and Sodium Dodecyl Sulfate - American Chemical Society

Apr 27, 2009 - The aggregation of the negatively charged complexes of hyperbranched poy(ethylenimine) (PEI) and sodium dodecyl sulfate (SDS) has been ...
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The Impact of Electrolyte on the Aggregation of the Complexes of Hyperbranched Poly(ethyleneimine) and Sodium Dodecyl Sulfate   Amalia Mezei, Agnes Abrah am, Katalin Pojjak, and Robert Meszaros* :: :: Laboratory of Interfaces and Nanosized Systems, Institute of Chemistry, Eotvos Lor and University, 1117 Budapest, P azm any P eter s et any 1/A, Hungary Received January 27, 2009. Revised Manuscript Received April 7, 2009 The aggregation of the negatively charged complexes of hyperbranched poy(ethylenimine) (PEI) and sodium dodecyl sulfate (SDS) has been investigated at different sodium chloride (NaCl) concentrations using coagulation kinetics, electrophoretic mobility and dynamic light scattering measurements. The observed variation of the initial rate of coagulation with NaCl concentration indicates the formation of kinetically stable colloid dispersions in the investigated composition and pH range. These dispersions are electrostatically stabilized due to the adsorption of excess dodecyl sulfate ions on the surface of the polyelectrolyte/surfactant particles. Because of the enhanced adsorption of the anionic surfactant, the kinetic stability of the PEI/SDS dispersions increases with increasing SDS concentration and decreasing pH. Finally, we rationalize the effect of salt on the phase behavior and surface properties of polyelectrolyte/surfactant mixtures in terms of the salt-induced aggregation features of polyelectrolyte/surfactant particles.

Introduction Ionic surfactants can strongly interact with oppositely charged polyelectrolytes because the electrostatic interaction between the surfactant and polyelectrolyte contributes to the hydrophobic interaction.1,2 The aqueous mixtures of these compounds are widely used in a variety of products such as hair conditioners, shampoos, coatings, cosmetics, and drug delivery systems.1 Because of their importance in applied as well as fundamental research, these systems have been the subject of intensive investigations over the last 2 decades.1-10 The complexation between oppositely charged macromolecules and amphiphiles results in a unique phase behavior of these mixtures.1-4 At low surfactant-to-polyelectrolyte ratios the mixtures are transparent. As the surfactant-to-polyelectrolyte ratio increases, the charges of the polyelectrolyte are compensated by the bound surfactants which results in associative phase separation. The associative term means that one of the formed phases is rich in both the polyelectrolyte and surfactant molecules while the other phase is a dilute solution.3,6 At even higher surfactantto-polyelectrolyte ratios, a charge reversal of the polyelectrolyte/ surfactant complexes occurs, and transparent systems might be formed again.2,3 These latter transparent mixtures are generally considered as one-phase systems according to various theoretical studies based on the classical framework of cooperative surfactant *Corresponding author. E-mail: [email protected]. (1) Goddard, E. D.; Ananthapadmanabhan, K. P. Interactions of Surfactants with Polymers and Proteins, 1st ed.; CRC Press: Boca Raton, FL, 1993; Chapter 4. (2) Kogej, K.; Skerjanz J. In Surfactant Science Series; Radeva, T., Ed.; Marcell Dekker Inc.: New York, 2001; Vol. 99, p 793. (3) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604–613. (4) Wei, Y. C.; Hudson, S. M. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1995, C35, 15–45. (5) Thalberg, K.; Lindman, B.; Bergfeldt, K. Langmuir 1991, 7, 2893–2898. (6) Bergfeldt, K.; Piculell, L.; Linse, P. J. Phys. Chem. 1996, 100, 3680–3687. (7) Ilekti, P.; Piculell, L.; Tournilhac, F.; Cabane, B. J. Phys. Chem. B 1998, 102, 344–351. (8) Rojas, O. J.; Claesson, P. M.; Berglund, D.; Tilton, R. D. Langmuir 2004, 20, 3221–3230. (9) Campbell, R. A.; Ash, P. A.; Bain, C. D. Langmuir 2007, 23, 3242–3253. (10) Matsuda, T.; Annaka, M. Langmuir 2008, 24, 5707–5713. (11) Hansson, P. Langmuir 2001, 17, 4167–4180.

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binding.1-4,11-13 The thermodynamic stability of this phase is explained by the considerable net charge of the individual polyelectrolyte/surfactant complexes due to the excess charges of the bound surfactant micelles.11-13 According to this reasoning, the precipitate formed at intermediate surfactant-to-polyelectrolyte ratios should be resolubilized in concentrated surfactant solutions. However, in many practical cases one-phase system cannot be achieved experimentally, e.g., the precipitate cannot be resolubilized, even at extremely large excess of the surfactant.5 Furthermore, the mentioned theoretical studies cannot explain the frequently observed nonequilibrium states in these systems. The effect of salt on the association between oppositely charged polyelectrolytes and surfactants is rather complex and still not well understood. Some binding isotherm studies indicate that the surfactant binding can be observed at higher surfactant concentrations in the presence than in the absence of an inert electrolyte.2,14 The effect of added electrolyte on the phase behavior seems to be controversial. In the classical work of Thalberg et al.15 the authors revealed that the addition of salt suppresses the association between the anionic polysaccharide sodium hyaluronate (NaHy) and the cationic surfactant tetradecyltrimethylammonium bromide (C14TAB). This means that above the critical electrolyte concentration (cec) no phase separation occurs in a very wide surfactant concentration range. However, at extremely high salt concentrations a phase separation into one polyelectrolyte-rich and one surfactant-rich phase was observed (segregative phase separation) .15 In contrast, Naderi et al. found only the broadening of the precipitated composition region with increasing concentration of the supporting electrolyte for the aqueous mixtures of the cationic polyelectrolyte poly{[2-(propionyloxy)ethyl]trimethylammonium chloride} (PCMA) and sodium dodecyl sulfate (SDS).16 (12) Allen, R. J.; Warren, P. B. Langmuir 2004, 20, 1997–2009. (13) Nguyen, T. T.; Shklovski, B. I. J. Chem. Phys. 2001, 114, 5905–5916. (14) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 99, 16684–16693. (15) Thalberg, K.; Lindman, B.; Karlstrom, G. J. Phys. Chem. 1991, 95, 6004– 6011. :: (16) Naderi, A.; Claesson, P. M.; Bergstrom, M.; Dedinaite, A. Colloids Surf. A 2005, 253, 83–93.

