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Ind. Eng. Chem. Res. 2009, 48, 2430–2435
Characterization of Surfactant Partitioning in Polyelectrolyte-Surfactant Nanorod Aggregates Observed with a Surfactant-Specific Electrode Daniel M. Kuntz and Lynn M. Walker* Department of Chemical Engineering, Center for Complex Fluids Engineering, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213
Self-assembled polymer-surfactant aggregates offer an inexpensive and robust approach to controlling the nanoscale structure of materials in solution. However, complete understanding of the partitioning of different species is required for design and engineering of these structures. In this work, the surfactant partitioning of an oppositely charged polyelectrolyte-surfactant (PES) aggregate system in aqueous solution is characterized using a surfactant-specific coated-wire electrode. Previous work has shown that the structure of these aggregates is sensitive to the surfactant partitioning behavior, so characterizing this behavior is vital in controlling the aggregate structure. Due to our synthesis procedure, no small counterions (salts) are present in the system; therefore, the electrolyte concentration can be controlled very accurately and the effect of background electrolyte on surfactant partitioning behavior characterized. A simple model describing the surfactant partitioning in the system is presented and compared to the observed results. Results provide the tools to control the length and surface charge of these rodlike nanoparticles through overall composition. Introduction In aqueous solution, polyelectrolytes (PE), or charged polymer chains, and surfactants (S) are driven by a combination of electrostatic, hydrophobic, and steric interactions to associate to form polyelectrolyte-surfactant aggregates (PES). In many oppositely charged PES systems, these associative interactions lead to the formation of highly nanostructured precipitates,1–5 gel phases,6 and immiscible liquid phases (coacervation)7–12 at the 1:1 molar stoichiometry ratio (with respect to charge) of the complexes. In other systems, the formation of stable “beadson-string” structures useful in solubilization in aqueous media is observed.4 These two extremes are reached by tuning the relative strengths of hydrophobic attraction and electrostatic interaction. Between these extremes, a rich region of behavior exists where aggregates are stable in solution with significant diversity in nanoscale structure. We have developed a series of PES aggregates that form rodlike aggregates which are stable in solution; the aggregates are rigid cylinders with diameters ranging from 3.5 to 5 nm and lengths ranging from tens to hundreds of nanometers. The aggregates maintain amphiphilic behavior,13 can be concentrated into liquid crystalline solutions,14 and have been used to make structured layers at solid-liquid interfaces.15–17 The partitioning of the different species in the system is vital to the control and manipulation of the nanoscale aggregate structure; the goal of the work presented here is to fully characterize the partitioning in this specific PES system. This partitioning is straightforward to characterize in a system with one self-assembling species. In pure surfactant systems, many experimental techniques, including conductivity and surface tension measurements, are employed to determine the partitioning behavior.4,18,19 When applied to PES systems, surface tension measurements are complicated by the existence of multiple surface active species in the system.4 Conductivity measurements provide a direct signal from the bulk phase but can be difficult to interpret when multiple charged species contribute to the observed signal. Recently, surfactant-specific * To whom correspondence should be addressed. E-mail: lwalker@ andrew.cmu.edu.
electrodes have been used to provide a direct characterization method of partitioning in PES systems.20–28 The electrode is tailored to respond only to the surfactant species and provides a direct measurement of surfactant concentration in the bulk phase, allowing the overall partitioning behavior in the PES system to be quantified. In most PES systems, the PE and surfactant are prepared separately and mixed in solution.24–26 Therefore, the original counterions (generally small-molecule inorganic salts) on the PE and surfactant species remain present in the system. As the concentration of either the PE or surfactant is varied, so is the electrolyte concentration due to the presence of these counterions. This changes both the intra-aggregate and interaggregate interactions in the system. In the work discussed here, we investigate the surfactant partitioning in a simpler system: an aqueous PES system prepared with no other counterions present. The aggregates (polymerized cetyltrimethylammonium 4-vinylbenzoate, or pC16TVB) are soluble and extremely stable in water over a wide concentration range. In this system, the PE chain is generated from a polymerizable hydrotrope that acts as the counterion to the surfactant template. The chemistry and structural characterization has been presented previously.14,29–32 Due to the synthesis method, we are able to tune