respectively, as a function of the analytical surfactant concentration (cSDS) at a fixed polyelectrolyte concentration (cPDADMAC = 500 mg/dm3) in the absence and presence of 2 g/dm3 PEO, PVP, and dextran. The sparse area indicates the composition range where uζ and dH were not determined because precipitated or highly turbid systems were formed in both the presence and absence of uncharged additives. (Because the observed results for the two dextran samples are very similar, for the sake of clarity the mobility and DLS data of the larger-molecular-weight dextran sample (Mw = 400 kDa) were omitted from Figure 1.) First, we discuss the characteristics of the mobility versus SDS concentration functions. In the absence of polymer additive, because the binding of the surfactant anions gradually compensates the initial positive charge of the PDADMAC molecules, the formation of precipitates can be observed (sparse area). In excess surfactant, negatively charged complexes are formed. At this composition range of PDADMAC/SDS mixtures, the addition of dextran, PEO, and PVP results in the reduction of the mobility data (in

absolute value). This effect is less significant in the case of dextran; however, it is notable in the presence of PEO and PVP, where roughly constant and considerably reduced uζ values can be measured at an intermediate SDS concentration range of surfactant excess. At larger surfactant concentrations, the uζ versus cSDS functions show a breakpoint after which the mobility starts to increase gradually. The equilibrium free surfactant concentrations (ce,SDS) belonging to the breakpoints were determined with the help of the experimental binding isotherm data of SDS on PDADMAC (Figure S2 of the Supporting Information). The calculated ce,SDS values are in good agreement with the critical aggregation concentrations of the neutral polymer/SDS systems: cacPEO/SDS = 4.5 mM SDS and cacPVP/SDS = 3 mM SDS, respectively (determined from electrical conductivity measurements, performed in the presence of 2 g/dm3 PEO and PVP, see Figure S3a,b in the Supporting Information). Therefore, it can be concluded that the breakpoint of the mobility versus surfactant concentration curves is ascribable to the onset of PEO/SDS and PVP/SDS association. This behavior was observable for PEO and PVP in the recently investigated PEI/SDS systems as well.20,21 In the presence of dextran, the mobility data do not show a breakpoint 10079


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nonionic polymer additive20,21) assuming that the composition, mean size, and structure of the formed PDADMAC/SDS nanoparticles are not affected by the presence of the uncharged polymers. The average adsorbed layer thickness values calculated from DLS measurements were ddextran = 1.0 ± 0.5 ads PEO nm, dPVP = 4 ± 1 nm, and d = 7 ± 1 nm at both PDADMAC ads ads concentrations (the calculations were restricted to the SDS composition region, where the PEO/SDS or PVP/SDS interactions are negligible; see Table S1 in the Supporting Information). The approximated dads values are comparable to the error in the DLS measurements in the case of dextran, indicating that the adsorption of this polymer on the PDADMAC/SDS nanoparticles is negligible. Similar results were observed for the dextran sample with Mw = 400 kDa. Furthermore, the calculated adsorbed layer thickness values are also relatively small for PVP and PEO. 3.2. Impact of the Nonionic Polymer Concentration. We have shown in section 3.1 that the changes in the mobility values are indicative of the adsorption of the neutral polymers on the PDADMAC/SDS nanoparticles if the interaction between SDS and the uncharged additives is negligible. To monitor the variation in the adsorbed amount of the nonionic macromolecules, in Figure 2 the electrophoretic mobility of the

