Segregation in Like-Charged Polyelectrolyte–Surfactant Mixtures Can

Mar 5, 2013 - ... Departamento de Física de la Materia Condensada, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain...
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Segregation in Like-Charged Polyelectrolyte−Surfactant Mixtures Can Be Precisely Tuned via Manipulation of the Surfactant Mass Ratio Peter W. Wills,† Sonia G. Lopez,‡ Jocelyn Burr,† Pablo Taboada,‡ and Stephen G. Yeates*,† †

School of Chemistry, The University of Manchester, Manchester, U.K. Group de Física de Coloides y Polímeros, Departamento de Física de la Materia Condensada, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain

‡

ABSTRACT: In this study, we consider segregative phase separation in aqueous mixtures of quaternary ammonium surfactants didecyldimethylammonium chloride (DDQ) and alkyl (C12, 70%; C14 30%) dimethyl benzyl ammonium chloride (BAC) upon the addition of poly(diallyldimethylammonium) chloride (pDADMAC) as a function of both concentration and molecular weight. The nature of the surfactant type is dominant in determining the concentration at which separation into an upper essentially surfactant-rich phase and lower polyelectrolyte-rich phase is observed. However, for high-molecular-weight pDADMAC there is a clear indication of an additional depletion flocculation effect. When the BAC/DDQ ratio is tuned, the segregative phase separation point can be precisely controlled. We propose a phase separation mechanism for like-charged quaternary ammonium polyelectrolyte/surfactant/water mixtures induced by a reduction in the ionic atmosphere around the surfactant headgroup and possible ion pair formation. An additional polyelectrolyte-induced depletion flocculation effect was also observed.

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INTRODUCTION Because of their commercial importance in cosmetics, detergents, and pharmaceutical and biotechnological applications,1−6 several polymer/surfactant/water mixtures have been investigated, and many of these have been shown to undergo isothermal phase separation into polyelectrolyte-rich and surfactant-rich phases. However, the area of like-charged polyelectrolyte−surfactant mixture interactions remains less well understood,7−16 particularly for formulations containing quaternary ammonium chloride polyelectrolytes. Comas-Rojas et al.7 studied the interaction of polyethyleneimine (PEI) and cetyltrimethylammonium bromide (CTAB) under dilute conditions, where the presence of PEI was found to result in a reduction in the critical micelle concentration (cmc) and the formation of meso-structured thin films at the air−water interface. This was rationalized in terms of weak interactions occurring between CTAB and PEI in solution, giving rise to surfactant templating leading to the suppression of the electric double layer, causing the counterions to be held more tightly.17 At higher concentrations, the segregative phase separation of like-charged polyelectrolyte−surfactants into an upper surfactant-rich phase and a lower polyelectrolyte phase has been reported,11,13−16 which could be further triggered at much lower polymer/surfactant concentrations by the addition of NaCl.13−15 Nilsson et al.15,16 illustrated that in these segregative types of systems the addition of a hydrophobic cosolute can affect the polyelectrolyte/surfactant compatibility and hence © 2013 American Chemical Society

the critical phase separation concentration. The addition of octane was observed to increase the compatibility, but the addition of octanol decreased the compatibility. These observations were justified in terms of the micelle aggregation number. Segregative phase separation observed in the polymer−surfactant systems is analogous to that in polymer− polymer mixtures, which also segregatively phase separate, and this phenomena is commonly referred to as polymer incompatibility. Polymer incompatibility occurs when the effective interactions between unlike polymers are repulsive or/and have different levels of affinity for the solvent.14 A detailed evaluation of the segregative phase separation of sodium hyaluronate/sodium dodecyl sulfate/water,13 poly(sodium-4-styrenesulfonate)/sodium dodecyl sulfate/water,11 and poly(diallyldimethylammonium) chloride (pDADMAC)/ cetyltrimethylammonium bromide/water11 showed that in each case the water was found to partition so as to maintain chemical potential neutrality between the two phases. Although this explains the final equilibrium state, it fails to explain what initially triggers the phase separation within these mixtures. The objective of this Article is to further study the processes triggering the isothermal phase separation in concentrated multicomponent aqueous mixtures of quaternary ammonium chloride polyelectrolyte and quaternary ammonium surfacReceived: October 29, 2012 Revised: March 4, 2013 Published: March 5, 2013 4434