the ratio of charges in the system through the initial composition of the surfactants and polymerizable counterion. The systems discussed here are all prepared so that there is one cationic surfactant for each anionic monomer group on the PE, a state denoted 1:1 or stoichiometrically matched. Since no other counterions are introduced during the synthesis, the resulting solution of aggregates is initially free of background electrolyte so that the electrolyte concentration can be accurately controlled and its effect on partitioning characterized. Previous results using NMR and surface tension to characterize pC16TVB solutions have indicated that some fraction of surfactant counterion (C16TA+) is dissociated from the aggregate phase and resides in the bulk aqueous phase.29,32 From the surface tension measurements it was initially suggested that the dissociated fraction of surfactant is equal to the critical micelle concentration (cmc) of the pure surfactant (0.7 mM, C16TA+); however this is an indirect measure of the concentra-
10.1021/ie8004572 CCC: $40.75 2009 American Chemical Society Published on Web 07/01/2008
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tion of dissociated surfactant. Determination of bulk surfactant concentration by surfactant-specific electrode potentiometry offers a direct method of observation. Here, we directly measure the bulk concentration of surfactant counterion using a surfactant-specific poly(vinyl chloride)-coated-wire electrode constructed using the method of Xu and Bloor.27 We determine the overall surfactant partitioning in this 1:1 polyelectrolytesurfactant system. In addition, the effects of added electrolyte on the partitioning are presented and the formation of surfactantonly micelles is discussed. A simple partitioning model is presented and compared to the observed results. Materials and Methods The surfactant/counterion pair, cetyltrimethylammonium 4-vinylbenzoate (C16TVB), used for this work is synthesized from commercially available cetyltrimethylammonium bromide, or C16TAB (98% purity, BDH Limited, Poole, U.K.) via two counterion exchange steps. After separation and purification, the pure complex salt C16TVB is polymerized using an aqueous phase initiator to yield the pC16TVB product. Detailed explanations of all steps involved in this synthesis are available in previous publications.29,31–33 All salts (NaCl, NaBr) were used as received (Fisher Scientific, Fair Lawn, NJ). The coated-wire electrode was constructed using the method provided by Xu and Bloor for a cationic surfactant.27 All steps were completed as provided by Xu and Bloor, except that a silver-silver chloride (Ag/AgCl) electrode was used for the construction of the surfactant-specific electrode. The method used to create the Ag/AgCl electrode is described elsewhere.34,35 The silver wire used is available commercially (99.99% silver, C. C. Silver and Gold, Inc., Phoenix, AZ). The nonactive wirecoating solutions were made from commercially available products. Uncharged poly(vinyl chloride) (Mw ∼ 233,000) was obtained from Sigma-Aldrich (St. Louis, MO). The tetrahydrofuran (99+% THF) used in all solutions was obtained from Sigma-Aldrich (St. Louis, MO). The surfactant used for the conditioning of the active electrode membrane was recrystallized once from the C16TAB discussed above. The charged poly(vinylchloride) (PVC, carboxylated, Mw ∼ 220000 g/mol) was obtained from Sigma-Aldrich (St. Louis, MO). The plasticizer Elvaloy 742 was donated by Dupont (Wilmington, DE). For the experiments, a commercial Ag/AgCl electrode was used as a reference, and the electromotive force (EMF) was measured using a Keithley 614 electrometer (Keithley Instruments, Cleveland, OH). Measurements were made with steady stirring of sample solutions. Fluctuations in the observed EMF signal were typically (1 mV or less, and the time stability of the observed signal was verified. The rapid and stable response performance of wire-coated surfactant-specific electrodes is detailed by Xu and Bloor.27 The temperature for all experiments was 19-20 °C. All calibration curves exhibited a slope consistent with a Nernstian response for this temperature and ion valency27,28 (50-58 mV/decade). In all cases, some electrolyte is added to the system as done in previous worksa 0.1 mM NaBr background electrolyte is used unless otherwise noted. The surfactant used for electrode calibration experiments and surfactant titration experiments was recrystallized from the source material. Results and Discussion Surfactant Partitioning in the 1:1 System. To ensure that the electrode was operating properly and exhibiting a Nernstian
Figure 1. Measured electromotive force (EMF) as a function of total surfactant concentration (Ctotal). The filled circles represent the “calibration curve” using a pure surfactant system, C16TAB. The Nernstian region is highlighted by the regression line. The open circles represent the dilution curve for the pC16TVB system. For this data set, the ratio of surfactant to polyelectrolyte charge groups (C16TA+:pVB-) is 1:1 for all concentrations. The figure inset shows a rendition of surfactant partitioning into the bulk phase from the aggregate (not to scale).
