Preparation of Stable Electroneutral Nanoparticles of Sodium Dodecyl

ARTICLE pubs.acs.org/Langmuir

Preparation of Stable Electroneutral Nanoparticles of Sodium Dodecyl Sulfate and Branched Poly(ethylenimine) in the Presence of Pluronic F108 Copolymer Katalin Pojj
ak and R
obert M
esz
aros* Laboratory of Interfaces and Nanosized Systems, Institute of Chemistry, E€otv€os Lor
and University, 1117 Budapest, P
azm
any P
eter s
et
any 1/A, Hungary

bS Supporting Information ABSTRACT: Mixing of polyelectrolyte solutions with solutions of oppositely charged surfactants usually leads to phase separation in a certain concentration range. However, since the chargeneutralized polyelectrolyte/surfactant nanoparticles might be utilized as versatile nanocarriers of different substances, it would be desirable to prevent their aggregation for some applications. As it was revealed in earlier investigations, the complete suppression of precipitation may be achieved only in mixtures of ionic surfactants and appropriate copolymer polyelectrolytes with nonionic and ionic blocks. In this work, we present a method that could prevent phase separation in mixtures of homopolyelectrolytes and oppositely charged surfactants. Specifically, it is shown that nonaggregating electroneutral nanocomplexes of branched poly(ethylenimine) (PEI) and sodium dodecyl sulfate (SDS) can be prepared in the presence of the amphiphilic triblock copolymer Pluronic F108, provided that an adequate mixing protocol is used for preparation of the PEI/SDS/F108 mixtures.

’ INTRODUCTION Oppositely charged polyelectrolytes and surfactants have become essential ingredients in a variety of industrial applications such as hair-care and pharmaceutical products and coatings, as well as water and sewage treatment.1
5 Therefore, the interactions between macromolecules and oppositely charged amphiphiles have been a subject of great interest in the last few decades.6
16 An important consequence of these interactions is that in a certain concentration range a precipitate or coacervate of polyelectrolyte/surfactant (P/S) complexes separates out from a dilute solution.13
16 However, for some applications it might be desirable to prevent the formation of precipitates. For instance, due to their hydrophobic interior, the slightly charged or uncharged P/S nanoparticles could act as ideal nanocontainers for various hydrophobic substances in aqueous medium.2,6 The first approach to achieve the above-mentioned goals is related to the pioneering work of Kabanov and co-workers.17
22 These authors have shown water-soluble stoichiometric nanocomplexes of different block ionomer copolymers (containing ionic and nonionic blocks) and oppositely charged surfactants. Depending on the nature of the block ionomers and surfactant ions, these self-assemblies reveal different types of supramolecular structures. For example, the electroneutral complexes of cationic surfactants with sodium poly(ethylene oxide)-b-polymethacrylate form vesiclelike aggregates, which are composed of a poly(ethylene glycol) corona and a surfactant-neutralized polyanion shell.18 Recently, Wang et al.23 observed the formation of similar vesiclelike selfassemblies of azobenzene-containing cationic surfactants and r 2011 American Chemical Society

poly(ethylene glycol)-b-poly(acrylic acid) block ionomers. Through their photocontrollable disassembly, these vesicles might be further exploited as versatile drug nanocarriers. In other systems of ionic surfactants and different block ionomers with poly(ethylene oxide) (PEO) or poly(acrylamide) blocks, the hydrophobic core of the complexes is a disordered microphase of polyelectrolyte blocks and surfactant micelles surrounded by a hydrophilic corona of the nonionic blocks of the copolymer.20,24
27 Similar to the block ionomer/surfactant self-assemblies, stable stochiometric P/S complexes can also be prepared via association of ionic surfactants with oppositely charged random copolymers28 or comb polymers.29
31 The most important criterion of solubility is usually connected to an appropriate number of nonionic groups that can lyophilize even the electroneutral P/S complexes. In recent work, Stepanek et al.32 have demonstrated that the nature of the polyelectrolyte block also plays a crucial role in the solution behavior of block ionomer/surfactant mixtures. These authors have shown that, despite its large nonionic block, the diblock cationic copolymer poly[bis(3,5-trimethylammoniummethyl)-4-hydroxystyreneiodide]-b-poly(ethylene oxide) cannot form water-soluble electroneutral complexes with sodium dodecyl sulfate (SDS). These results are possibly attributable to the

Received: September 26, 2011 Revised: November 3, 2011 Published: November 03, 2011 14797