with increasing surfactant concentration because this polymer does not interact with SDS (See the electrical conductivity vs SDS concentration curves measured for pure SDS and in the presence of 2 g/dm3 dextran, respectively, in Figure S3c of the Supporting Information.) A similar impact of the investigated polymer additives on the mobility versus surfactant concentration graphs was observed at cPDADMAC = 50 mg/dm3 (Figure S4a of the Supporting Information). According to DLS and mobility measurements, none of the studied uncharged polymers interact considerably with the PDADMAC molecules in the absence of surfactant. Therefore, in the SDS concentration range where the nonionic polymer/ anionic surfactant association is negligible, the difference in the mobility values of the PDADMAC/SDS nanoparticles measured in the absence and presence of the neutral polymer additives is attributable to the binding of the uncharged polymer to these particles. It has been reported for PEI/SDS dispersions that the mobility reduction in the presence of PEO and PVP is due to the adsorption of these polymers on the surface of PEI/SDS nanoparticles, which have the same amount of bound SDS in both the absence and presence of the uncharged added polymers.20,21 Therefore, it can be presumed, that PEO and PVP also adsorb on the surface of the formed PDADMAC/SDS nanoparticles, which causes an outward shift in the shear plane of the double layer around these negatively charged particles and hence decreases their mobility similarly to that of other fuzzy nanoparticle dispersions.25,26 The adsorption of polymers can also be monitored by lightscattering measurements as demonstrated in Figure 1b. Exceeding the precipitated composition region, the apparent mean hydrodynamic diameter of the PDADMAC/SDS nanoassemblies decreases with increasing SDS concentration in the absence of additives. At high surfactant concentrations, the size of the nanoassemblies is reduced (compared to that of the SDSfree polyelectrolyte) whereas they have considerable negative charge according to the electrophoretic mobility data. This finding is consistent with the formation of charge-stabilized PDADMAC/SDS colloidal dispersions in excess of surfactant in accordance with recent studies.13 The kinetic stability of these dispersions is attributable to the adsorption of the dodecylsulfate anions on the surface of the PDADMAC/SDS nanoparticles. In the presence of dextran, PEO, and PVP, the variation of the apparent mean diameter with the surfactant concentration is similar to that of the neutral polymer-free systems; however, the measured dH values are somewhat larger, especially in the case of PEO. A similar effect of the added nonionic polymers on the mean size of the PDADMAC/SDS nanoparticles was found at cPDADMAC = 50 mg/dm3 as shown in Figure S4b of the Supporting Information. In principle, the reduced mobility and increased mean size of the PDADMAC/SDS nanoparticles might be explained by the increased viscosity of the medium resulting from the presence of the uncharged polymers. However, according to our viscosity measurements, in the applied nonionic polymer concentration range of the DLS and mobility measurements (cpolymer ≪ c*) the local viscosity of the medium around the nanoparticles is not affected significantly, similarly to other neutral-polymercoated nanoparticle systems.20,21,25,26 A rough approximation of the adsorbed dextran, PEO, and PVP layer thickness (dads) on the PDADMAC/SDS nanoparticles can be calculated from the apparent hydrodynamic diameter data (as half of the difference between the dH values of the nanoparticles obtained in the presence and absence of the

Figure 2. Electrophoretic mobility of the negatively charged PDADMAC/SDS nanoparticles as a function of dextran (▲), PEO (■), and PVP (○) concentrations 1 day after solution preparation at a fixed polyelectrolyte-to-surfactant ratio of cPDADMAC = 500 mg/dm3, cSDS = 5.25 mM.

PDADMAC/SDS nanoparticles is plotted as a function of the polymer additive concentration at a fixed polyelectrolyte-tosurfactant ratio in the charge-stabilized colloidal dispersion composition range, cPDADMAC = 500 mg/dm3 and cSDS = 5.25 mM. Figure S5 in the Supporting Information shows similar graphs at cPDADMAC = 50 mg/dm3, cSDS = 2.0 mM. Because the measured mobility values are identical within experimental error for the two dextran samples, only the results obtained in the presence of the smaller polysaccharide is presented in Figures 2 and S5. According to Figures S2 and S3, neither the dextran nor the PVP and PEO samples interact with the anionic surfactant at the given PDADMAC and SDS concentrations in Figures 2 and S5. The negligible nonionic polymer/SDS association was also supported by the observation that the conductivity values of these PDADMAC/SDS/nonionic polymer mixtures were independent of the concentration of added dextran, PVP, or PEO within experimental error. Therefore, the variation of the 10080


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mobility of the PDADMAC/SDS nanoparticles with the nonionic polymer concentration reflects the changes in the adsorbed amounts of these polymers on the surface of the polyelectrolyte/surfactant nanoparticles. For all investigated polymers, the electrophoretic mobility of the particles becomes less negative with increasing polymer concentration. The addition of PEO and PVP leads to a remarkable reduction in the mobility, whereas the impact of dextran is much smaller as a result of its low extent of adsorption on the PDADMAC/SDS nanoparticles. The effect of increasing the amount of uncharged additive on the stability of the PDADMAC/SDS dispersions is shown in Figure 3, where the turbidity of the PDADMAC/SDS systems

Figure 4. Effect of PVP concentration on the aggregation of PDADMAC/SDS nanoparticles. The mixtures on the top row were prepared without PVP. The systems in the middle and bottom rows contain 2 and 6 g/dm3 PVP, respectively. The mixtures were prepared by stopped-flow mixing in the presence of PVP. cPDADMAC = 500 mg/ dm3, and the concentration of SDS increases from left to right as follows: 3.00, 4.00, 4.25, 4.50, and 5.00 mM SDS.