response, measurements were made on aqueous solutions of the pure surfactant (C16TAB) system. In addition to demonstrating electrode suitability, these measurements serve as the calibration for determining bulk surfactant concentration in an unknown solution from the measured EMF. A typical C16TAB calibration curve with 0.1 mM NaBr background electrolyte is shown in Figure 1, and similar calibration measurements were made prior to each experiment. At low surfactant concentrations (C16TA+ concentration < 0.4 µM) a constant EMF value is observed indicating the lower sensitivity limit of the electrode. This is followed by a linear Nernstian region (roughly 0.4 µM to 0.7 mM) that corresponds to the operating range of the electrode. At a surfactant concentration of about 0.7 mM an inflection is observed in the EMF value. This point marks the critical micelle concentration, cmc, of the surfactant, and this cmc is in agreement with literature values for C16TA+ in the presence of small, hard counterions like Br-.36,37 Below the cmc, the electrode responds in a Nernstian fashion, indicating that the bulk surfactant activity can be approximated as being equal to the bulk surfactant concentration.28 Above the cmc, a plateau in the EMF signal is observed with concentration, indicating a constant bulk surfactant concentration. For this reason, the electrode is only used to determine the bulk surfactant concentration below the cmc. To determine the partitioning of surfactant in the pC16TVB polyelectrolyte-surfactant system (PES), the EMF response was observed over a range of pC16TVB concentrations. These data are presented as a function of total surfactant (C16TA+) present in the system (Ctotal) in Figure 1. By comparing the pC16TVB data to the calibration curve, the bulk concentration of surfactant in solution (Cbulk) is determined. From Cbulk, the concentration of surfactant bound to the PE chain in the PES aggregate phase (Cbound) can be calculated from a simple molar balance of surfactant. As shown in Figure 1, the pC16TVB curve lies to the right of the calibration curve for all concentrations measured, indicating less surfactant is free in solution at the same overall surfactant concentration, and, hence, a fraction must be bound to the aggregates as bound surfactant does not contribute to the EMF signal. These pC16TVB data also indicate that the critical aggregation concentration (cac) is not observable in this
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aggregate. A third phase of surfactant-only micelles is neglected here for reasons that will be discussed in a following section. The PE is hydrophobic and does not partition into the bulk aqueous phase.32,33 The surfactant partitioning can be described quantitatively as a competition between a dissociation driving force (free surfactant in the bulk water phase) and a binding driving force (bound surfactant in the aggregate phase). It is not unreasonable to expect that the bulk water phase will tend toward maintaining a constant amount of surfactant in equilibrium with that in the aggregate phase much like the typical cmc or cac behavior. If we assume that a fixed, equilibrium amount of surfactant, C*bulk, partitions into the bulk phase regardless of the pC16TVB concentration, we predict the binding fraction as βpredict ) Figure 2. Surfactant binding fraction, β, as a function of pC16TVB concentration. The filled circles represent the binding curve of pC16TVB in the presence of 0.1 mM NaBr. The solid line represents the model with C*bulk ) 17 µM, while the dashed line represents the model with C*bulk ) 3.3 µM. Model binding with C*bulk ) cmc of the pure surfactant (0.7 mM) is also provided for comparison and is represented by the dashed-dotted line.
concentration range and must lie at a concentration below the lower sensitivity limit of the electrode. To quantify the surfactant partitioning in the system, a binding fraction is defined as β)
Ctotal - Cbulk Cbound ) Ctotal Ctotal
(1)
where Ctotal, Cbulk, and Cbound are as defined above and measured relative to C16TAB as demonstrated in Figure 1. A plot of β as a function of PES concentration represents the binding curve of the system. It should be noted that typical binding curves presented in the literature are formed by holding the polymer or PE concentration constant and titrating surfactant into the solution while observing the EMF.23,25,26 In our work, the surfactant and PE concentration change throughout the experiment, but the ratio between the two is held constant (at a ratio of 1 surfactant to 1 monomer). Therefore, the limit of β ) 1 represents complete binding of the surfactant (Cbound ) Ctotal). The binding curve for our 1:1 PES system is shown in Figure 2. The curve contains two regimes of behavior based on the observed binding curve: regime 1 in which the binding is a strong function of pC16TVB concentration and regime 2 in which the binding has reached a plateau and is only a weak function of the pC16TVB concentration. In regime 1 (pC16TVB concentration < 0.25 mg/mL) the binding fraction is sensitive to the pC16TVB concentration. As the pC16TVB concentration is decreased in this regime, the binding fraction decreases and, hence, the net aggregate charge increases. In regime 2 (pC16TVB concentration > 0.25 mg/mL), the value of β is less sensitive to changes in pC16TVB concentration and approaches a relatively constant value as concentration is increased. In this regime, the pC16TVB aggregates will be near charge neutrality (β close to 1) and nearly all of Ctotal resides in the bound phase on the PES aggregate. On the basis of these observations, we propose a simple surfactant partitioning model. Due to the net 1:1 stoichiometry of the system, there exists one PE charge group (binding site) for each oppositely charged surfactant molecule present in the system. The surfactant must partition into one of two possible phases: the bulk water phase (including the air-water interface) and the bound phase where the surfactant is bound to the PES
Ctotal - C*bulk Ctotal
(2)