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Langmuir dicationic monomer units and the limited mobility of the polyelectrolyte blocks.32 In all the above-mentioned P/S systems, the lack of precipitation is related to the specific copolymer nature of the charged macromolecules. In principle, there is an alternative approach that can be utilized to avoid phase separation in the mixtures of common homopolyelectrolytes and oppositely charged amphiphiles. Recently, it was shown that the frequently observed nonequilibrium character of P/S association33
35 is related to the formation of kinetically stable colloidal dispersions of P/S nanoparticles, which are the nanophases of the insoluble P/S salt (e.g., the concentrated phase in the two-phase composition range).36
40 The aggregation of these P/S nanoparticles is considerably hindered at low ionic strengths if they have large enough surface charge density.41,42 The concentration range of kinetically stable P/S dispersions is strongly dependent on the concentration and charge of the homopolyelectrolytes as well as on the method of solution preparation.37,43,44 Nonionic surfactant or polymer additives could also affect the kinetic stability of P/S colloidal dispersions.45
48 For instance, it was shown that the addition of n-dodecyl β-D-maltoside considerably suppresses the precipitation concentration range of polycation/SDS mixtures.45,46 This finding can be rationalized by the synergistic adsorption of ionic and nonionic surfactants on the surface of the P/S nanoparticles, which greatly increases the kinetically stable composition range of these systems.45 In recent work, it was shown that uncharged linear polymers like PEO or poly(vinylpyrrolidone) (PVP) may adsorb on the surface of nanoparticles of branched poly(ethylenimine) (PEI) and SDS.47,48 At high surface coverage, the thick adsorbed layers of PEO or PVP considerably hinder aggregation of the PEI/SDS nanoparticles, which leads to enhanced kinetic stability of the PEI/SDS mixtures even at high salt concentrations. However, none of the mentioned nonionic additives could prevent the aggregation of electroneutral polycation/SDS nanoparticles and therefore completely suppress the precipitation concentration range.45
48 In the present study we demonstrate that utilization of a rapid solution mixing method and addition of a suitable neutral amphiphilic block copolymer could hinder the coagulation of uncharged nanoparticles of homopolyelectrolytes and oppositely charged surfactants. For this aim, the mixtures of a high molecular weight hyperbranched PEI sample with SDS are investigated in the presence of Pluronic-type triblock copolymer F108 by use of dynamic light scattering, electrophoretic mobility, and turbidity measurements.

’ EXPERIMENTAL SECTION Materials. Hyperbranched poly(ethylenimine) (PEI, Sigma
Aldrich, Mw = 750 000) was purchased in the form of a 50 wt % aqueous solution. The supplier’s polyelectrolyte solution was purified via dialysis by use of Amicon Ultracel regenerated cellulose filters with a cutoff molecular weight of 30 000. ACS reagent-grade HCl was used to adjust the initial pH of the PEI solutions (that is, the pH of the PEI solutions prior to addition of the surfactant, which is denoted as pHin throughout the paper). The experiments were carried out at pHin = 6, where approximately 40% of the amine groups are protonated.49 The sodium dodecyl sulfate sample (SDS, Sigma
Aldrich) was recrystallized twice from a 1:1 benzene/ethanol mixture. The critical micelle concentration (cmc) of SDS after purification was 8.2 mmol 3 dm
3 at 25.0 ( 0.1 °C as determined from conductivity measurements.

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The investigated nonionic polymer additive was the Pluronic-type triblock copolymer F108 (Sigma
Aldrich, Mw = 14 600) with the following mean composition of poly(ethylene oxide) and poly(propylene oxide) (PPO) units: (PEO)129-(PPO)56-PEO129. This polymer sample was used without further purification. Due to their amphiphilic nature, self-association of the Pluronic-type copolymer leads to the formation of micellelike self-assemblies under appropriate experimental conditions. According to surface tension measurements, the cmc of the investigated F108 sample was found to be 51 g 3 dm
3 at 25.0 ( 0.1 °C, in good agreement with the cmc data of F108 based on other experimental methods.50,51 The experiments were carried out at copolymer concentrations far below the cmc of F108, therefore, its self-association is negligible in the investigated systems. Solution Preparation Methods. The majority of the mixtures were prepared by one of the following two methods: Slow Mixing. Equal volumes of PEI and SDS solutions were mixed in such a way that the surfactant solution was added slowly, drop by drop, to the polyelectrolyte solution under continuous stirring with a magnetic stirrer (for further details see ref 44). Stopped-Flow Mixing. Equal volumes of SDS (or SDS/F108) and PEI (or PEI/F108) solutions were mixed by means of the stopped-flow mixing apparatus of Applied Photophysics (Model RX 1000). This mixing method is highly efficient since the two solutions are mixed within 10 ms (for further details see refs 37 and 43). In the case of added F108, both polyelectrolyte and surfactant solutions contained the neutral polymer at the same concentration. In the case of the multicomponent mixtures of PEI, SDS, and F108, a few experiments were also carried out in which the final composition of the systems was attained in two steps: Two-Step Mixing. First, equal volumes of SDS and PEI solutions were mixed by means of the slow mixing protocol and then the mixtures were left to stand for 24 h. Next, these PEI/SDS mixtures were mixed with F108 solutions of equal volume to attain the final composition of the system and stirred by magnetic stirrer for a day. Preparation of PEI/SDS and PEI/SDS/F108 mixtures via the different mixing methods is illustrated in Figure S1 of the Supporting Information. All the mixtures were prepared and stored at t = 25.0 ( 0.5 °C. Measurement Methods. Electrophoretic Mobility Measurements. The mean electrophoretic mobility (uζ) of P/S complexes in the presence and absence of F108 was determined at 25.0 ( 0.1 °C and 24 h, as well as 2 weeks after solution preparation, by use of a Malvern Zetasizer Nano Z instrument. The apparatus uses the M3-PALS technique, which is a combination of laser Doppler velocimetry and phase analysis light scattering. The relative standard error of mean electrophoretic mobility values was around 5
10%. Dynamic Light Scattering Measurements. Dynamic light scattering (DLS) measurements were performed on Brookhaven equipment consisting of a BI-200SM goniometer system and a BI-9000AT digital correlator. The measurements were made at θ = 90° scattering angle and at 25.0 ( 0.1 °C and 24 h, as well as 2 weeks after preparation of the systems. 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 by means of the Siegert relation to normalized electric field autocorrelation functions. These autocorrelation functions were analyzed by second-order cumulant expansion and CONTIN methods. The investigated PEI sample was found to be polydisperse with a wide unimodal distribution. As indicated by the CONTIN analysis, addition of SDS and/or F108 does not change the character of the size distribution significantly. This also means, however, that due to the significant polydispersity of the PEI sample, an accurate size distribution of the PEI/SDS/ F108 complexes cannot be determined from DLS measurements. In the investigated concentration range, addition of the F108 copolymer does not affect considerably the relative intensity (Irel) of the scattered light 14798