Published on Web 04/27/2009

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Voisin et al. observed first an enhancement and then a complete suppression of the two-phase area with increasing NaCl concentration for the mixtures of the anionic surfactant sodium lauryl ether sulfate with the cationic polysaccharide “Jaguar”.17 Similar effect of the added NaBr on the phase properties was reported by Wang et al.18 for the mixtures of dodecyl trimethylammonium bromide and sodium carboxymethyl cellulose. The main intention of the present paper is to shed new light onto the effect of salt on the association between oppositely charged macromolecules and surfactants. The focus is on the aqueous mixtures of branched poly(ethleneimine) (PEI) and sodium dodecyl sulfate. The investigated PEI sample is a hyperbranched polyamine with an approximate 1:2:1 ratio of the primary, secondary, and tertiary amine groups. Because of the amine groups, the polyelectrolyte charge can be set by changing solution pH and ionic strength. Recently, the bulk association between the hyperbranched PEI molecules and SDS has been investigated intensively by a variety of experimental techniques.19-28 The structure of the precipitates as well as of the aggregates formed in concentrated PEI/SDS solutions reveals a peculiar dependence on the pH of the solution. 20,21 Winnik et al. investigated the PEI/ SDS interaction at high pH.22 The authors revealed that the pH of the PEI solution increases with increasing surfactant concentration as well as that the PEI/SDS mixtures exhibit unusually large conductivity compared to the pure SDS solutions. Li et al. investigated the effect of pH on the surfactant binding isotherms.23 A two-step binding mechanism was proposed, in which the initial monomer binding is followed by cooperative binding of the dodecyl sulfate ions. Recently, a novel interpretation was proposed for the association between the hyperbranched PEI and SDS based on binding isotherm, dynamic light scattering, and electrophoretic mobility measurements in a wide pH, polyelectrolyte and surfactant concentration range.24-26 At low surfactant concentrations (and at constant PEI concentration) the surfactant binds to the protonated amine groups in monomer form, which results in a decreasing mean size and net positive charge of the polyelectrolyte/ surfactant complexes. In this concentration region, the system is described as a thermodynamically stable solution of the solvated PEI/SDS complexes. At a critical amount of bound dodecyl sulfate ions the PEI/SDS complexes collapse and an unstable colloid dispersion of the polyelectrolyte/surfactant particles is formed. The aggregation of these particles results in precipitation at the intermediate SDS concentration region. At higher surfactant concentrations, in a second type of binding process, the anionic surfactant molecules adsorb on the surface of the neutral PEI/SDS particles. Depending on the applied solution preparation method this process may lead to an electrostatically stabilized colloid dispersion of the negatively charged particles.24,26

The variation of pH with SDS concentration at a fixed concentration of PEI was quantitatively described by making use of a thermodynamic model. This model takes into account the effect of surfactant binding on the protonation equilibrium of the amine groups.24 The formation of kinetically stable colloid dispersions at large excess of the surfactant was also reported for the aqueous mixtures of linear poly(vinylamine) (PVAm) and SDS in a wide pH range.27 The fact that at certain compositions an electrostatically stabilized colloid dispersion of the polyelectrolyte/surfactant nanoparticles is formed explains well the observed nonequilibrium nature of oppositely charged polyelectrolyte/surfactant mixtures. In our recent studies, it was shown that the kinetically stable composition range of the colloid dispersion of PEI/SDS or PVAm/SDS particles considerably depends on the mixing methods used for the preparation of the solutions. 24,26,27 The effect of mixing can be understood in terms of the local rate of coagulation of the polyelectrolyte/surfactant particles which is largely dependent on the efficiency of the applied mixing protocol.27 The kinetically stable composition range of PEI/SDS mixtures may also be manipulated by nonionic surfactants. In recent work, it was shown that the addition of dodecyl maltoside (C12G2, a nonionic sugar surfactant) to the PEI/SDS mixtures results in a colloid dispersion of the composite PEI/SDS/C12G2 particles at high SDS concentrations.28 In comparison with the PEI/SDS system, the kinetically stable SDS concentration range was found to increase considerably in the presence of dodecyl maltoside. This finding was attributed to the synergistic adsorption of the surfactants on the surface of the polyelectrolyte/surfactant particles.28 There are also a few studies on the interfacial association between hyperbranched PEI and SDS. In the case of mica/water29 and silica/water30 interface the characteristics of the surface layer were explained according to the adsorption properties of the surfactant and that of the PEI molecules as well as by the adsorption features of the PEI/SDS complexes. For the free aqueous surface of PEI/SDS mixtures, peculiar adsorption features such as the formation of multilayers and surface aggregates were reported recently. 31-33 In the present paper, we study the effect of added salt on the association between PEI and SDS. Coagulation kinetics, electrophoretic mobility and dynamic light scattering measurements are used to monitor the aggregation process of the PEI/SDS complexes as well as the phase behavior of PEI/SDS mixtures in the presence of NaCl. We exploit the recently introduced concept of colloid dispersion formation to interpret our results. Finally, we discuss the implications of our findings with respect to earlier results about the effect of salt on the phase behavior and surface properties of oppositely charged macromolecules and amphiphiles.