PDADMAC/SDS mixtures are shown as a function of SDS concentration without neutral polymer as well as when 2 and 6 g/dm3 PVP is added to the mixtures (cPDADMAC = 500 mg/ dm3). It is clearly visible that the higher the concentration of PVP, the more extended the precipitated and/or highly turbid composition range becomes. Similar observations were made in the presence of the same amounts of PEO, whereas in accordance with the turbidity measurements in Figure 3 neither of the dextran samples affects the visual appearance of the PDADMAC/SDS mixtures at 2 or 6 g/dm 3 dextran concentration. On the basis of the results of Figures 1−4 and S4 and S5, it can be concluded that the adsorption of both dextran samples on the PDADMAC/SDS nanoparticles is very low and does not significantly influence the bulk behavior of PDADMAC/SDS mixtures in the investigated dextran concentration range except at cdextran > c*. The low adsorption of the high-molecular-weight dextran sample (Mw = 400 kDa) was observed for the PEI/SDS systems as well.21 However, all the above-presented results demonstrate that the effect of PEO and PVP addition on the PDADMAC/SDS dispersions significantly differs from the features of PEI/SDS/neutral polymer systems. Although an appropriate amount of adsorbed PEO or PVP could form thick adsorbed layers and thus sterically stabilize the PEI/SDS nanoparticles, the adsorption of these polymers on the PDADMAC/SDS nanoparticles cannot induce this stabilizing effect. On the contrary, exceeding a critical (and low) polymer concentration, enhanced aggregation can be observed in the PDADMAC/SDS system. High-molecular-weight nonionic polymers (under good solvent conditions) may induce flocculation in colloidal dispersions in two ways. On one hand, the adsorption of polymers in a small amount might induce bridging flocculation among the individual particles. Polymer bridging is quite well understood and has been thoroughly studied in many colloidal mixtures.27−29 However, a relatively high level of nonadsorbing or free polymer may also induce phase separation, whose effect

Figure 3. Turbidity (100 − T %) of the PDADMAC/SDS systems as a function of dextran Mw = 100−200 kDa (green ▲, green △) and Mw = 400 kDa (yellow solid ⧫, yellow ◊), PEO (red ■, red □), and PVP (blue ●, blue ○) concentrations at a fixed polyelectrolyte-to-surfactant ratio: cPDADMAC = 500 mg/dm3, cSDS = 5.25 mM. The solid and open symbols designate the turbidity of the same systems measured 24 h and 8 days after solution preparation, respectively. The red and blue sparse areas indicate the formation of precipitated or highly turbid systems in the presence of PEO and PVP, respectively.

is shown as a function of dextran, PEO, and PVP concentrations at cPDADMAC = 500 mg/dm3 and cSDS = 5.25 mM. The solid and open symbols designate measurements performed 24 h as well as 8 days after solution preparation, respectively. One day after solution preparation, the turbidity is low, and increases only slightly in time over a wide polymer concentration range in the case of both dextran samples. However, at polymer concentrations close to the critical overlap concentration of the dextran samples (c*, see Table 1), the observed turbidity values are much larger and increase in time, indicating pronounced aggregation in the system. However, with an increasing amount of PEO or PVP the turbidity of the PDADMAC/SDS systems markedly increases in time from very low polymer concentrations (such as 1 to 2 g/dm3). Above 12 g/dm3 PEO concentration as well as 30 g/dm3 PVP concentration, highly turbid and/or precipitated systems were formed after 24 h (which are denoted by the red and blue sparse areas, respectively). After 8 days, very high turbidity and/ or precipitation was observed from 10 g/dm3 PEO as well as 20 g/dm3 PVP concentration. These observations unanimously indicate that even a small amount of these latter polymers largely accelerates the aggregation of the PDADMAC/SDS nanoparticles. The enhancement of aggregation is also demonstrated by a photograph in Figure 4, where the 10081


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is designated as depletion flocculation.30−33 Depletion is an entropy effect that originates from the exclusion of the free polymer molecules from a narrow gap around the dispersed nanoparticles (the so-called depletion layer). The overlapping of two depletion layers generates an osmotic pressure gradient that induces attraction between the particles above a critical volume fraction, and phase separation into a colloid-enriched and a polymer-enriched phase occurs. Depending on the characteristics of the system in question, both bridging and depletion mechanisms might be present in dispersions. For example, McFarlane et al. have shown that the type of flocculation of cationically modified silica nanoparticles significantly depends on the applied polymer additive.34 They have found that the adsorption driving force of PEO is much weaker than that of PVP in the investigated system (based on isothermal titration calorimetry measurements). Accordingly, PEO molecules could only induce depletion flocculation in the dispersion; however, in the presence of increasing amounts of PVP, both bridging and depletion flocculation could be observed in the aqueous medium.34 In our experiments, the adsorption of dextran was found to be nearly negligible on the PDADMAC/SDS nanoparticles. Therefore, the observed aggregation of PDADMAC/SDS nanoparticles in the presence of this macromolecule in a larger amount (cdextran > c*) is presumably due to depletion flocculation. The depletion effect of dextran was not observed in the PEI/SDS system because the maximum applied dextran concentration was much lower than in the present study (cdextran ≪ c*). In the case of PEO and PVP, the DLS and mobility measurements indicate that these polymers adsorb on the surface of the PDADMAC/SDS nanoparticles. Furthermore, according to Figures 2 and 3 the addition of PEO and PVP largely accelerates the aggregation of the polyelectrolyte/ surfactant nanoparticles even at such low nonionic polymer concentrations where the adsorption is far from saturation. This finding suggests that bridging flocculation occurs at low and moderate PEO and PVP concentrations. At higher concentrations of PEO and PVP, however, the depletion mechanism is likely to be the dominant factor resulting from the large free polymer concentration and the increased level of adsorption, as in the case of PVP/cationically modified silica systems.34 However, it should be noted that according to the presented results only, it is not possible to draw a clear distinction between destabilization induced by bridging or depletion flocculation via the addition of PVP or PEO to the PDADMAC/SDS system. 3.3. Effect of Solution Preparation Methods. To explore further the differences in the flocculation mechanisms induced by the addition of various nonionic polymers, the impact of the solution preparation method and order of addition of the components was also studied for all investigated polymer additives. In Figure 5, the turbidity of the PDADMAC/SDS/uncharged polymer mixtures (24 h after their preparation) is plotted against the nonionic polymer concentration for the two dextran samples as well as for PEO and PVP, respectively. The final composition of the systems (e.g., the final concentration of PDADMAC, SDS, and the uncharged additive) was attained in three different ways. Apart from the stopped-flow mixing method applied for the preparation of all of the mixtures discussed so far throughout the Article, the mixtures were also prepared by two other types of mixing methods (two-step-mixing-I and two-step-mixing-II