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Figure 1. Electrical conductivity (k) as a function of the analytical concentration of SDS in the absence of neutral polymer (9) as well as in the presence of 2.0 g 3 dm
3 F108 (Δ). (Inset) Break point of the k vs cSDS curve in the presence of 2.0 g 3 dm
3 F108. (with respect to benzene, Irel = I/Ibenzene) of the PEI/SDS mixtures. The apparent diffusion coefficient (Dapp) of the PEI/SDS complexes in the presence and absence of F108 was derived from the mean relaxation rate ̅ (q), first cumulant): (Γ Dapp ðqÞ ¼

̅ ðqÞ Γ q2

ð1Þ

where q is the scattering vector (q = (4πn/λ0) sin (θ/2), n is the refractive index of the solution, and λ0 is the wavelength of the incident light). The apparent mean hydrodynamic diameter (dH) of the complexes was calculated from Dapp by use of the Einstein
Stokes equation: Dapp ¼

kB T 3πηdH

ð2Þ

where kB is the Boltzmann constant, T is the absolute temperature, and η is the viscosity of the medium. In the applied composition range, the viscosity of the medium can be well approximated by the viscosity of water.47,48 Prior to the measurements the solutions were filtered through 0.45 μm pore-size membrane filters. Turbidity Measurements. The turbidity was determined at 25.0 ( 0.1 °C from the transmittance (T) of the mixtures which was measured at 480 nm by a UV/vis (Perkin-Elmer Lambda 2) spectrophotometer. The turbidity is given as (100
T)%, and the measurements were carried out 24 h as well as 2 weeks after preparation of the mixtures. Conductivity Measurements. The electrical conductivity (k) of SDS solutions and F108/SDS mixtures (at 1 and 2 g 3 dm
3 F108 concentration) was measured at 25.0 ( 0.1 °C by use of a Radelkis conductometer. The conductivity values measured at a given composition immediately and 24 h after the solution preparation were found to be identical within experimental error.

’ RESULTS AND DISCUSSION Before analyzing the multicomponent mixtures of PEI, SDS, and F108, it is important to consider all the possible interactions between the solution components. Electrophoretic mobility and DLS measurements do not indicate any association between PEI and F108 macromolecules. However, as is well-known, anionic surfactants interact strongly with Pluronic-type triblock copolymers as well as with oppositely charged polyelectrolytes. Therefore, in the following sections a short account is given of the features of F108/SDS and PEI/SDS association.

Figure 2. Mean electrophoretic mobility (uζ) and apparent mean hydrodynamic diameter (dH) of PEI/SDS complexes plotted against analytical surfactant concentration for cPEI = 50 mg 3 dm
3. The experiments were carried out 24 h after solution preparation. Key: stoppedflow-mixing (9, green); slow mixing (4, red). The green and red hatched areas indicate the composition range of precipitated systems belonging to the mixtures made by stopped-flow and slow mixing, respectively. Standard error of mobility values is commensurate with the size of the symbols.

F108/SDS Interaction. Due to their PPO block, the Pluronic copolymers interact more strongly with SDS than the PEO homopolymers.7,52,53 In order to monitor the association between SDS and F108, in Figure 1 the electrical conductivity of F108/SDS mixtures is plotted against the analytical concentration of SDS at cF108 = 2 g 3 dm
3. As shown in the figure, surfactant binding onto the F108 molecules occurs roughly from ∼4 mM SDS concentration. Furthermore, at cSDS = 4.5 ( 0.4 mM, there is a breakpoint in the k versus cSDS curve that is possibly related to the onset of surfactant binding on the PEO blocks of the F108 molecules (critical aggregation concentration, cacPEO/SDS; see Figure 1, inset graph).54,55 Above this SDS concentration, the conductivity values measured in the presence and absence of F108 largely deviate, indicating that a considerable amount of surfactant is bound to the copolymer. Similar results were observed at cF108 = 1 g 3 dm
3 (see Figure S2 of the Supporting Information). PEI/SDS Complexes Formed via Application of the StoppedFlow Mixing Protocol. In Figures 2
4, the mean electrophoretic mobility (uζ) and the apparent mean hydrodynamic diameter (dH) of the PEI/SDS complexes formed via the stoppedflow-mixing protocol are plotted as a function of the analytical surfactant concentration at three polyelectrolyte concentrations (cPEI = 50, 250, and 1000 mg 3 dm
3). At all three PEI concentrations, rapid mixing of the polyelectrolyte and surfactant solutions leads to transparent mixtures in the presence of polyelectrolyte excess, where binding of the 14799