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H NMR was conducted on a Bruker Ultrashield 400 MHz spectrometer. Samples were rehydrated in D2O (Sigma-Aldrich) before measurement. Gravimetric analysis was conducted by a weighed sample being transferred to a sample pan and placed in a vacuum oven (80 °C). Every 24 h the sample was weighed until no further weight loss was observed. Three repeat measurements were conducted per sample, with the average result recorded. Dilute solution viscometry was carried out using an Ubbelohde (Rheotek - VIS3300) capillary viscometer at 25 °C. Individual samples were formulated from different concentrations in 1 M NaCl, with each solution being left for 30 min prior to determination and the average of three measurements taken. Samples were formulated by mass with the addition of a pDADMAC stock solution, a 2 M NaCl stock solution, and Millipore-filtered water. Huggins Kraemer plots were produced for each polymer sample, and extrapolation to zero concentration provided the intrinsic viscosity (η). Surface tension measurements were carried out using a Kibron Delta 8 tensiometer, which is based on the DeNouy method with a platinum rod probe. Serial dilutions of 0.5 from a concentrated stock solution were performed using an Epmotion 5075 liquid handler (Eppendorf) in a 12 × 8 well plate. Each well was filled with a total sample volume of 100 μL. A total of eight replicate measurements were made for each concentration. A purified distilled water control was present on each plate. Photon correlation spectroscopy (PCS) particle sizing was performed using a Zetasizer-Nano from Malvern Instruments with a He−Ne laser having a wavelength of 632.8 nm and a back-scattering detector set at 173°. Solutions were filtered twice with a 0.22 μm poly(ethersulfone) syringe filter and then placed into a polystyrene 10 × 10 × 45 mm3 cuvette. The results are an average of four measurements made at 25 °C. A CONTIN program was used to deconvolute the size distribution.18 To identify a suitable concentration of NaCl to be added to the surfactant solution, a series of solutions were formulated for each surfactant type at constant surfactant concentration (100 mM). The NaCl concentration below the surfactant cloud points were chosen for the PCS experiment: 5 × 10−3 M DDQ and 0.3 M BAC. The samples for the phase separation studies were measured by mass from the appropriate stock solution. Samples were stirred for 1 h with a magnetic stirrer and then inverted 10 times by hand. Visual observations were made 48 h after the last agitation to determine if phase separation had occurred.

tant(s). In this study, we focus solely on pDADMAC of varying molecular weight and its interactions with quaternary ammonium surfactants didecyldimethylammonium chloride (DDQ) and alkyl (C12, 70%; C14 30%) dimethyl benzyl ammonium chloride (BAC) and their mixtures.

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EXPERIMENTAL SECTION

Reagents and Sample Preparation. pDADMAC, Mv = 8.5 kDa, was obtained from Poly Sciences Inc. as a 28 wt % solution in water, and pDADMAC, Mv = 21 and 140 kDa, was obtained from SigmaAldrich as 35 and 20 wt % solutions, respectively. DDQ was obtained from Lonza (trade name - Lonza Bardac 2240) and received as a 40 wt % solution in water. BAC was obtained from Thor (trade name Acticide BAC 50 M) and received as a 50 wt % solution in water. The concentrations of the above stock solutions were verified gravimetrically by drying in a vacuum oven at 80 °C until no further mass loss was recorded. Three repeat measurements were conducted per sample with the average result recorded. Both the surfactants and polyelectrolyte were used as received without further purification unless otherwise stated (Table 1).