(17) Voisin, D.; Vincent, B. Adv. Colloid Interface Sci. 2003, 106, 1–22. (18) Wang, X.; Li, Y.; Li, J.; Wang, J.; Wang, Y.; Guo, Z.; Yan, H. J. Phys. Chem. B 2005, 109, 10807–10812. (19) Wang, H.; Wang, Y.; Yan, H.; Zhang, J.; Thomas, R. K. Langmuir 2006, 22, 1526–1533. (20) Zhou, S.; Burger, C.; Chu, B. J. Phys. Chem. B 2004, 108, 10819–10824. :: (21) Bastardo, L.; Garamus, V. M.; Bergstrom, M.; Claesson, P. M. J. Phys. Chem. B 2005, 109, 167–174. (22) Winnik, M. A.; Bystryak, S. M.; Chassenieux, C.; Strashko, V.; Macdonald, P. M.; Siddiqui, J. Langmuir 2000, 16, 4495–4510. (23) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 3093–3100. (24) Meszaros, R.; Thompson, L.; Bos, M.; Varga, I.; Gilanyi, T. Langmuir 2003, 19, 609–615. (25) Mezei, A.; Meszaros, R. Langmuir 2006, 22, 7148–7151. (26) Mezei, A.; Meszaros, R.; Varga, I.; Gilanyi, T. Langmuir 2007, 23, 4237– 4247. (27) Mezei, A.; Pojjak, K.; Meszaros, R. J. Phys. Chem. B 2008, 112, 9693–9699. (28) Mezei, A.; Meszaros, R. Soft Matter 2008, 4, 586–593.

Experimental Section

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Materials. The hyperbranched poly(ethyleneimine) (PEI) with a mean molecular weight of 750 000 g/mol was purchased from Sigma-Aldrich in the form of a 50 wt % aqueous solution. The polymer solution was purified by mixed bed cation/anion exchange and dialysis. The sodium dodecyl sulfate sample (29) Dedinaite, A.; Meszaros, R.; Claesson, P. M. J. Phys. Chem. B 2004, 108, 11645–11653. (30) Meszaros, R.; Thompson, L.; Varga, I.; Gilanyi, T. Langmuir 2003, 19, 9977–9980. (31) Penfold, J.; Tucker, I.; Thomas, R. K.; Zhang, J. Langmuir 2005, 21, 10061– 10073. (32) Penfold, J.; Tucker, I.; Thomas, R. K.; Taylor, D. J. F.; Zhang, J.; Zhang, X. L. Langmuir 2007, 23, 3690–3698. (33) Tonigold, K.; Varga, I.; Nylander, T.; Campbell, R. A. Langmuir, in press.

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(SDS, Sigma-Aldrich) was recrystallized twice from a 1:1 benzene-ethanol mixture. The critical micelle concentration (cmc) of the recrystallized SDS was 8.2 mmol/dm3 without added salt as determined by surface tension measurements at 25 °C. ACS reagent-grade HCl and NaOH were used to adjust the initial pH of the PEI solutions (e.g., the pH of the PEI solutions prior to the addition of the surfactant, which is denoted by pHin throughout the paper). The applied coagulating electrolyte was NaCl in each experiment. All of these chemicals were provided by SigmaAldrich. During the experiments, double distilled water was used for the solution preparation. Mixing Protocol. For the preparation of the PEI/SDS mixtures, equal volumes of pH preadjusted polyelectrolyte and surfactant solutions were mixed by means of the stop-flow mixing apparatus of Applied Photophysics Model RX. 1000 (the so-called stop flow mixing protocol in refs 26 and 27). This mixing method is very efficient since the two solutions are mixed within 10 ms. Electrophoretic Mobility Measurements. The mean electrophoretic mobility of the PEI/SDS complexes (uζ) was measured at 25 ( 0.1 °C using a Malvern Zetasizer Nano Z instrument. The instrument uses a combination of laser Doppler velocimetry and phase analysis light scattering (PALS) in a technique called M3-PALS. The details of this method can be found elsewhere.25 Prior to the measurements, the instrument was always tested with the Malvern Zeta Potential Transfer Standard. The standard error in the values of the mean electrophoretic mobility was found to be around 5%. Despite of the polydispersity of the PEI molecules, the measured uζ values are good indicators of the charged nature of the PEI/SDS complexes and also consistent with the binding characteristics of SDS on the PEI molecules.25 In some cases, the electrophoretic mobility (uζ) was converted to electrokinetic potential according to the Henry equation:34 uζ ¼

ζεr ε0 f ðKaÞ 1:5η

ð1Þ

where ζ is the electrokinetic potential of the particles, ε0 and εr are the permittivity of the vacuum and that of the medium and η is the viscosity of the medium. (Both εr and η were approximated with the corresponding values for water at 25 °C). a is the radius of the :: colloid particle, and κ is the Debye-Huckel parameter 2 2 1/2 (κ = (2000F celz /ε0εrRT) ; F is the Faraday number, R is the universal gas constant, T is the absolute temperature, z is the valence of the ions of the symmetrical supporting electrolyte, and cel is the electrolyte concentration in mol/dm3). f(κa) is a correction factor which varies between 1.0 and 1.5 for small and large κa values, respectively.34 Dynamic Light Scattering Measurements (DLS). The light scattering measurements were performed at 25 ( 0.1 °C at different scattering angles (from 40 to 140°) by means of a Brookhaven equipment consisting of a BI-200SM goniometer system and a BI-9000AT digital correlator. An argon-ion laser (Omnichrome, model 543AP) operating at 488 nm wavelength and emitting vertically polarized light was used as the light source. Prior to the measurements the solutions were cleaned of dust particles by filtering through 0.8 μm pore-size membrane filters. 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. These autocorrelation functions were analyzed by the cumulant expansion and CONTIN methods. The investigated PEI sample was found to be polydisperse having a wide unimodal distribution. The addition of SDS did not change significantly the character of the size distribution as it was indicated by the CONTIN analysis. Therefore, the first cumulant of the second (34) Henry, D. C. Proc. R. Soc. London 1931, A133, 106.