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Figure 3. Mean electrophoretic mobility (uζ) and apparent mean hydrodynamic diameter (dH) of PEI/SDS complexes plotted against analytical surfactant concentration for cPEI = 250 mg 3 dm
3. The experiments were carried out 24 h after solution preparation. The meaning of the symbols is the same as in Figure 2. Standard error of mobility values is commensurate with the size of the symbols.

Figure 4. Mean electrophoretic mobility (uζ) and apparent mean hydrodynamic diameter (dH) of PEI/SDS complexes plotted against analytical surfactant concentration for cPEI = 1000 mg 3 dm
3. The experiments were carried out 24 h after solution preparation. The meaning of the symbols is the same as in Figure 2. Standard error of mobility values is commensurate with the size of the symbols..

anionic surfactant results in decreasing net positive charge and size of the PEI/SDS complexes with increasing surfactant concentration. From a given SDS concentration, the apparent mean diameter of the complexes starts to increase with increasing cSDS, and as the mobility of the PEI/SDS complexes becomes sufficiently low, precipitation can be observed. In the presence of surfactant excess, a significant effect of polyelectrolyte concentration is observable. At cPEI = 50 and 250 mg 3 dm
3, transparent systems of negatively charged complexes are formed at high surfactant-to-polyelectrolyte ratios. In this composition range the mean size of the complexes decreases, whereas their negative charge density increases with increasing SDS concentration. In accordance with the earlier studies, at large excess of surfactant the mean size of the PEI/SDS complexes is roughly constant (45 ( 2 nm for both PEI concentrations) and significantly reduced compared to surfactant-free polyelectrolyte molecules (86 ( 5 nm).43,45 However, in sharp contrast to the observations at cPEI = 50 and 250 mg 3 dm
3, at the largest applied PEI concentration even the rapid homogenization of the mixtures could not prevent precipitation of the PEI/SDS systems in the investigated composition range of surfactant excess. Impact of Mixing Methods on Phase Properties of PEI/SDS Mixtures. In order to test the thermodynamic stability of PEI/ SDS mixtures, in Figures 2
4 the impact of the slow mixing protocol44 on precipitation concentration range and mean size of the PEI/SDS complexes is also shown. At very low surfactant-topolyelectrolyte ratios, the two mixing protocols give roughly the same mean diameter of the complexes. However, with a further

increase of SDS concentration, the mean size of the complexes becomes significantly larger; also, precipitation can be observed from lower surfactant concentration for the slow mixing than for the stopped-flow mixing protocol. The deviation between the systems formed by the two mixing methods increases with increasing polyelectrolyte concentration. At cPEI = 1000 mg 3 dm
3, similar to the stopped-flow mixing protocol, the slow mixing method leads to phase separation in the presence of surfactant excess. However, for cPEI = 50 and 250 mg 3 dm
3, a dramatic difference between the results of the two mixing methods is observable at high surfactant-to-polyelectrolyte ratios. Contrary to the transparent systems of negatively charged complexes formed at these PEI concentrations by the stopped-flow mixing protocol, precipitation occurs for the PEI/SDS mixtures prepared via the slow mixing procedure in the same concentration region. These precipitates cannot be dissolved even by continuous stirring with magnetic stirrer for a long time (weeks). These results can be explained by taking into account that in a certain concentration range the transparent PEI/SDS mixtures— prepared via the stopped-flow mixing procedure—are trapped in the nonequilibrium colloidal dispersion state.36
38,41,42 In the presence of polyelectrolyte excess, the colloidal dispersions of the PEI/SDS nanoparticles are stabilized by the uncompensated charges of the PEI molecules.38 At high surfactant-to-polyelectrolyte ratios, the kinetic stability of these dispersions is ensured via the excess charge of the dodecyl sulfate ions adsorbed on the surface of the PEI/SDS nanoparticles.41,42 14800

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Figure 5. Mean electrophoretic mobility (uζ) and apparent mean hydrodynamic diameter (dH) of PEI/SDS/F108 nanocomplexes, as well as turbidity [(100
T)%] of the systems, as a function of surfactant concentration at cPEI = 50 mg 3 dm
3. Mixtures in the absence of Pluronic (9, green) as well as in the presence of 2 g 3 dm
3 F108 (b, red) were prepared via the stopped-flow mixing procedure. The green hatched area indicates the composition range of precipitated systems without added F108. Solid and open symbols designate the measurements carried out 24 h and 2 weeks after solution preparation, respectively. (---) Roughly constant mean size of PEI/SDS/F108 complexes formed in the intermediate SDS concentration range; ( 3 3 3 ) apparent mean diameter of PEI/SDS nanoparticles formed at large excess of SDS without added F108. For the sake of clarity, the values of uζ and dH measured 2 weeks after solution preparation are not plotted (since they were identical within experimental error with the values measured after 24 h). Standard error of the mobility and turbidity values is commensurate with the size of the symbols.