Table 1. Chemical Structures of the Polymer and Surfactants Used in the Study

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RESULTS AND DISCUSSION Poly(diallyldimethylammonium) Chloride (pDADMAC). Molecular weight characterization of the pDADMAC samples used in the study is given in Table 2. SEC Table 2. Molecular Weight Determination of pDADMAC Polymers

When specifically stated, the 140 kDa pDADMAC was dialyzed against Millipore-filtered water using benzoylated dialysis tubing (2 kDa molecular weight cutoff, Sigma-Aldrich, D7884-1FT) for four days with the water changed twice every day, at which point the conductivity of the water within the beaker had returned to a level comparable to that of Millipore-filtered water. The conductivity was measured on a Jenway 4010 conductivity meter at 25 °C. Analytical-grade sodium chloride was obtained from Fisher Scientific. Millipore-filtered water was used for all solutions. Fresh solutions were always used for all experiments. Stock solutions were prepared in polypropylene bottles (Nalgene bottles, style 200, 100 mL, purchased from Sigma-Aldrich), with individual samples prepared in 10 mL polypropylene graduated sample tubes. Procedures and Techniques. Size exclusion chromatography (SEC) was carried out using TSK gel columns. Two different SEC systems were used: low-molecular-weight (G2000PW and G3000PW) and high-molecular-weight (G4000PW and G5000PW) in combination with a Gilson 132 refractive index detector at a flow rate of 0.5 mL/min with a 0.1 M citric acid buffer at 25 °C, calibrated using poly(ethylene oxide) standards.

SEC

viscometry

supplied pDADMAC

Mw (kDa)

Mn (kDa)

Mw/Mn (PDI)

Mv (kDa)

Poly Sciences Inc. Sigma-Aldrich Sigma-Aldrich

12.3 37.6 403

4.6 4.1 19

2.7 9 21

8.5 21 140

chromatograms in all cases were monomodal of varying polydispersity. Viscosity-average molecular weights Mv, used subsequently to define the polymers, were obtained using the Kuhn−Mark−Houwink−Sakurada relationship η = KMa, the constant K = 4.71 × 10−3, and exponent α = 0.83 being an average from a range of references under similar experimental conditions.19−21 Quaternary Ammonium Chloride Surfactant. Surface tension profiles of the pure surfactants and BAC/DDQ (2:3 4435

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Figure 1. (a) Surface tension profiles in water at 25 °C for (□) BAC, (○) DDQ, and (Δ) BAC/DDQ (2:3 mol/mol). (b) PCS intensity profiles at 25 °C as a function of concentration: (i) BAC (0.3 M NaCl), (ii) BAC/DDQ 2:3 mol/mol (0.05 M NaCl), and (iii) DDQ (5 × 10−3 M NaCl).

show a monomodal distribution over the concentration range of interest, with the mean Z average and the monomodal distribution suggestive of ellipsoidal or spherical micelle geometry. DDQ shows a bimodal/trimodal distribution, suggesting micellar structures with a large aggregation number (Table 4). This can be explained either by cylindrical geometry that has a radial length and an axial length or spherical micelles in equilibrium with unimellar or multimellar vesicles.28 Both scenarios are consistent with the findings of an earlier fluorescence probe study of DDQ, which observed an increase in micelle aggregation number from 20 to 86 with increasing concentration from 10 to 81 mM.29 Phase Separation in Mixtures of p(DADMAC) with DDQ and BAC. In an attempt to quantify the critical ionic strength required for phase separation within each system, a series of samples were formulated containing 100 mM (∼3.6 wt %) surfactant plus differing concentrations of polyelectrolyte, including by way of comparison the effect of NaCl and the first sample within the series to phase separate identified. A phase separation boundary diagram was constructed to illustrate the phase boundary between non-phase-separating and phaseseparating mixtures for 8.5 kDa pDADMAC/DDQ (Figure 2a), with an example of the phase-separating behavior shown in Figure 2b. For selected pDADMAC/surfactant/water mixtures that exhibited segregative phase separation, aliquots from the top and bottom phases were taken 24 h from the last agitation. The volume of each phase was measured, and a predicted weight % was calculated that assumed complete phase separation between the two phases. Gravimetric analysis was performed, which indicated a concentrated top phase and a more dilute

mol/mol), chosen for particular attention because of its recently reported beneficial performance in antimicrobial formulations,22 were determined (Figure 1a). The cmc and calculated surface area per headgroup were determined using the Gibbs adsorption isotherm17 (Table 3), with both BAC and Table 3. Tabulated CMC and Surface Area per Headgroup Values for BAC, DDQ, and BAC/DDQ 2:3 mol/mol at the Air−Water Interface at 25 °C experimental surfactant