Figure 5. Effect of mixing on the turbidity (100 − T %) of the PDADMAC/SDS systems (measured 1 day after solution preparation) in the presence of (a) the investigated dextran samples, (b) PEO, and (c) PVP at a fixed polyelectrolyte-to-surfactant ratio: cPDADMAC = 500 mg/dm3 and cSDS = 5.25 mM. The mixtures were prepared by stoppedflow mixing as well as by two-step mixing protocols, during which either the neutral polymer solution was added slowly to the PDADMAC/SDS mixture in equal volumes (two-step-mixing-I) or vice versa (two-step-mixing-II). The red and blue sparse areas denote precipitated and/or highly turbid mixtures formed by stopped-flowmixing and two-step-mixing-I protocols in the presence of PEO and PVP, respectively. The blue and brown boxes indicate the formation of precipitated systems via the two-step-mixing-II protocol in the presence of PEO and PVP, respectively.

protocols). First, equal volumes of PDADMAC and SDS solutions were mixed with the stopped-flow apparatus, but this time none of the solutions contained uncharged polymer 10082


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the application of stopped-flow mixing, precipitation occurs. However, salt-induced phase separation was found to be prevented in PEI/SDS dispersions if the adsorption of PEO or PVP could sterically stabilize the PEI/SDS nanoparticles.20,21 In this section of the Article, we discuss the kinetic stability of the PDADMAC/SDS dispersion in salt in the presence of the investigated uncharged polymers. In Figure 6, the electrophoretic mobility of the PDADMAC/ SDS complexes is shown in 0.1 M NaCl against the SDS

additive. These mixtures were left to stand for 24 h at room temperature. Next, to attain the final composition, in the twostep-mixing-I method the neutral polymer solutions were added to equal volumes of PDADMAC/SDS mixtures in a dropwise manner under stirring with a magnetic stirrer. However, during the application of the two-step-mixing-II procedure, the order of addition was reversed: the PDADMAC/SDS mixtures were added to the uncharged polymer solutions in equal volumes drop by drop under continuous stirring. The preparation of the systems via the different mixing protocols is demonstrated in Figure S6 of the Supporting Information. Figure 5 indicates that both two-step mixing methods induce further aggregation of the PDADMAC/SDS particles even by the slow mixing of the polymer-free solution (water) and the PDADMAC/SDS dispersion. The aggregation is more pronounced when the dispersion is added to water (two-stepmixing-II protocol). These findings suggest limited stability of the PDADMAC/SDS dispersion at the given composition even though the PDADMAC/SDS nanoparticles are negatively charged. Figure 5 also reveals that the addition of dextran does not significantly affect the turbidity of the PDADMAC/ SDS systems for a given type of mixing method. Furthermore, none of the applied mixing procedures resulted in precipitation in the investigated concentration range of dextran samples (cdextran< c*). This observation is in agreement with the expectations that both dextran samples may cause only depletion flocculation in the PDADMAC/SDS system in the vicinity of their critical overlap concentrations. In the presence of PVP or PEO, the effect of mixing methods is much more pronounced than in the case of dextran. At low and intermediate PEO or PVP concentrations (0−10 g/dm3), the two-step-mixing-I protocol results in a similar trend but enhanced turbidity compared to those of stopped-flow mixing. However, by the application of two-step-mixing-II procedure, the turbidity values are remarkably higher even at small polymer additive concentrations, and massive precipitation can be observed from much lower PEO or PVP concentrations than by the application of the other two mixing protocols. This finding indicates that in contrast to the mixtures with added dextran, upon mixing a pronounced bridging between the polyelectrolyte/surfactant nanoparticles could occur when the PDADMAC/SDS solution is added slowly to the PVP or PEO solution even at low polymer concentrations. The observed impact of the solution preparation on the flocculation mechanism is complex but definitely reveals the distinguished role of the dynamics of the adsorbed polymer chains in the stability of the PDADMAC/SDS dispersions. Specifically, one may expect that bridging is more effective in the initial stage of adsorption where the fraction of the adsorbed train segments of the uncharged polymer is far below its equilibrium value.35,36 Furthermore, the high local nonionic polymer concentration can also initiate the formation of bridges between the PDADMAC/SDS nanoparticles in the initial stage of the two-step-mixing-II protocol. 3.4. Effect of Nonionic Additives on the Stability of the PDADMAC/SDS Dispersions in 0.1 M NaCl. The addition of inert electrolyte to electrostatically stabilized dispersions reduces their kinetic stability because the diminishing electrostatic repulsion between the particles induces coagulation in their dispersions. In accordance with this reasoning, in the presence of an appropriate amount of salt charge-stabilized PDADMAC/SDS or PEI/SDS dispersions cannot be prepared: even at a very large excess of SDS and via