The mean size of the formed P/S nanoparticles and the width of the kinetically stable composition range of their colloidal dispersion are determined by local coagulation of the primary P/S complexes, which is induced by the local inhomogeneities developed during the homogenization of the system.37,43 Since the initial rate of this local aggregation process is roughly proportional to the square of the polyelectrolyte concentration, even very rapid mixing of the components cannot prevent coagulation of the PEI/ SDS nanoparticles at cPEI = 1000 mg 3 dm
3 in the presence of surfactant excess. In the slow mixing procedure, the SDS solution is added in a dropwise manner to the PEI solution and the rate of mixing is smaller compared to the stopped-flow mixing protocol. At very low surfactant-to-polyelectrolyte ratios, both procedures lead to PEI/ SDS complexes of the same size and charge since the PEI/SDS mixtures are thermodynamically stable solutions in this composition range.36
38 However, above a certain surfactant-to-polyelectrolyte

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Figure 6. Mean electrophoretic mobility (uζ) and apparent mean hydrodynamic diameter (dH) of the PEI/SDS/F108 nanocomplexes, as well as turbidity [(100
T)%] of the systems, as a function of surfactant concentration at cPEI = 250 mg 3 dm
3. The meaning of the symbols is the same as in Figure 5. For the sake of clarity, the values of uζ and dH measured 2 weeks after solution preparation are not plotted (since they were identical within experimental error with the values measured after 24 h). Standard error of the mobility and turbidity values is commensurate with the size of the symbols.

ratio, the slightly charged PEI/SDS nanoparticles—formed in the intermediate stage of the slow mixing procedure—coagulate. This aggregation process results in phase separation. Further addition of SDS solution and subsequent homogenization of the system cannot disaggregate the irreversibly coagulated PEI/SDS nanoparticles, which leads to a diminishing concentration range of kinetically stable PEI/SDS dispersions. Thus, in contrast to the rapid homogenization of the system, the slow mixing procedure leads to precipitation in the investigated composition range of surfactant excess even at cPEI = 50 and 250 mg 3 dm
3. Furthermore, the lack of kinetically stable PEI/SDS dispersions in the presence of polyelectrolyte excess leads to precipitation from considerably smaller surfactant-to-polyelectrolyte ratios compared to the mixtures prepared by the stopped-flow mixing protocol. PEI/SDS/F108 Supramolecular Nanocomplexes. Addition of the amphiphilic block copolymer F108 to the PEI/SDS mixtures considerably changes the properties of the formed nanocomplexes. This is shown in Figures 5
7, where the mean electrophoretic mobility and apparent mean hydrodynamic diameter of the PEI/SDS/F108 nanocomplexes as well as the turbidity of the mixtures are plotted as a function of analytical surfactant concentration at cF108 = 2 g 3 dm
3 as well as at cPEI = 50, 250, and 1000 mg 3 dm
3, respectively. The PEI/SDS/F108 multicomponent systems were prepared by the stopped-flow mixing protocol. For the sake of comparison, the experimental 14801

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Figure 7. Mean electrophoretic mobility (uζ) and apparent mean hydrodynamic diameter (dH) of PEI/SDS/F108 nanocomplexes, as well as turbidity [(100
T)%] of the systems, as a function of surfactant concentration at cPEI = 1000 mg 3 dm
3. The meaning of the symbols is the same as in Figure 5. For the sake of clarity, the values of uζ and dH measured 2 weeks after solution preparation are not plotted (since they were identical within experimental error with the values measured after 24 h). Standard error of the mobility and turbidity values is commensurate with the size of the symbols.

data of PEI/SDS mixtures in the absence of added neutral copolymer (prepared by stopped-flow mixing) are also given in Figures 5
7. Variation of mean mobility of the complexes with surfactant concentration is largely different in the absence than in the presence of Pluronic copolymer F108 at each PEI concentration. In the presence of polyelectrolyte excess, uζ decreases with increasing SDS concentration; however, the observed mobility values are considerably smaller at cF108 = 2 g 3 dm
3 than without added neutral copolymer. Charge reversal of the PEI/SDS/F108 complexes occurs roughly in the same SDS concentration range as in the case of the PEI/SDS mixtures at each polyelectrolyte concentration. In the presence of surfactant excess, a plateau with slightly negative mobility values can be observed in a relatively wide SDS concentration range of the PEI/SDS/F108 system, whereas in the same composition range of PEI/SDS mixtures, large mobility values (in absolute values) were detected for the negatively charged complexes. With a further increase of surfactant concentration, the electrophoretic mobility of PEI/SDS/ F108 nanocomplexes starts to markedly decrease again with increasing SDS concentration. According to Figures 5
7, the analytical SDS concentrations corresponding to this latter break point of the mobility versus surfactant concentration curves are at cSDS = 4.7 ( 0.5; 6.6 ( 0.6, and 14 ( 1 mM for cPEI = 50, 250,