CMC (mM)

BAC

3.5

DDQ BAC/DDQ (2:3 mol/mol)

1.6 1.9

literature

experimental

CMC (mM)

area of headgroup (Ǻ 2 ± 1)

C12 − 8.1,25 8.824 C14 − 1.9,27 2.024 1.228 1.929 N/A

83

79 78

DDQ having comparable headgroup surface areas. For BAC, this arises from the phenyl group contained within BAC orientating itself next to the quaternary ammonium headgroup instead of inside the micellar interior, thus creating a more bulky headgroup,23−27 whereas in DDQ the two decyl tails of the surfactant cause greater tail packing, constraining the surfactant’s ability to adsorb efficiently at the air−water interface. Photon correlation spectroscopy (PCS) was used to determine the hydrodynamic diameter of the micelles. The PCS profiles of BAC and BAQ/DDQ 2:3 mol/mol (Figure 1b) 4436

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studies.11 As such, the terms surfactant-rich and polyelectrolyte-rich layers will be used to describe the different phases. For the surfactants and surfactant mixtures studied, increasing the molecular weight of pDADMAC from 8.5 to 140 kDa has a negligible effect on the phase separation boundary (Figure 4a). The effect on the phase separation boundary of surfactant type at constant polyelectrolyte molecular weight was more dramatic with the DDQ phase separating at a much lower concentration compared to that for BAC, with mixtures, exemplified by BAC/DDQ (2:3 mol/mol), lying systematically between the two extremes (Figure 4b). It should be noted that the tensiometry profile of BAC showed a minimum around the cmc that is indicative of impurities present, with the common explanation being that hydrophobic cosolutes are present, namely, long-chain alcohols. As discussed previously, long-chain alcohols can increase the polyelectrolyte/ surfactant compatibility but are present at concentrations where this will have a small effect. To investigate fully the effect of the BAC/DDQ surfactant ratio on the phase boundary, a series of mixtures were formulated at constant surfactant concentration (100 mM ≈ 3.6 wt %) with differing pDADMAC (21 kDa) concentration and as a comparison to differing NaCl concentrations. The first sample within the series to phase separate was identified as the phase separation concentration for that system (Figure 5). The NaCl/surfactant/water mixtures that showed segregative phase separation were observed to give a surfactant-rich upper phase analogous to the phase separation within the polyelectrolyte/surfactant/ water mixtures. This illustrates that the phase boundary can be manipulated by altering the ratio of BAC/DDQ for both the pDADMAC (21 kDa) and NaCl systems.

Figure 2. (a) Phase separation boundary for pDADMAC (8.5 kDa)/ DDQ/water at 25 °C: (□) one phase and (■) two phases. (b) Phase separation image for sample pDADMAC (3 wt %)/DDQ (3 wt %)/water (t = time since last agitation).

bottom phase. In Table 5 for pDADMAC 8.5 kDa (3 wt %)/DDQ (3 wt %)/water, the predicted and actual weight % Table 4. Tabulated PCS Z-Average Values of the Respective Surfactants at 25 °C intensity - Z average (nm) surfactant concentration (mM)

BAC

BAC/DDQ (2:3 mol/mol)