Within the framework of this simple model, β is aphysical when Ctotal < C*bulk, so the model will fail in regime 1, but is useful for extracting information in regime 2. The only parameter that varies in this simple model is the equilibrium bulk surfactant concentration, C*bulk, so comparisons of the model to data allow us to estimate C*bulk. Model curves with varied C*bulk values are shown in Figure 2. For reference, a model curve with C*bulk ) cmc of the pure surfactant (0.7 mM C16TAB) is also shown. Previous surface tension results32 suggested that C*bulk is equivalent to the cmc of the pure surfactant. However, in the figure it is clear that the observed C*bulk values are much smaller than the cmc of the pure surfactant. These results demonstrate the error associated with extracting partitioning information from surface tension data involving PES systems. In regime 2 (pC16TVB concentration > 0.25 mg/mL) the data show excellent agreement with the C*bulk ) 17 µM model curve. In this regime, the system adheres to the simple model and maintains an equilibrium amount of surfactant in the bulk phase equal to 17 µM. In regime 1 (pC16TVB concentration < 0.25 mg/mL) the model fails as expected. Even though the model is too simple for this regime, we can allow C*bulk to be a parameter and fit the model to estimate the decrease in the bulk surfactant concentration; a comparison with the model provides an estimate of C*bulk ) 3.3 µM. The observed partitioning demonstrates the relative strengths of driving forces in regime 1. Here, the balance of the driving forces for binding and dissociation are shifted more toward binding to the aggregate when compared to the balance observed in regime 2. As a result, the amount of surfactant in the bulk phase is less than C*bulk. We postulate that this shift in the balance of driving forces in the system is controlled by electrostatic interactions. Impact of Background Electrolyte on Partitioning. In regime 1, as the aggregate concentration decreases, the bound fraction of the surfactant decreases significantly which will lead to an increase in the net charge of the aggregate. This suggests that the shift in the surfactant partitioning is controlled by electrostatic interactions in regime 1. If this is the case, increasing the ionic strength of the solvent by adding salt should restore the partitioning balance to that observed in regime 2. To test this hypothesis, pC16TVB dilution curves were measured in the presence of 5 and 100 mM NaCl background electrolyte concentrations. For each electrolyte concentration, a new C16TAB calibration curve was measured, since these have the same functional form as that shown in Figure 1, these data are not shown. For both salt concentrations, the observed lower sensitivity limit (0.4 µM C16TA+) is the same as that observed
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Figure 3. Surfactant binding fraction, β, as a function of pC16TVB concentration. The open circles represent the binding curve of pC16TVB in the presence of 5 mM NaCl. The dashed line represents the model with C*bulk ) 50 µM. The binding curve in the presence of 0.1 mM NaBr (filled circles) and the model with C*bulk ) 17 µM (solid line) are provided for comparison.
Figure 4. Surfactant binding fraction, β, as a function of pC16TVB concentration. The open squares represent the binding curve of pC16TVB in the presence of 100 mM NaCl. The dashed line represents the model with C*bulk ) 30 µM. The binding curve in the presence of 0.1 mM NaBr (filled circles) and the model with C*bulk ) 17 µM (solid line) are provided for comparison.
in the presence of 0.1 mM salt (Figure 1). Above the lower limit, a linear Nernstian region extending from the lower limit to the cmc is observed in both cases. However, the observed cmc (upper limit) decreases with increasing background salt concentration. This is expected, as the addition of salt to ionic surfactant solutions is known to reduce the observed cmc.38,39 In our experiments, cmc values of about 0.4 and 0.1 mM C16TAB were observed in 5 and 100 mM NaCl, respectively. These values are in general agreement with literature values for C16TAB in the presence of KCl as measured by surface tension.38 The binding fraction of surfactant as a function of pC16TVB concentration was determined for systems with background electrolyte concentrations of 5 and 100 mM. The data with 5 mM background electrolyte are shown in Figure 3 along with the predicted binding curve and the comparison to the 0.1 mM background electrolyte data from Figures 1 and 2. Qualitatively, the system with 5 mM background electrolyte exhibits behavior similar to that shown in Figure 2, displaying both regimes of binding behavior. Although the onset of regime 1 occurs at roughly the same pC16TVB concentration as that seen in 0.1 mM electrolyte (pC16TVB concentration < 0.25 mg/mL), the 5 mM case exhibits a lower surfactant binding fraction in regime 1. Here, the increased electrolyte concentration has resulted in an increased Cbulk, or the amount of dissociated surfactant. In this regime, the data show excellent agreement with the model binding for C*bulk ) 17 µM, compared to 3.3 µM for the 0.1 mM electrolyte case (Figure 2). This trend is further exhibited in regime 2 (pC16TVB concentration > 0.25 mg/mL). Here, the data show excellent agreement with model binding for C*bulk ) 50 µM, compared to 17 µM for the 0.1 mM case. While the increased electrolyte concentration has increased Cbulk in both regimes, two regimes of behavior are still observed. The increased electrolyte concentration has not significantly altered the nature of the binding. When the background electrolyte concentration is increased to 100 mM (Figure 4), the binding curve shows improved agreement with model binding and the two regimes of behavior are not as distinct. Here, the data are compared to model binding for C*bulk ) 30 µM at all pC16TVB concentrations. Deviations from model binding do not occur until the pC16TVB concentration is decreased below about 0.1 mg/mL, marking regime 1.