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and 1000 mg 3 dm
3, respectively. If it is assumed that up to these SDS concentrations the anionic surfactant binds only to the PEI molecules, the free surfactant concentration can be roughly estimated from the binding isotherm of SDS on PEI, which is shown in Figure S3 of the Supporting Information. According to these calculations, the free surfactant concentrations belonging to the break points of the mobility versus SDS concentration curves are at ce,SDS = 4.2 ( 0.5, 4.2 ( 0.6, and 4 ( 1 mM for cPEI = 50, 250, and 1000 mg 3 dm
3, respectively. These values are in good agreement with the cacPEO/SDS values determined from conductivity measurements (see Figure 1 and Figure 2 of the Supporting Information), indicating the onset of surfactant binding on the PEO blocks of the amphiphilic copolymer. uζ versus cSDS curves similar to the ones in Figures 5
7 were observed recently for complexation between PEI and SDS in the presence of PEO.47 Variation of mean diameter of the complexes and turbidity of the systems with SDS concentration also markedly deviate in the absence and presence of the amphiphilic neutral copolymer. The most important difference compared to PEI/SDS mixtures is that the formation of precipitates cannot be observed in the investigated composition range of PEI/SDS/F108 mixtures. In the presence of F108, apparent mean diameter of the complexes decreases and turbidity of the mixtures increases with increasing SDS concentration. With a further increase of the SDS concentration, both quantities become roughly constant over a wide surfactant concentration range including the composition region of positively charged, electroneutral, and negatively charged nanocomplexes. In this latter composition range, the mean size of PEI/SDS/F108 nanoparticles is similar at each polyelectrolyte concentration: 59 ( 2, 59 ( 2, and 61 ( 3 nm for cPEI = 50, 250, and 1000 mg 3 dm
3, respectively. These values are significantly higher compared to the reduced mean size of the PEI/SDS nanoparticles formed at large excess of surfactant at cPEI = 50 and 250 mg 3 dm
3 (45 ( 2 nm). At even higher SDS concentrations, exceeding the analytical surfactant concentration that corresponds to the break point of the uζ versus cSDS curves, the apparent mean diameter of the complexes starts to slightly decrease again. An important finding of Figures 5
7 is that turbidity of the PEI/SDS/F108 mixtures as well as the values of uζ and dH of the formed complexes, measured 24 h and 2 weeks after solution preparation, did not reveal any time dependence. Adsorption of F108 Molecules on the Surface of PEI/SDS Nanoparticles. In principle, the addition of F108 may change the amount of dodecyl sulfate ions bound to the PEI molecules at a given polyelectrolyte and surfactant concentration, which might explain the observed deviations between the mean size and mobility of the complexes measured in the absence and presence of F108. It has been shown recently that the pH change induced by addition of surfactants to the solutions of oppositely charged weak polyelectrolytes can be used to monitor surfactant binding onto the oppositely charged groups of the macromolecules. Specifically, in the framework of a simple model, it was revealed that the pH of the PEI/SDS mixtures (at constant PEI concentration) increases monotonously with increasing amount of dodecyl sulfate ions bound to the protonated amine groups of PEI.36 In Figure 8, the pH of the PEI/SDS mixtures is shown in the absence and presence of F108 (cF108 =2 g 3 dm
3) at cPEI = 250 mg 3 dm
3. The shape of this curve reflects the fact that surfactant binding leads to a significant increase in protonation degree of PEI due to its weak polyelectrolyte nature.36,56 As is clearly indicated by the figure, the pH of the PEI/SDS mixtures is not affected significantly by addition of amphiphilic neutral 14802

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Table 2. Thickness of Adsorbed Pluronic Layer on PEI/SDS Nanoparticles at Different SDS Concentrations for cPEI = 250 mg 3 dm
3 and cF108 = 2 g 3 dm
3 cSDS (mM)

dF108a ((1 nm) ads

dF108b ((1 nm) ads

4.5

6

8

5.0

7

7

5.5

7

7

6.0

6

6

a

From dynamic light scattering (DLS) measurements. b From electrophoretic mobility measurements.

Figure 8. pH of the PEI/SDS mixtures as a function of the total surfactant concentration without added Pluronic (b) and in the presence of 2 g 3 dm
3 F108 (O). Mixtures were prepared by stoppedflow mixing. Standard error in the pH values is commensurate with the size of the symbols. cPEI = 250 mg 3 dm
3.