10 20

N/A 7.8

N/A 8.0

50 100

6.8 7.1

9.2 9.1

DDQ (1) 3.3, (2) 235.0 (1) 3.1, (2) 13.0, (3) 263.4 N/A (1) 3.1, (2) 48.1

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Table 5. Gravimetric Analysis of Top and Bottom Layers of pDADMAC 8.5 kDa (3 wt %)/DDQ (3 wt %)/Water 24 h after the Last Agitation layer

volume (mL) (±0.05)

predicted (wt %)

actual (wt %)

top bottom

0.60 4.40

24.0 ± 2.0 3.3 ± 0.1

22.7 ± 0.1 4.4 ± 0.1

DISCUSSION A direct comparison could be made between the critical pDADMAC and NaCl phase-separating concentrations with respect to the number of moles of electrolyte; consequently, the number of chloride ions (NCl− phase separation) required for phase separation was calculated (Table 6). In Figure 5b, we compare NCl− phase separation for the pDADMAC and NaCl systems as a function of the DDQ mass fraction. We see that for a surfactant mixture where the mass fraction of DDQ is greater than 0.4 the ionic strength required for segregative phase separation is the

values, although in close agreement, are not statistically comparable. 1H NMR (Figure 3) showed that the upper phase was surfactant-rich whereas the lower phase was found to be polyelectrolyte-rich, which is consistent with earlier

Figure 3. 1H NMR of the top phase and the bottom phase for the sample: (a) pDADMAC 8.5 kDa (3 wt %)/BAC (3 wt %)/water sample and (b) pDADMAC 8.5 kDa (3 wt %)/DDQ (3 wt %)/water sample, 24 h since the last agitation. (Red) top phase (surfactant) and (blue) bottom phase (pDADMAC). 4437

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Figure 4. (a) Phase separation boundary for pDADMAC/DDQ/water as a function of pDADMAC molecular weight: (□) 8.5 kDa, (○) 21 kDa, and (Δ) 140 kDa at 25 °C. (b) Phase separation boundary for pDADMAC (21 kDa)/surfactant/water for (○) DDQ, (□) BAC, and (Δ) BAC/DDQ (2:3 mol/mol) at 25 °C. Dashed lines indicate approximate locations of phase boundaries.

Figure 5. (a) Critical phase separation concentration as a function of the BAC/DDQ mass ratio. The surfactant (BAC/DDQ) concentration was kept constant (100 mM ≈ 3.6 wt %): (○) NaCl and (□) pDADMAC (21 kDa). (b) Number of chloride ions from the electrolyte required to induce phase separation (NCl− phase separation) as a function of the BAC/DDQ ratio. The surfactant (BAC/DDQ) concentration was kept constant (100 mM/ ≈ 3.6 wt %). Electrolyte type: (○) NaCl and (□) pDADMAC (21 kDa).

reasoning behind this is that the distance between the surfactant headgroups is larger within a spherical geometry compared to that within a cylindrical geometry.30−32 PCS results suggest that over the concentration regime studied BAC micelles are ellipsoidal/spherical in shape with a nominal change in micelle size compared to that of DDQ, which is either cylindrical in geometry or a combination of spherical micellar and vesicle structures. A difference in the micelle counterion dissociation constant of the two surfactants seems likely to be a major factor in the differing critical ionic strengths required to induce phase separation within the above mixtures. When the mass fraction of DDQ is less than 0.4, a deviation is observed between the NaCl and pDADMAC systems, with the polyelectrolyte system inducing phase separation at a lower ionic strength, with the effect being most pronounced the higher the pDADMAC molecular weight. To explore this effect further, a range of different molecular weight pDADMAC molecules were studied a over a range of BAC/DDQ ratios at a constant overall surfactant concentration. The critical phase separation concentration was identified, and then NCl− phase separation was calculated. This can be explained as a complementary depletion flocculation effect, where in the case of a polyelectrolyte that does not adsorb onto the surfactant micelle entropic depletion interactions have been known to induce phase separation. Entropic depletion interactions results from changes in the conformational entropy of the polymer chains, which prevents polymers from getting too close to a