Figure 6. Electrophoretic mobility of the PDADMAC/SDS complexes in 0.1 M NaCl without uncharged additive (□) as well as in the presence of 2 g/dm3 dextran (green ▲), PEO (red ■), and PVP (blue ●) immediately after mixing with salt. The sparse area indicates the composition range where precipitated or highly turbid systems are formed 2 h after solution preparation. cPDADMAC = 500 mg/dm3.

concentration in the absence and presence of dextran, PEO, and PVP (2 g/dm3) measured immediately after the addition of salt. As is indicated in the figure, in excess SDS negatively charged complexes are formed, the mobility of which decreases in absolute value in the presence of neutral polymers, especially by the addition of PEO or PVP, similar to the results in Figure 1a. This finding reveals that the adsorption of these latter two polymers on the PDADMAC/SDS nanoparticles is more pronounced compared to the adsorption of dextran even in 0.1 M NaCl. However, contrary to the salt-free PDADMAC/SDS systems and the PEI/SDS mixtures, 2 h after the addition of NaCl all mixtures above 2 mM SDS, with or without added neutral polymer, were very turbid and/or precipitated (sparse area of the graph). Similarly, even at larger concentrations of all investigated uncharged macromolecules the presence of 0.1 M NaCl induced precipitation in excess surfactant. These observations suggest that in the presence of salt the adsorption of PEO or PVP cannot retard the aggregation of the negatively charged PDADMAC/SDS nanoparticles. The initial rate of aggregation in 0.1 M NaCl with increasing polymer additive concentration was monitored by means of coagulation kinetics measurements at a fixed PDADMAC/SDS ratio: cPDADMAC = 500 mg/dm3, cSDS = 5.25 mM. The results are plotted in Figure 7a, where the experimental coagulation rate constant (kx) of PDADMAC/SDS nanoparticles is shown against the polymer additive concentration for the investigated uncharged macromolecules. (The results obtained in the presence of high-molecular-weight dextran are not shown for the sake of clarity.) In the presence of dextran, kx increases only slightly in the given nonionic polymer concentration range in a manner similar to that for the PEI/SDS system. This behavior 10083


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macromolecules, which could form bridges between the individual particles as the electrostatic repulsion between the particles decreased in the presence of salt. Therefore, in line with our previous results, bridging flocculation might explain the largely accelerated aggregation of PDADMAC/SDS nanoparticles in salt at low PEO or PVP concentration compared to that of the nonionic-polymer-free dispersion. As demonstrated in Figure 7b, the application of 5 g/dm3 PEO or PVP significantly reduces the aggregation rate of PEI/ SDS nanoparticles in salt as a result of the steric repulsion between the thick PEO or PVP adsorbed layers on the nanoparticles.21 Thus, in 0.1 M NaCl stable PEI/SDS dispersions could be prepared over a wide concentration range of excess surfactant.21 In contrast, a remarkable reduction of the initial aggregation rate is not observed in the PDADMAC/SDS dispersions in the investigated extended PEO and PVP concentration ranges, indicating that thick protective adsorbed layer cannot be developed on the surface of the PDADMAC/SDS nanoparticles. Accordingly, in 0.1 M NaCl precipitation occurs over the whole concentration range of excess SDS regardless of the presence or amount of uncharged additives (Figure 6). The only difference between the impact of dextran, PVP, or PEO is that faster precipitation of the PDADMAC/SDS mixtures occurs in the presence of the latter two polymers as a result of the accelerated aggregation caused by bridging. This difference in the initial rate of coagulation is also illustrated in the photographs in Figure S7 of the Supporting Information, where the visual appearance of the PDADMAC/SDS mixtures was monitored in time in 0.1 M NaCl as well as in the presence and absence of the investigated nonionic additives. All of our measurements indicate that PEO and PVP induce different changes in the kinetic stability of PDADMAC/SDS and PEI/SDS dispersions. This observation clearly reveals the marked deviation in the adsorption features of these nonionic polymers on the surface of the various polycation/SDS nanoparticles. The different adsorption behavior of PEO and PVP on the PEI/SDS and PDADMAC/SDS nanoparticles might be attributable to the different structure of the primary polyelectrolyte/surfactant nanoparticles. The PEI/SDS nanoparticles formed by the complexation of SDS and highly charged PEI are reported to be compact with an ordered lamellar inner structure.37 However, in the PDADMAC/SDS systems nanoparticles with ordered hexagonal internal structure are formed according to SAXS measurements.38 It is possible that the adsorption driving force of PEO and PVP is lower for the PDADMAC/SDS nanoparticles than for the PEI/SDS nanoparticles because of the different surface characteristics of these particles. One may also expect that the activation energy and kinetics of adsorption for a given uncharged polymer could be largely dependent on the interfacial structure of the polycation/SDS nanoparticles. However, further investigations are necessary to explore the detailed mechanism of nonionic polymer adsorption on the surfaces of various PE/S nanoparticles.