Table 1. Thickness of Adsorbed Pluronic Layer on PEI/SDS Nanoparticles at Different SDS Concentrations for cPEI = 50 mg 3 dm
3 and cF108 = 2 g 3 dm
3 cSDS (mM)

dF108a ((1 nm) ads

dF108b ((1 nm) ads

2.5

6

7

3.0

7

8

3.5

7

7

4.0

7

6

a

From dynamic light scattering (DLS) measurements. b From electrophoretic mobility measurements.

copolymer in the investigated composition range. Similar results were observed at cPEI = 50 and 1000 mg 3 dm
3 (see Figure S4 in the Supporting Information). These observations suggest that the amount of dodecyl sulfate ions bound to the PEI molecules at a given composition is not dependent on the concentration of F108 (in the investigated concentration range of the copolymer). In light of this result, one possible interpretation of the effect of F108 is related to its adsorption on the surface of formed PEI/ SDS nanoparticles. Since the internal part of the PEI/SDS nanoparticles is considerably hydrophobic, adsorption of the amphiphilic block copolymer is mainly driven by hydrophobic interactions. The thick adsorbed F108 layers largely increase the mean hydrodynamic diameter and significantly reduce the measured mean electrophoretic mobility values of the P/S nanoparticles, similarly to the effect of PVP or PEO additives.47,48 Adsorption of the amphiphilic block copolymer is most pronounced if the interaction between F108 and SDS is negligible, that is, below the total SDS concentration belonging to the break point of the uζ versus cSDS curves. At higher SDS concentrations (i.e., at ce,SDS > cacPEO/SDS), due to increasing surfactant binding on the nonionic copolymer, the amount of F108 molecules adsorbed on the PEI/SDS nanoparticles decreases. Therefore, the difference between the mean size and mobility of PEI/SDS and PEI/SDS/F108 nanocomplexes also decreases. Similar to the earlier studies on PEI/SDS systems with linear neutral homopolymers, the thickness of adsorbed F108 layers

Figure 9. Mean electrophoretic mobility (uζ) of PEI/SDS nanoparticles plotted against concentration of Pluronic copolymer. (Insets) Thickness of adsorbed F108 layer (dF108 ads ) calculated from plotted uζ data as a function of cF108. (a) cPEI = 50 mg 3 dm
3 and cSDS = 3.0 mM SDS; (b) cPEI = 250 mg 3 dm
3 and cSDS = 5.0 mM SDS .

can be roughly approximated from DLS and mobility measurements carried out in the absence and presence of F108. The calculations are limited to that specific SDS concentration range of surfactant excess, where the F108/SDS interaction is negligible. It is also assumed that the internal and surface structure as well as the size of the primary PEI/SDS nanoparticles is not affected by adsorption of the neutral copolymer. (Further details and assumptions of the calculations can be found in the Supporting Information.) The calculated adsorbed layer thickness data for various 14803

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Figure 10. Effect of mixing methods and addition of F108 block copolymer on the phase properties of the PEI/SDS system. The photos show the mixtures prepared by (left) slow mixing without F108; (middle) stopped-flow mixing without F108; and (right) stopped-flow mixing in the presence of 2 g 3 dm
3 F108. The concentration of SDS increases from left to right in each panel as follows: 2.0, 3.0, 3.5, and 4.0 mM SDS. cPEI = 250 mg 3 dm
3 and pHin = 6.

surfactant-to-polyelectrolyte ratios at cF108 = 2 g 3 dm
3 are shown in Tables 1 and 2 for cPEI = 50 and 250 mg 3 dm
3, respectively. The values of dF108 ads determined from mobility and DLS measurements are in good agreement at a given composition and indicate the formation of thick adsorbed polymer layers (dF108 ads = 7 nm). Figure 9 shows the mean electrophoretic mobility of PEI/SDS nanoparticles as a function of F108 concentration at fixed surfactant-to-polyelectrolyte ratios for cPEI = 50 and 250 mg 3 dm
3. In the insets, the adsorbed layer thickness calculated from plotted mobility data is also shown against the concentration of uncharged polymer. With increasing concentration and therefore surface coverage of F108, the electrophoretic mobility of P/S nanoparticles becomes less negative and then levels off at high concentrations of F108. Similar mobility versus nonionic polymer concentration curves were observed for the PEI/SDS/PEO and PEI/ SDS/PVP systems.47,48 The shape of these curves can be rationalized by taking into account that the adsorption of neutral copolymer on PEI/SDS nanoparticles tends to saturation with increasing F108 concentration. It should be noted that according to Figure 9 the adsorption of F108 molecules could be considerable at copolymer concentrations as low as 0.2 g 3 dm
3. This is illustrated in Figure S5 of the Supporting Information, where mean size and mobility of the complexes, as well as turbidity of the PEI/ SDS/F108 mixtures, are shown in the presence of 0.2 g 3 dm
3 F108 at cPEI = 250 mg 3 dm
3. Figure S5 (Supporting Information) as well as the estimated layer thickness data of Table S1 (Supporting Information) indicate significant adsorption of F108 molecules even at this low Pluronic concentration. This leads to the complete suppression of precipitation in the investigated