same irrespective of whether it is in the form of NaCl or polyelectrolyte. We ascribe this electrolyte driving force to the ionic atmosphere around the surfactants quaternary ammonium headgroup decreasing in size, resulting in the condensing of the counterion. Upon phase separation, the surfactant becomes insoluble in water, forming an immiscible oil that due to its low density will cream to the top of the solution and coalesce to form a surfactant-rich phase. The surfactant molecules become soluble within this surfactant-rich phase because of the absence/reduction in polyelectrolyte concentration and hence the ionic atmosphere around the headgroup increases. From the data presented, it is not possible to determine whether the reduced ionic atmosphere around the quaternary nitrogen was enough to induce phase separation on its own or if an actual intimate ion pair is formed. An intimate ion pair is when the charged moiety, in this case, the quaternary ammonium group, is in direct contact with its corresponding counterion, subsequently neutralizing the ionic charge, and further investigation of this aspect is required to elucidate the mechanism fully. We believe the reason that surfactant mixtures containing a greater mass fraction of DDQ require a lower ionic strength to induce phase separation is due to the different micellar properties of surfactants DDQ and BAC. It has been noted previously in the literature that the degree of counterion dissociation within a micellar structure is smaller for cylindrical micelles compared to that for spherical micelles, and the 4438

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polyelectrolyte depletion flocculation effect. The nature of the surfactant type is dominant in determining the onset of segregative phase separation. For mixtures having a high mass fraction of BAC, there is an indication of an additional depletion flocculation effect particularly for high-molecularweight pDADMACs. To our knowledge, this Article is the first to report the ability to tune the phase separation point of likecharged polyelectrolyte−surfactant mixtures by varying the surfactant ratio (BAC/DDQ).

Table 6. Tabulated Results Showing the Number of Chloride Ions (NCl−) Added to the Ternary Mixtures by Electrolyte to Induce Phase Separationa surfactant type DDQ DDQ DDQ DDQ BAC/ DDQ BAC/ DDQ BAC/ DDQ BAC/ DDQ BAC/ DDQ BAC BAC BAC

weight % of polyelectrolyte to induce phase separation

NCl− phase separation (× 1020)

NaCl pDADMAC (8.5 kDa) pDADMAC (21 kDa) pDADMAC (140 kDa) NaCl

0.04 M 0.7

2.4 2.6

0.7

2.6

0.6

2.2

0.20 M

12

pDADMAC (8.5 kDa) pDADMAC (21 kDa) pDADMAC (140 kDa) pDADMAC (140 kDa)b NaCl pDADMAC (21 kDa) pDADMAC (140 kDa)

3.0

11

2.9

11

2.2

8

2.5

9

1.10 M 12.5

66 47

10.0

37

electrolyte type

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was funded via an EPSRC CASE award grant involving the collaboration of The Organic Materials Innovation Centre, University of Manchester, and Byotrol Plc. We are also grateful to Dr. B. Carter for his help with the Epmotion 5075 liquid handler (Eppendorf) and Kibron tensiometer at The Centre of Materials Discovery, University of Liverpool.

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a Surfactant concentrations remained constant at 100 mM (∼3.6 wt %). Surfactant mixture BAC/DDQ 2:3 mol/mol. bpDADMAC was dialyzed against water to remove low-molecular-weight impurities.