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order cumulant expansion is used to monitor the changes in the mean hydrodynamic size of the polyelectrolyte/surfactant complexes caused by the addition of SDS. The apparent diffusion coefficient (Dapp) was derived from the mean relaxation rate (Γh(q), first cumulant): Dapp ðqÞ ¼

ΓðqÞ q2

ð2Þ

where q is the scattering vector (q = (4nπ/λ0) sin(θ/2), n is the refractive index of the solution, λ0 is the wavelength of the incident light, and θ is the scattering angle). In order to obtain the translational collective diffusion coefficient, Dcoll, the apparent diffusion coefficient was extrapolated to zero scattering vector: Dcoll = Dapp(q f 0).35 In principle, the translational diffusion coefficient D0, can be determined only at infinite dilution of the polyelectrolyte solutions when the interaction between the particles becomes negligible:35 Dcoll ¼ D0 ð1 þ kD cÞ

ð3Þ

where kD is a constant and c is the concentration of the polyelectrolyte. Assuming that at the investigated PEI concentration (0.05 wt %) the second term can be neglected in eq 3 and that the polyelectrolyte/surfactant complexes are spherical, the mean apparent hydrodynamic diameter of the complexes (dH) was calculated from Dcoll by means of the Einstein-Stokes relation. On the basis of repeated measurements, the standard error in dH can be estimated as 10-15% in the preprecipitation (low) and 3-4% in the postprecipitation (high) SDS concentration range. Coagulation Kinetics Measurements. The rate of coagulation was measured at 25 ( 0.1 °C by a Perkin-Elmer (Lambda 2) spectrophotometer coupled with a stop-flow mixing unit (Model RX.1000, Applied Photophysics Ltd.). First, the PEI/SDS solutions were prepared via the stop-flow mixing protocol and left to stand for 24 h. Next, these PEI/SDS solutions were mixed with NaCl solutions in equal volumes and the absorbance (Ab) vs time (t) curves were monitored at 480 nm. In the case of monodisperse particles the initial rate of coagulation can be approximated by second order kinetics:17 -

dn ¼ kn2 dt

ð4Þ

where k is the absolute coagulation rate constant and n is the number of particles per unit volume. The experimental coagulation rate constant (kx) can be determined from the initial slope of the absorbance versus time curves (Ab - t) provided that the size of the particles is small compared to the wavelength of the light beam:17 

 dAb ¼ Ck ¼ kx dt tf0

ð5Þ

where C is a constant (which depends on the volume and concentration of the particles as well as on the characteristics of the spectrophotometer cell).17 It should be noted that in the applied surfactant concentration range the recrystallization of SDS may be observed in concentrated NaCl solutions (cNaCl > 0.6 M). In separate test experiments a 10 mM SDS solution (which was the highest applied surfactant concentration in the coagulation kinetics experiments) was mixed with NaCl solutions of different concentrations. The absorbance of these solutions was observed to be constant for a couple of hours for cNaCl < 0.5 M. Therefore, the coagulation (35) Sedlak, M. In Surfactant Science Series; Radeva, T., Ed.; Marcel Dekker Inc.: New York, 2001; Vol. 99, pp 1-58.

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kinetics experiments were carried out at salt concentrations below 0.5 M NaCl. For the characterization of the kinetic stability of electrostatically stabilized colloid dispersions, the so-called stability ratio (W) is usually introduced:36 W ¼

kfast k

ð6Þ

where kfast refers to the initial coagulation rate constant in the fast coagulation regime. Reerink and Overbeek derived an approximate interrelation between the stability ratio and the coagulating electrolyte concentration.36 At 25 °C the Reerink-Overbeek equation can be given in the following form: logðWÞ ¼ b -2:06  109

aγ2 logðcel Þ z2

ð7Þ

where b is a constant and a is the radius of the colloid particles. The factor γ in eq 7 depends on the Stern potential of the colloid particles (Ψd):36 γ ¼

expðzFψd =2RTÞ -1 expðzFψd =2RTÞ þ 1

ð8Þ

Results and Discussion Before starting to analyze the effect of supporting electrolyte on the association between PEI and SDS, we reiterate the previously investigated characteristics of PEI/SDS complex formation24,26 without added salt. In Figure 1 the electrophoretic mobility and the mean apparent hydrodynamic diameter of the PEI/SDS complexes are plotted against the analytical surfactant concentration at pHin = 8 (cPEI = 0.05 wt %). According to Figure 1, at low surfactant concentrations both the mean size and net positive charge of the PEI/SDS complexes decrease with increasing cSDS due to the binding of the dodecyl sulfate ions to the charged amine groups of the PEI molecules. At intermediate SDS concentrations associative phase separation can be observed. On the other hand, at high surfactant concentrations transparent systems of negatively charged PEI/SDS complexes with a considerably reduced size are formed. Similar features of the PEI/ SDS complex formation were observed in a wide pHin range provided that the most efficient stop-flow mixing protocol was used to prepare the mixtures.26 In this case at large excess of the surfactant, the dH values of the complexes are close to the mean size of the collapsed PEI molecules in 1 M NaCl (56 ( 3 nm).26 These results suggest that the system can be considered as a colloid dispersion of the collapsed PEI/SDS particles rather than a thermodynamically stable solution phase. The colloid dispersion is unstable at intermediate SDS concentrations due to the lack of stabilizing repulsive forces between the polyelectrolyte/ surfactant particles. However, at higher surfactant concentrations the adsorption of dodecyl sulfate ions takes place on the surface of PEI/SDS particles.24,26 This process may result in a considerable negative surface charge density of the PEI/SDS particles and therefore in an electrostatically stabilized colloid dispersion of the polyelectrolyte/surfactant particles 24,26 (the region of kinetically stable colloid dispersion is roughly indicated in Figure 1). Aggregation Kinetics of Negatively Charged PEI/SDS Particles at Different NaCl Concentrations. The kinetic stability of electrostatically stabilized colloid sols largely depends (36) Reerink, H.; Overbeek, J. Th. G. Discuss. Faraday Soc. 1954, 18, 74–84.