Figure 7. (a) Experimental coagulation rate constant (kx) of PDADMAC/SDS nanoparticles in the presence of 0.1 M NaCl as a function of dextran (green ▲), PEO (red ■), and PVP (blue ●) concentration at a fixed polyelectrolyte-to-surfactant ratio. cPDADMAC = 500 mg/dm3 and cSDS = 5.25 mM. The red and blue sparse areas indicate the composition ranges where highly turbid systems are formed before the addition of salt in the presence of PEO and PVP, respectively. (b) The experimental coagulation rate constant of the PEI/SDS nanoparticles as a function of the polymer additive concentration. The data were taken from the Supplementary Data in ref 21. cPEI = 0.025 wt %, cSDS =4.0 mM, cNaCl = 0.1 M, and pH 10.

is consistent with the negligible adsorption of dextran on the PDADMAC/SDS nanoparticles. (At very high dextran concentrations, the coagulation kinetics measurements cannot be carried out because of the depletion flocculation of the PDADMAC/SDS/dextran system before the addition of salt.) However, with the addition of PEO and PVP the experimental coagulation rate constant first remarkably increases and then starts to decrease slightly. Above a given PEO or PVP concentration, the second mixing step with 0.2 M NaCl could not be performed because the systems were very turbid and/or precipitated after the first mixing step; therefore, kx could not be determined. (The polymer additive composition range, where enhanced turbidity or precipitation was observed before mixing with salt, is denoted by the red and blue sparse areas for PEO and PVP, respectively.) The increase in the experimental coagulation rate constant in 0.1 M NaCl at low PEO and PVP concentrations was also observed for the dispersions of PEI/SDS nanoparticles (in the composition ranges where the SDS/uncharged polymer interaction is negligible), as illustrated in Figure 7b from ref 21. This behavior was attributed to the small number of adsorbed

4. CONCLUSIONS The effect of neutral polymers on the oppositely charged polyelectrolyte/surfactant association has been relatively unexplored. Recently, it was reported that high-molecular-weight nonionic polymers such as PEO and PVP could form thick adsorbed layers on the surfaces of PEI/SDS nanoparticles, resulting in sterically stabilized colloidal dispersions of these 10084