composition range of PEI/SDS mixtures, similar to the results at cF108 = 2 g 3 dm
3 (see Figure 6). These results suggest that, for a suitably large amount of adsorbed F108 molecules, they form thick adsorbed layers on the surface of PEI/SDS nanoparticles. At hydrophobic substrate/ water interfaces, the adsorbed molecules of F108 can be found in a brushlike conformation where the hydrophobic PPO block is positioned at the interfacial region and the hydrophilic PEO blocks are extended into the solution.57 The thickness of the adsorbed F108 layers at high surface coverage on two different types of hydrophobically modified silica surface57 (=10 nm, measured under experimental conditions similar to those in the present study) is in good agreement with the results of Figure 9. It should be emphasized, however, that the structural features of the formed PEI/SDS/F108 nanoparticles cannot be completely elucidated solely on the basis of electrophoretic mobility and DLS measurements. Therefore, further investigations are necessary to explore the internal structure of PEI/SDS/F108 complexes as well as the composition of their surface layer. Impact of F108 on Kinetic Stability of the PEI/SDS System. The above-mentioned observations clearly indicate that, similarly to PEO or PVP, the F108 molecules could adsorb on the surface of PEI/SDS nanoparticles. However, a very important deviation compared to the impact of the earlier studied linear neutral homopolymer additives is that, in the presence of an appropriate amount of F108, precipitation of PEI/SDS systems can be completely suppressed even at PEI concentrations as large as 1000 mg 3 dm
3. In the case of PEO or PVP, very rapid mixing of the solution components cannot prevent the aggregation of slightly charged or 14804

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Langmuir electroneutral PEI/SDS nanoparticles even at polyelectrolyte concentrations as low as cPEI = 50 mg 3 dm
3.47,48 This marked difference in the stability of multicomponent mixtures of PEI and SDS with nonionic polymer additives is related to different adsorption affinity, adsorbed layer structure, and adsorption kinetics of the investigated uncharged macromolecules. On one hand, in the case of PEO and PVP, only the high molecular weight samples (Mw = 100 000) can create an adsorbed layer on the surface of PEI/SDS nanoparticles thick enough to provide steric repulsion between the approaching nanoparticles.47,48 The adsorption rate of these long polymer chains is quite low at the applied concentrations of PVP or PEO, whereas coagulation of PEI/SDS nanoparticles of low surface charge density is highly accelerated. Therefore, even the rapid homogenization of the mixtures cannot ensure enough time for the development of a protective adsorbed polymer layer on the surface of slightly charged or electroneutral PEI/SDS nanoparticles before their aggregation. Thus, in the lack of extra steric stabilization, precipitation occurs in the intermediate SDS concentration range of PEI/SDS mixtures in both the presence and absence of PEO or PVP. On the other hand, despite the considerably lower molecular weight of F108 compared to the earlier investigated PEO or PVP stabilizers, adsorption of this Pluronic-type copolymer could provide steric stabilization of the PEI/SDS nanoparticles through the formed brushlike adsorbed polymer layer structure depicted schematically in Figure 9. Because of the considerably higher adsorption rate of F108 in the investigated polymer concentration range and due to its significantly higher adsorption affinity compared to the PEO and PVP samples discussed in refs 47 and 48, upon application of the stopped-flow mixing procedure, the amphiphilic block copolymer could adsorb to a large extent on the surface of the uncharged or charged PEI/SDS nanoparticles before they coagulate. Therefore, addition of F108 leads to transparent systems in the investigated composition range of PEI/SDS mixtures at each PEI concentration. It is important to note that, apart from a narrow concentration range at very low surfactant-to-polyelectrolyte ratios, these multicomponent systems are not thermodynamically stable solutions but sterically stabilized colloidal dispersions of the PEI/SDS/ F108 nanoparticles with hydrophobic core and hydrophilic shell. In order to demonstrate the nonequilibrium character of the PEI/ SDS/F108 mixtures, they were also prepared by the two-step mixing protocol. During this mixing procedure, the final composition was attained in two steps. First, PEI/SDS mixtures were prepared via the slow mixing protocol, and then after a day they were mixed with F108 solutions. In this case, the PEI/SDS/F108 mixtures remain precipitated over a wide composition range (see Figures S6 and S7 of the Supporting Information) even if they were stirred for a long time (weeks). This observation is attributable to the fact that there was no chance for the adsorption of amphiphilic copolymer on the surface of primary PEI/SDS nanoparticles prior to their aggregation.

’ CONCLUSIONS The major results of our study are illustrated in Figure 10. In the absence of an uncharged polymer, less rapid mixing of PEI and SDS solutions (for instance, the slow mixing protocol) results in two-phase systems over a very wide composition range. The very rapid homogenization of PEI/SDS mixtures (stoppedflow mixing protocol) could largely decrease the two-phase

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composition region via increasing the concentration range of charge-stabilized PEI/SDS dispersions. Finally, stopped-flow mixing of polyelectrolyte and surfactant solutions containing the amphiphilic copolymer F108 leads to complete suppression of precipitation due to the formation of sterically stabilized dispersions of uncharged and charged PEI/SDS nanoparticles with hydrophobic core and hydrophilic corona. The results of the present study have revealed that, as an alternative to block ionomer/surfactant systems, stable electroneutral nanocomplexes of oppositely charged homopolyelectrolytes and surfactants can be prepared by combined highly efficient and rapid mixing methods and suitable nonionic amphiphilic block copolymer additives.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional text, seven figures, and one table with experimental information as discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected].

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