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Figure 1. Electrophoretic mobility and mean apparent hydrodynamic diameter of the PEI/SDS complexes as a function of the surfactant concentration. The gray area indicates the intermediate surfactant concentration range, where precipitation or high turbidity can be observed. The measurements were performed 24 h after the preparation of the mixtures. cPEI = 0.05 wt %, pHin = 8. The standard error in the values of uζ is commensurable with the size of the symbols.

on the presence of supporting electrolyte. In order to test the effect of salt on the properties of PEI/SDS mixtures, the coagulation kinetics of negatively charged PEI/SDS particles (which are formed in the presence of excess SDS) were monitored in the case of different NaCl concentrations at pHin = 4, 6, 8, and 10. According to ref 37, the ionization degree of the PEI molecules is 0.60, 0.40, 0.13, and 0.05 at pHin = 4, 6, 8, and 10, respectively.37 In Figures 2 and 3 the logarithm of the experimental coagulation rate constant is plotted against the logarithm of NaCl concentration at pHin = 4 and 10 for different surfactant concentrations at cPEI = 0.025 wt %. The rate of coagulation is low at low ionic strengths due to the negative charge of the PEI/ SDS particles. kx increases with increasing NaCl concentration (slow coagulation regime) up to the critical coagulation concentration ccc. At electrolyte concentrations above the ccc the repulsive potential barrier between the particles diminishes, therefore kx remains constant (rapid coagulation regime). Figures 2 and 3 also demonstrate that the ccc increases with increasing SDS concentration. Similar log (kx) vs log (cNaCl) curves were observed for pHin = 6 and 8 in a wide range of SDS concentrations. The kinetic stability of PEI/SDS dispersions can also be analyzed through the variation of the stability ratio with the salt concentration. Figure 4 shows the dependence of log(W) on log (cNaCl) for PEI/SDS mixtures at pHin = 8 and cPEI = 0.025 wt %. According to Figure 4, log(W) linearly decreases with log(cNaCl) up to the ccc. At NaCl concentrations above the ccc log(W) becomes zero. The same types of log(W) vs log(cNaCl) curves were observed in a wide surfactant concentration range (in the postprecipitation regime) at pHin = 4, 6, and 10 as well. The shape of these curves resembles the experimental stability plots of different kinds of electrostatically stabilized colloid sols, which are in qualitative agreement with the predictions of DLVO theory.36,38,39 (37) Meszaros, R.; Thompson, L.; Bos, M.; de Groot, P. Langmuir 2002, 18, 6164–6169. (38) Ottewill, R. H.; Shaw, J. N. Discuss. Faraday Soc. 1966, 42, 154–163. (39) Grolimund, D.; Elimelech, M.; Borkovec, M. Colloids Surf. A 2001, 191, 179–188.

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Figure 2. Logarithm of the experimental coagulation rate constant (kx) against the logarithm of NaCl concentration for different surfactant concentrations: cSDS = 4.0 mM (9); 5.0 mM (0) and 6.0 mM (b). cPEI = 0.025 wt %, pHin = 4. The standard error in the values of log(kx) is commensurable with the size of the symbols. Additional measurements were also carried out at other SDS concentrations. For the sake of clarity, only some representative measurements are shown in the figure.

Figure 3. Logarithm of the experimental coagulation rate constant (kx) against the logarithm of NaCl concentration for different surfactant concentrations: cSDS = 2.5 mM (9); 3.5 mM (0); 10.0 mM (b). cPEI = 0.025 wt %, pHin = 10. The standard error in the values of log (kx) is commensurable with the size of the symbols. Additional measurements were also carried out at other SDS concentrations. For the sake of clarity, only some representative measurements are shown in the figure.

It should be emphasized, however, that similarly to the classical inorganic sols,36,38 the polydispersity of the PEI/SDS dispersions is high. Therefore, further investigations on model dispersions of monodisperse polyelectrolyte/surfactant particles are necessary to test quantitatively the kinetic stability of these kinds of colloid sols with the predictions of DLVO theory. The slope of the experimental stability plots is a characteristic of the applied electrolytes in the case of moderately charged colloid particles. The values of -d(log(W ))/d(log(cNaCl)) for PEI/SDS dispersions are summarized in Table 1 in the investigated pHin and surfactant concentration range. As it can be seen -d(log(W ))/d (log(cNaCl)) varies between 1.3 and 6.3 in the investigated pHin and SDS concentration range. These values are in good agreement with the values of -d(log(W ))/d(log(cel)) (cel is the electrolyte 7308 DOI: 10.1021/la9003388

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Figure 4. Logarithm of the stability ratio as a function of the logarithm of the NaCl concentration for different surfactant concentrations: cSDS = 2.5 mM (b); 3.0 mM (0); 4.0 mM (9), and 8.0 mM (O). pHin = 8, cPEI = 0.025 wt %. The standard error in the values of log (W) is commensurable with the size of the symbols. For the sake of clarity, not all the measurements are shown.