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(5) Santos, S.; Gustavsson, C.; Gudmundsson, C.; Linse, P.; Piculell, L. When Do Water-Insoluble Polyion−Surfactant Ion Complex Salts “Redissolve” by Added Excess Surfactant? Langmuir 2011, 27, 592− 603. (6) Shulevich, Y. V.; Petzold, G.; Navrotskii, A. V.; Novakov, I. A. Properties of Polyelectrolyte−Surfactant Complexes Obtained by Polymerization of an Ionic Monomer in a Solution of an Oppositely Charged Surfactant. Colloids Surf., A 2012, 415, 148−152. (7) Liu, J. Y.; Wang, J. G.; Li, N.; Zhao, H.; Zhou, H. J.; Sun, P. C.; Chen, T. H. Polyelectrolyte−Surfactant Complex as a Template for the Synthesis of Zeolites with Intracrystalline Mesopores. Langmuir 2012, 28, 8600−8607. (8) Hajduová, J.; Procházka, K.; Šlouf, M.; Angelov, B.; Mountrichas, G.; Pispas, S.; Štěpánek, M. Polyelectrolyte−Surfactant Complexes of Poly[3,5-bis(dimethylaminomethyl)-4-hydroxystyrene]-block-poly(ethylene oxide) and Sodium Dodecyl Sulfate: Anomalous SelfAssembly Behavior. Langmuir 2013, 29, 5443−5449. (9) Naderi, A.; Claesson, P. M. Association between Poly(vinylamine) and Sodium Dodecyl Sulfate: Effects of Mixing Protocol, Blending Procedure, and Salt Concentration. J. Dispersion Sci. Technol. 2005, 26, 329−340. (10) Naderi, A.; Claesson, P. M.; Bergstrom, M.; Dedinaite, A. Trapped Non-equilibrium States in Aqueous Solutions of Oppositely Charged Polyelectrolytes and Surfactants: Effects of Mixing Protocol and Salt Concentration. Colloids Surf., A 2005, 253, 83−93. (11) Mezei, A.; Mészáros, R.; Varga, I.; Gilányi, T. Effect of Mixing on the Formation of Complexes of Hyperbranched Cationic Polyelectrolytes and Anionic Surfactants. Langmuir 2007, 23, 4237− 4247. (12) Mezei, A.; Á brahám, Á .; Pojják, K.; Mészáros, R. The Impact of Electrolyte on the Aggregation of the Complexes of Hyperbranched Poly(ethyleneimine) and Sodium Dodecyl Sulfate. Langmuir 2009, 25, 7304−7312. (13) Á brahám, Á .; Mezei, A.; Mészáros, R. The Effect of Salt on the Association between Linear Cationic Polyelectrolytes and Sodium Dodecyl Sulfate. Soft Matter 2009, 5, 3718−3726. (14) Pojják, K.; Bertalanits, E.; Mészáros, R. Effect of Salt on the Equilibrium and Nonequilibrium Features of Polyelectrolyte/Surfactant Association. Langmuir 2011, 27, 9139−9147. (15) Li, D.; Kelkar, M. S.; Wagner, N. J. Phase Behavior and Molecular Thermodynamics of Coacervation in Oppositely Charged Polyelectrolyte/Surfactant Systems: A Cationic Polymer JR 400 and Anionic Surfactant SDS Mixture. Langmuir 2012, 28, 10348−10362. (16) Štěpánek, M.; Hajduová, J.; Procházka, K.; Šlouf, M.; Nebesárǒ vá, J.; Mountrichas, G.; Mantzaridis, C.; Pispas, S. Association of Poly(4-hydroxystyrene)-block-poly(ethylene oxide) in Aqueous Solutions: Block Copolymer Nanoparticles with Intermixed Blocks. Langmuir 2012, 28, 307−313. (17) Mezei, A.; Mészáros, R. Novel Nanocomplexes of Hyperbranched Poly(ethyleneimine), Sodium Dodecyl Sulfate and Dodecyl Maltoside. Soft Matter 2008, 4, 586−592. (18) Pojják, K.; Mészáros, R. Preparation of Stable Electroneutral Nanoparticles of Sodium Dodecyl Sulfate and Branched Poly(ethylenimine) in the Presence of Pluronic F108 Copolymer. Langmuir 2011, 27, 14797−14806. (19) Janiak, J.; Bayati, S.; Galantini, L.; Pavel, N. V.; Schillén, K. Nanoparticles with a Bicontinuous Cubic Internal Structure Formed by Cationic and Nonionic Surfactants and an Anionic Polyelectrolyte. Langmuir 2012, 28, 16536−16546. (20) Pojják, K.; Mészáros, R. Novel Self-Assemblies of Oppositely Charged Polyelectrolytes and Surfactants in the Presence of Neutral Polymer. Langmuir 2009, 25, 13336−13339. (21) Pojják, K.; Mészáros, R. Association between Branched Poly(ethyleneimine) and Sodium Dodecyl Sulfate in the Presence of Neutral Polymers. J. Colloid Interface Sci. 2011, 355, 410−416. (22) Rhodes, S. K.; Lambeth, R. H.; Gonzales, J.; Moore, J. S.; Lewis, J. A. Cationic Comb Polymer Superdispersants for Colloidal Silica Suspensions. Langmuir 2009, 25, 6787−6792.

nanoparticles. In this Article, we have shown that the impact of PEO and PVP on the stability of the PDADMAC/SDS dispersions significantly differs from the observations on the PEI/SDS/neutral polymer systems. Although both PEO and PVP adsorb on the surfaces of the PDADMAC/SDS nanoparticles, the addition of these polymers does not result in steric stabilization but leads to enhanced aggregation of the PDADMAC/SDS nanoparticles even at low uncharged polymer concentrations. The observed aggregation in the presence of PEO or PVP becomes even more pronounced if the PDADMAC/SDS mixture is added slowly to the nonionic polymer stock solution as compared to the rapid homogenization of the mixtures at the same total composition. The accelerated aggregation of the PDADMAC/SDS nanoparticles is attributed to bridging at lower nonionic polymer concentrations and to depletion flocculation at higher nonionic polymer concentrations. The contrasting effect of PEO and PVP adsorption on the stability of PEI/SDS and PDADMAC/ SDS dispersions can be rationalized by taking into account the different structure and stability of the bare polyelectrolyte/ surfactant nanoparticles. Similar to the PEI/SDS system, the adsorption of dextran was found to be negligible on the surface of PDADMAC/SDS nanoparticles. In line with this finding, the dextran molecules do not affect the kinetic stability of PDADMAC/SDS dispersions at low and moderate concentrations; however, they induce depletion flocculation around their critical overlap concentration. Our results clearly demonstrate that the nonequilibrium association of oppositely charged polyelectrolytes and surfactants can be tuned by the addition of appropriate nonionic polymers. The supramolecular assemblies of charged macromolecules, ionic surfactants, and nonionic polymers might also be further exploited in novel personal care applications and they can serve as templates in the development of new mesoporous substances.