REFERENCES

(1) Oliveira, C. P.; Ribeiro, M.; Ricardo, N.; Souza, T. V. D.; Moura, C. L.; Chaibundit, C.; Yeates, S. G.; Nixon, K.; Attwood, D. The effect of water-soluble polymers, PEG and PVP, on the solubilisation of griseofulvin in aqueous micellar solutions of Pluronic F127. Int. J. Pharm. 2011, 421, 252−257. (2) Ricardo, N.; Costa, F.; Bezerra, F. W. A.; Chaibundit, C.; Hermida-Merino, D.; Greenland, B. W.; Burattini, S.; Hamley, I. W.; Nixon, S. K.; Yeates, S. G. Effect of water-soluble polymers, polyethylene glycol and poly(vinylpyrrolidone), on the gelation of aqueous micellar solutions of Pluronic copolymer F127. J. Colloid Interface Sci. 2012, 368, 336−341. (3) Goddard, E. D. Polymer surfactant interaction 0.1. Uncharged water-soluble polymers and charged surfactants. Colloids Surf. 1986, 19, 255−300. (4) Goddard, E. D. Polymer surfactant interaction 0.2. Polymer and surfactant of opposite charge. Colloids Surf. 1986, 19, 301−329. (5) Hansson, P.; Lindman, B. Surfactant-polymer interactions. Curr. Opin. Colloid Interface Sci. 1996, 1, 604−613. (6) Mace, C. R.; Akbulut, O.; Kumar, A. A.; Shapiro, N. D.; Derda, R.; Patton, M. R.; Whitesides, G. M. Aqueous multiphase systems of polymers and surfactants provide self-assembling step-gradients in density. J. Am. Chem. Soc. 2012, 134, 9094−9097. (7) Comas-Rojas, H.; Aluicio-Sarduy, E.; Rodriguez-Calvo, S.; PerezGramatges, A.; Roser, S. J.; Edler, K. J. Interactions and film formation in polyethylenimine-cetyltrimethylammonium bromide aqueous mixtures at low surfactant concentration. Soft Matter 2007, 3, 747−753. (8) Bromberg, L.; Temchenko, M.; Colby, R. H. Interactions among hydrophobically modified polyelectrolytes and surfactants of the same charge. Langmuir 2000, 16, 2609−2614. (9) Colby, R. H.; Plucktaveesak, N.; Bromberg, L. Critical incorporation concentration of surfactants added to micellar solutions of hydrophobically modified polyelectrolytes of the same charge. Langmuir 2001, 17, 2937−2941. (10) Kosacheva, E. M.; Kudryavtsev, D. B.; Bakeeva, R. F.; Kuklin, A. I.; Islamov, A. K.; Kudryavtseva, L. A.; Sopin, V. F.; Konovalov, A. I. The aggregation of branched polyethylenimine and cationic surfactants in aqueous systems. Colloid J. 2006, 68, 713−720. (11) Kalwarczyk, E.; Golos, M.; Holyst, R.; Fialkowski, M. Polymerinduced ordering and phase separation in ionic surfactants. J. Colloid Interface Sci. 2010, 342, 93−102.

micelle. As a result of this, when two micelles are close enough together to prevent the polyelectrolyte from separating them the region between the micelles is said to be depleted of polymer. The polyelectrolyte outside the depletion zone between micelles induces an osmotic pressure pushing micelles together and encouraging phase separation within the solution mixture.11,33 A number of studies have shown that in colloid− polymer mixtures of like charge and low salinity an enhanced depletion interaction is observed at much lower concentrations than in neutral systems. Long-range repulsive electrostatic forces were found to be behind this enhanced depletion interaction, with the Debye length of the solvent reported to be a major factor. It was also noted that at higher electrolyte concentrations the radius of gyration of the polymer became an important factor in increasing the range of the depletion force.34−37 For the pDADMAC mixtures studied here, the proposed depletion effect is more significant for the mixtures containing a lower mass fraction of DDQ. The depletion effect is also more pronounced at higher molecular weight, with 8.8 kDa pDADMAC in 1 M NaCl reported to have a radius of gyration of 8.6 nm38 whereas 114 kDa pDADMAC is reported to have a radius of gyration of 64 nm.19 We speculate that this is because these mixtures are more concentrated. In more concentrated mixtures, a smaller Debye length would be expected but also entropic interactions will become more dominant and increased depletion interactions would be expected.

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CONCLUSIONS Within this Article, we propose a segregative phase separation mechanism for mixtures of like-charged quaternary ammonium polyelectrolyte/surfactant/water induced by a reduction in the ionic atmosphere between the quaternary nitrogen and chloride counterion of the ionic surfactant and an additional 4439

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dx.doi.org/10.1021/la400130x | Langmuir 2013, 29, 4434−4440