concentration) reported for various kinds of electrostatically stabilized sols with different added 1:1 electrolytes.36,38,39 According to Table 1, the slope of the log(W ) vs log(cNaCl) curves increases with increasing surfactant concentration at a given pHin value. With increasing cSDS the negative charge density of the PEI/SDS particles increases at constant pHin (see Figure 1), which leads to higher values of γ and therefore higher values of -d(log(W )/d log(cNaCl) according to eqs 7 and 8. We emphasize, however, that the Reerink-Overbeek equation is approximate in nature and might be used only to qualitatively interpret the variation of the stability ratio with the electrokinetic potential of a given type of colloid particle. The comparison of the presented -d(log(W ))/d(log(cNaCl)) data belonging to different pHin values is not really straightforward since the composition and structure of the polyelectrolyte/surfactant particles are different and not exactly known in the given pHin and composition range. The experimental results of Figures 2-4 and Table 1 clearly indicate that the transparent systems of negatively charged PEI/ SDS particles are kinetically stable colloid dispersions and not thermodynamically stable solutions. It is expected that the kinetic stability of the PEI/SDS dispersions plays a distinguished role in understanding the effect of salt on the phase and surface properties of PEI/SDS mixtures. The presented results together with our recent findings on the linear PVAm/SDS system28 also indicate that the colloid dispersion formation at conditions of excess surfactant is a general characteristic of the mixtures containing oppositely charged hydrophilic polyelectrolytes and surfactants. It should be noted, however, that the mixtures of surfactants and oppositely charged macromolecules with hydrophobic functionalities are special cases. In these systems, one may obtain thermodynamically stable solutions of overcharged polyion/surfactant complexes at high surfactant concentrations due to the surfactant binding in the intramolecular hydrophobic domains of these macromolecules.40 Effect of Surfactant Concentration and Polyelectrolyte Charge on the Kinetic Stability of PEI/SDS Dispersions. In Figure 5 the ccc is plotted against the analytical SDS concentration at different pHin values and at cPEI = 0.025 wt %. (40) Deo, P.; Deo, N.; Somasundaran, P.; Moscatelli, A.; Jockusch, S.; Turro, N. J.; Ananthapadmanabhan, K. P.; Ottaviani, M. F. Langmuir 2007, 23, 5906– 5913.

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Table 1. -d(log(W))/d(log(cNaCl)) Values of PEI SDS Dispersions at Different pHin and Surfactant Concentrations for cPEI = 0.025 wt % CSDS (mM)

-d(log(W ))/d(log(cNaCl))

(a) pHin = 4 4.0 5.0 5.5 6.0

1.3 2.8 3.2 4.1

(b) pHin = 6 3.5 3.8 4.0 5.0

2.9 3.0 3.0 3.7

(c) pHin = 8 2.5 3.0 4.0 5.0 8.0

2.0 2.7 4.2 4.5 5.0

(d) pHin = 10 2.5 3.5 5.0 8.0 10.0

3.1 4.7 5.1 5.2 6.3

Figure 5. Critical coagulation concentration as a function of the total SDS concentration at different protonation degrees of the PEI molecules: pHin = 4 (9); 6 (O); 8 (b); 10 (4). cPEI = 0.025 wt %.

At a given ionization degree of PEI, the ccc first increases with increasing surfactant concentration and then it roughly levels off at higher SDS concentrations except for pHin = 4 where only a steep increase of ccc in cSDS can be seen. This finding can be interpreted in terms of the variation of the attractive and the repulsive forces, acting between the PEI/SDS particles, with increasing surfactant activity and NaCl concentration at a given PEI concentration and pHin. As a first approximation, the attractive forces depend on the size (dH) and effective Hamaker constant (Aeff) of the polyelectrolyte/surfactant particles. In recent studies, it was shown that at a given pHin and PEI concentration the mean size of the PEI/SDS complexes is roughly constant at high surfactant concentrations26 (see, for instance, Figure 1). Because of the constant size of the particles, Aeff can be considered as a monotonously increasing function of the bound Langmuir 2009, 25(13), 7304–7312

amount of the surfactant (B). A moderate increase of the bound amount of SDS as a function of surfactant activity was observed up to the cmc at different ionic strengths (in the concentration range of negatively charged PEI/SDS particles).25 The effect of the electrolyte concentration on the bound amount of surfactant is very complex. The increasing salt concentration may cause a slight increase of B at a given surfactant activity due to the elevated PEI charge at high ionic strengths.25 At extremely high salt concentrations, however, due to the largely reduced cmc of SDS a decrease in the bound amount of surfactant might be expected at a given surfactant concentration and pHin. Ignoring the possible variation of B with the salt concentration, it is likely that Aeff as well as the strength of the attractive dispersion interactions are only slightly affected in the investigated SDS concentration range at a given pHin. The dependence of the repulsive forces on the surfactant and salt concentration is reflected in the variation of the electrokinetic potential of the PEI/SDS particles (ζ) with cSDS and cNaCl. This is demonstrated in Figure 6 where the electrokinetic potential of the PEI/SDS particles is shown as a function of SDS concentration at pHin = 4 and 8 in the presence and absence of 0.1 M NaCl. According to Figure 6, ζ increases considerably (in absolute values) with increasing cSDS at a given pHin and NaCl concentration. This observation is attributable to the enhanced adsorption of dodecyl sulfate ions on the surface of the PEI/SDS particles with increasing surfactant activity. At even higher SDS concentrations, where free micelles appear in the mixture, a plateau value of ζ (ζplat) can be observed due to the roughly constant surfactant activity in this composition region. The addition of electrolyte reduces the cmc, therefore ζ becomes roughly constant at smaller total SDS concentrations in 0.1 M NaCl compared to the mixtures without added salt. Another important effect of the electrolyte is that the plateau values of ζ are smaller (in absolute value) in 0.1 M NaCl than without added salt. Namely, ζplat changes from = -90 mV to -70 mV and from = -74 mV to -53 mV at pHin=4 and 8, respectively, due to the addition of 0.1 M NaCl to the mixtures. In the case of constant surface charge density the same increase in the 1:1 electrolyte concentration (e.g., from the cmc of SDS in water (=0.008 M) to =0.1 M) would result in a much more pronounced decrease of |ζplat| values according to the DLVO theory. Therefore, the surface charge density of the PEI/SDS particles may also increase with increasing salt concentration due to the enhanced surfactant adsorption on their surface. The above-mentioned reasoning indicates that in contrast to the attractive dispersion forces, the double layer forces are expected to increase largely with increasing surfactant activity, which results in an enhanced kinetic stability of the PEI/SDS dispersion at large SDS concentrations and at a given protonation degree of PEI. However, the kinetic stability cannot be increased without limit, since at a given SDS concentration free micelles appear in the system. Above this surfactant concentration the activity of the surfactant molecules becomes approximately constant, therefore neither ζ nor Aeff vary considerably with a further increase of SDS concentration. This explains the approximately constant ccc values at the highest investigated surfactant concentrations. It should be noted, however, that at pHin = 4 a plateau value of ccc cannot be attained experimentally because of the limited solubility of SDS in extremely concentrated NaCl solutions. An interesting feature of Figure 5 is the dependence of the kinetic stability on the protonation degree of PEI molecules. At high surfactant concentrations, the PEI/SDS dispersion becomes more stable at low pHin values. This finding can be qualitatively DOI: 10.1021/la9003388