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ASSOCIATED CONTENT

S Supporting Information *

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

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Hungarian Scientific Research Fund (OTKA-K-81380), by TÁ MOP-4.2.2/B-10/1-2010-0030, and by European Commission COST Action CM1101, which are gratefully acknowledged.

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REFERENCES

(1) Goddard, E. D. Polymer-Surfactant Interaction Part II. Polymer and Surfactant of Opposite Charge. Colloids Surf. 1986, 19, 301−329. (2) Wei, Y. C.; Hudson, S. M. The Interaction between Polyelectrolytes and Surfactants of Opposite Charge. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1995, 35, 15−45. (3) Holmberg, K.; Jönsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution, 2nd ed; John Wiley & Sons: Chichester, U.K., 2002. (4) Norman, J.; Lynch, I.; Piculell, L. Phase Behavior of Aqueous Polyion-Surfactant Ion Complex Salts: Effects of Polyion Charge Density. J. Phys. Chem. B 2007, 111, 8402−8410. 10085


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(23) Krause, W. E.; Bellomo, E. G.; Colby, R. H. Rheology of Sodium Hyaluronate under Physiological Conditions. Biomacromolecules 2001, 2, 65−69. (24) Voisin, D.; Vincent, B. Flocculation in Mixtures of Cationic Polyelectrolytes and Anionic Surfactants. Adv. Colloid Interface Sci. 2003, 106, 1−22. (25) Gittings, M. R.; Saville, D. A. Electrophoretic Behavior of Bare and Polymer-Coated Latices. Langmuir 2000, 16, 6416−6421. (26) Hill, R. J.; Saville, D. A.; Russel, W. B. Electrophoresis of Spherical Polymer-Coated Colloidal Particles. J. Colloid Interface Sci. 2003, 258, 56−74. (27) Swenson, J.; Smalley, M. V.; Hatharasinghe, H. L. M. Mechanism and Strength of Polymer Bridging Flocculation. Phys. Rev. Lett. 1998, 81, 5840−5843. (28) Glover, S. M.; Yan, Y.; Jameson, G. J.; Biggs, S. Bridging Flocculation Studied by Light Scattering and Settling. Chem. Eng. J. 2000, 80, 3−12. (29) Bhosale, P. S.; Berg, J. C. The Dynamics of Polymer Bridge Formation and Disruption and Its Effect on the Bulk Rheology of Suspensions. Langmuir 2012, 28, 16807−16811. (30) Asakura, S.; Oosawa, F. On Interaction between Two Bodies Immersed in a Solution of Macromolecules. J. Chem. Phys. 1954, 22, 1255−1256. (31) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall: London, 1993. (32) Jenkins, P.; Snowden, M. Depletion Flocculation in Colloidal Dispersions. Adv. Colloid Interface Sci. 1996, 68, 57−96. (33) Tuinier, R.; Rieger, J.; de Kruif, C. G. Depletion-Induced Phase Separation in Colloid−Polymer Mixtures. Adv. Colloid Interface Sci. 2003, 103, 1−31. (34) McFarlane, N. L.; Wagner, N. J.; Kaler, E. W.; Lynch, M. L. Poly(ethylene oxide) (PEO) and Poly(vinyl pyrolidone) (PVP) Induce Different Changes in the Colloid Stability of Nanoparticles. Langmuir 2010, 26, 13823−13830. (35) de Gennes, P. G. New Trends in Physics and Physical Chemistry of Polymers; Lee, L. H., Ed.; Plenum: New York, 1989. (36) Mubarekyan, E.; Santore, M. M. Influence of Molecular Weight and Layer Age on Self-Exchange Kinetics for Saturated Layers of PEO in a Good Solvent. Macromolecules 2001, 34, 4978−4986. (37) Bastardo, L. A.; Garamus, V. M.; Bergström, M.; Claesson, P. M. The Structures of Complexes between Polyethylene Imine and Sodium Dodecyl Sulfate in D2O: A Scattering Study. J. Phys. Chem. B 2005, 109, 167−174. (38) Nizri, G.; Magdassi, S.; Schmidt, J.; Cohen, Y.; Talmon, Y. Microstructural Characterization of Micro- and Nanoparticles Formed by Polymer-Surfactant Interactions. Langmuir 2004, 20, 4380−4385.

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