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Figure 7. Mean apparent hydrodynamic diameter of the PEI/SDS complexes as a function of the surfactant concentration. The gray and sparse areas indicate the intermediate surfactant concentration range, where precipitation or high turbidity can be observed in the absence and presence of 0.1 M NaCl, respectively. The measurements were performed 1 h after the preparation of the mixtures. cPEI = 0.05 wt %, pHin = 10. The relative standard error of the dH values (not shown for the sake of clarity) is 10-15% in the preprecipitation (low) and 3-4% in the postprecipitation (high) SDS concentration range.

Figure 6. The electrokinetic potential of the PEI/SDS particles as a function of SDS concentration without added salt (0) and in 0.1 M NaCl (b) at cPEI = 0.025 wt %. Key: (a) pHin = 4; (b) pHin = 8. The ζ values were calculated from the measured electrophoretic mobility data according to the Henry equation (eq 1). The details of these calculations as well as of the measurements of uζ are given in the Supporting Information.

explained by the pHin dependent variation of the dH, Aeff and ζ values of the PEI/SDS particles at a given surfactant activity and salt concentration. As it was shown recently, the average size of the negatively charged PEI/SDS particles only slightly depends on pHin in a wide SDS concentration range.26 In contrast, the bound amount of dodecyl sulfate ions increases considerably with decreasing pHin at a given surfactant activity and ionic strength due to the increasing charge density of PEI molecules.,25 Therefore, the effective Hamaker constant of the polyelectrolyte/surfactant particles increases with decreasing pHin at a fixed surfactant activity and NaCl concentration. On the other hand, with increasing bound amount of the surfactant the particles become more hydrophobic which results in an enhanced adsorption of the dodecyl sulfate ions in the case of highly charged PEI molecules at the same surfactant activity and ionic strength. This explains the observed more negative values of ζplat at pHin = 4 compared to the values at pHin =8 (see Figure 6, parts a and b). Therefore, both the attractive and the repulsive forces acting between the polyelectrolyte/surfactant particles are expected to increase with decreasing pHin at a given surfactant activity. The enhanced stability at high ionization degree of the PEI molecules 7310 DOI: 10.1021/la9003388

indicates that with decreasing pHin the changes in the double layer forces become dominant over the variation of the strength of the attractive dispersion forces. It is important to emphasize that in the previous reasoning it was assumed implicitly that the structure of the PEI/SDS particles does not depend on pHin. In a recent small angle neutron scattering study, Bastardo et al. revealed that at low pHin compact PEI/SDS particles with ordered inner structure were formed. In contrast, at high pHin values looser aggregates with less ordered structure were detected.21 Therefore, the enhanced kinetic stability of PEI/SDS dispersions in the case of highly charged PEI molecules might be attributed to the formation of more compact and charged PEI/SDS particles at low pHin values compared to the particles formed at high pHin values at the same surfactant activity. Nevertheless, other factors, such as the surface charge heterogeneities of the particles41,42 or the possible variation of the bound amount of surfactant with salt concentration, may also play a significant role in the interpretation of the kinetic stability of PEI/SDS dispersions. Effect of Salt on the Phase Properties. The colloid dispersion nature of PEI/SDS mixtures in a given composition range can be utilized to understand the effect of electrolyte on the phase behavior of this system. In Figure 7, the mean apparent hydrodynamic diameter of the PEI/SDS complexes is plotted as a function of SDS concentration at pHin =10 in the absence of added salt and in 0.1 M NaCl, respectively. (The measurements were carried out 1 h after the solution preparation.) In 0.1 M NaCl the mean size of the PEI/SDS particles is considerably increased at high SDS concentrations compared to the PEI/SDS mixtures without added NaCl. Another important feature of Figure 7 is that the two-phase area is considerably larger in the presence of supporting electrolyte with respect to the mixtures without added salt. Similar results were observed in a wide pHin and PEI concentration range.26 The formation of huge aggregates in (41) Bouyer, F.; Robben, A.; Li Yu, W.; Borkovec, M. Langmuir 2001, 17, 5225– 5231. (42) Lin, W.; Galletto, P.; Borkovec, M. Langmuir 2004, 20, 7465–7473.

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0.1 M NaCl and at pHin = 4 is also demonstrated in Figure S2 of the Supporting Information. Clearly, these findings can be rationalized in terms of the enhanced coagulation rate of the PEI/SDS particles in the presence of the supporting electrolyte. This also means that in the applied NaCl concentration range, the primary effect of the added salt is not the enhancement of the equilibrium two-phase area, but rather the enlargement of the kinetically unstable composition range of the system. An important consequence of the addition of electrolyte to the solutions of ionic surfactants is the reduced cmc, which means an upper limit of the surfactant activity. The significantly reduced surfactant activity may result in the suppression of the surfactant binding to the oppositely charged macromolecules at high salt concentrations. This may lead to the lack of associative phase separation at electrolyte concentrations above the so-called critical electrolyte concentration cec.15,17,18 However, the salt-induced suppression of the surfactant binding25 and the lack of associative phase separation cannot be observed for the mixtures of the investigated hyperbranched PEI sample and SDS. In the electrolyte concentration range cNaCl < 0.5 M the main effect of the added salt is the reduction of the kinetically stable concentration range of the PEI/ SDS dispersion. The application of higher NaCl concentrations leads to seggregative phase separation due to the limited solubility of SDS. It is important to note that the salt-induced suppression of the polyelectrolyte/surfactant association was detected only for mixtures with largely soluble polyelectrolytes (cationic17 or anionic15,18 polysaccharides) and/or ionic surfactants (sodium lauryl ether sulfate17). In these mixtures, the lack of precipitation was observed in a wide surfactant concentration range at salt concentrations above the cec.15 At low salt concentrations (e.g., csalt < cec) the observed dominant effect of the added electrolyte was the increased turbidity of the investigated polyelectrolyte/surfactant mixtures. 16-18 On the other hand, at very high salt concentrations (csalt . cec) seggregative phase separation was observed by Thalberg et al. 15 In the light of the colloid dispersion character of the mixtures of oppositely charged hydrophilic polyelectrolytes and surfactants, the effect of salt on the phase behavior of these mixtures can be interpreted as follows. Without added salt, associative phase separation occurs at intermediate surfactant-to-polyelectrolyte ratios, whereas kinetically stable colloid dispersions of the polyelectrolyte/surfactant particles are formed at higher surfactant-to-polyelectrolyte ratios. With increasing salt concentration, the primary effect of the electrolyte is the reduction of the kinetically stable composition range of the polyelectrolyte/ surfactant mixtures. A further significant increase in the electrolyte concentration might result in the lack of associative phase separation due to the considerably reduced surfactant binding. At even higher electrolyte concentrations a seggregative phase separation into one polyelectrolyte-rich and one surfactantrich phase occurs. However, if the solubility of the polyelectrolyte and/or the surfactant is low, (like in the case of PEI/SDS system) then the suppression of the associative phase separation may not be detected at any concentration of the supporting electrolyte. The Impact of Electrolyte on the Mixed Adsorption Layers of PEI and SDS at the Air/Water Interface. The concept of colloid dispersion formation is also useful for the interpretation of the recently published results on the adsorption behavior of PEI/SDS mixtures at the free aqueous interface. Langmuir 2009, 25(13), 7304–7312

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Based on neutron reflection measurements, Penfold and coworkers determined the composition of the mixed surface layer of branched PEI molecules and SDS at the air/water interface under different experimental conditions.31,32 The authors found that the adsorption of SDS is most pronounced at high pHin. The studies also revealed that multilayers of the polyelectrolyte and surfactant molecules might be observed. The multilayer formation becomes more significant with increasing pHin and ionic strength as well as with decreasing molecular mass of the branched PEI molecules.31,32 According to the present study the increased level of multilayer formation correlates with the increased aggregation rate of the PEI/SDS particles. According to Figures 2-6, the coagulation rate of the PEI/SDS particles is enhanced with increasing electrolyte concentration and decreasing PEI charge density. It is also clear that the smaller the PEI molecules, the higher the aggregation rate of the PEI/SDS particles becomes under the same experimental conditions. In another communication, Tonigold et al. investigated the mixed PEI/SDS layers at the air/water interface by ellipsometry at different pHin values and in 0.1 M NaCl.33 The study revealed the appearance of lateral inhomogeneities in the surface layer due to the presence of PEI/SDS surface aggregates of macroscopic size. The authors demonstrated that the trapped surface aggregates could be removed by appropriate surface cleaning in the case of low and intermediate pHin. However, in the case of high pHin values even after surface cleaning and filtering of the PEI/SDS solutions, the surface aggregates reappear in the surface layer. The authors argued that the adsorption and surface aggregation of the PEI/SDS particles is more pronounced at high pHin because of the low surface charge density of the particles compared to the ones formed at low pHin values. These findings are also consistent with our observations, e.g., with the enhanced kinetic stability of the PEI/SDS dispersions at low pHin values.

Conclusions The addition of electrolyte has a significant impact on the aggregation of the PEI/SDS complexes. The shape and characteristics of the stability ratio vs NaCl concentration curves strongly resemble the earlier observed experimental stability plots of electrostatically stabilized inorganic and organic colloid sols. These results provide an unambiguous evidence for the formation of kinetically stable colloid dispersions in the investigated composition and pHin range. These dispersions are electrostatically stabilized due to the adsorption of dodecyl sulfate ions on the surface of the PEI/SDS particles. The kinetic stability of the PEI/SDS dispersions at a given pHin value increases with increasing surfactant activity due to the enhanced adsorption of the surfactant on the surface of the PEI/ SDS particles. The kinetic stability at large excess of the surfactant increases with increasing ionization degree of the PEI molecules because of the considerably increased surface charge of the PEI/ SDS particles at low pHin values. The effect of salt on the phase behavior of the aqueous mixtures of oppositely charged hydrophilic polyelectrolytes and surfactants can be reinterpreted in the light of the colloid dispersion nature of these systems. We have shown that at moderate salt concentrations, the primary effect of the increasing electrolyte concentration is the reduction of the kinetically stable composition range of the polyelectrolyte/surfactant mixtures. Finally, we have also demonstrated that the nature of multilayer and surface aggregate formation in the mixed surface layer of hyperbranched PEI and SDS is consistent with the kinetic stability of PEI/SDS dispersions under the same experimental conditions. DOI: 10.1021/la9003388

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Acknowledgment. This work was supported by the European Commission under the sixth Framework Program, MRTN-CT2004-512331-Project SOCON as well as under COST Action D43. R. Meszaros is a Bolyai Janos fellow of the Hungarian Academy of Sciences which is gratefully acknowledged. The work was also sponsored by the Hungarian Scientific Research Fund (OTKANKTH K-68027).

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Supporting Information Available: Text giving additional experimental information as discussed in the text of the paper and figures showing the mean electrophoretic mobility against the SDS concentration and the mean apparent hydrodynamic diameter as a function of the surfactant concentration. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2009, 25(13), 7304–7312