The binding curve does not deviate as strongly from the simple model in the high electrolyte solution as observed in the 0.1 and 5 mM electrolyte concentration cases. The 100 mM electrolyte concentration has restored the driving force balance to some degree and has extended the concentration range that shows agreement with our simple model. Regime 2 has been extended and covers a concentration range of 0.1 mg/mL pC16TVB and greater. This result supports our postulate that the shift in regime 1 balance is dominated by electrostatic interactions. When these interactions are screened, the system behaves as if there is only a dissociation driving force. A quantitative comparison of C*bulk values measured in regime 2 presents an interesting result. As the electrolyte concentration is increased from 0.1 to 5 to 100 mM, the observed C*bulk first increases from 17 to 50 µM and then decreases to 30 µM. While the two cases of increased background electrolyte concentration (5 and 100 mM) yield larger C*bulk values than that observed at an electrolyte concentration of 0.1 mM, they do not follow a monotonic trend with increasing electrolyte concentration. Formation of Surfactant Micelles. The onset of surfactantonly micelle formation in the pC16TVB system is observed through similar methods. In all experiments presented so far, there was an equimolar ratio of surfactant to PE charge groups. In these pC16TVB systems, no evidence for surfactant micelle formation has been observed at any concentration studied.14,32 However, by adding excess surfactant to a 1:1 system, the onset of micelle formation is observed. Here, C16TAB was titrated into a 1:1 system of fixed aggregate concentration and the EMF (Cbulk) was observed. This was done at two concentrations of pC16TVB, 1.0 and 0.1 mg/mL, and the data for these titration experiments are shown in Figure 5. Here, the titration data are shown along with the dilution curve for the 1:1 system and the appropriate surfactant calibration curve. As excess surfactant is added to the system, the observed EMF increases significantly and approaches a constant value. Further increase in added C16TAB results in an observed plateau in the measured EMF, indicating a constant Cbulk value. While this Cbulk value (∼0.15 mM) is less than the cmc of the pure surfactant (0.7 mM), it suggests the formation of surfactant micelles. The formation of surfactant micelles is further demonstrated by converting the data shown in Figure 5 to binding curves as
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Figure 5. Measured electromotive force (EMF) as a function of total surfactant concentration (Ctotal). The filled circles represent results for the pure surfactant system, C16TAB. The open circles represent the dilution curve for the pC16TVB system. The closed squares represent data from a titration of 0.1 mg/mL pC16TVB with excess surfactant. The open squares represent data from a titration of 1.0 mg/mL pC16TVB with excess surfactant.
relative amount of surfactant is increased. This suggests the aggregate phase and the bulk phase are saturated. In oppositely charged PES systems, saturation of the aggregate phase can be observed prior to the formation of surfactant-only micelles.26 It appears that this is the case in the pC16TVB system. This suggests the formation of surfactant micelles above the critical ratio for each system. Much like the behavior of the cmc seen in the calibration curve, the value of the EMF signal reaches a constant value because the bulk concentration of surfactant is constant. However, the added surfactant forms micelles, and the binding fraction increases. An alternate explanation for the observed inflection point is that an additional aggregate binding mode has appeared, such as the formation of surfactant bilayers.5 However, dynamic light scattering results for a 1.0 mg/mL pC16TVB with excess surfactant (C16TA+:pVB- monomer group > 1.5) does show evidence of the formation of a population of surfactant micelles. Therefore, we interpret the observed critical point as the critical point for formation of surfactant micelles. As the critical ratio lies well above the 1:1 point, these results demonstrate the absence of surfactant micelles in 1:1 pC16TVB systems. In addition, we note that the aggregates in our system remain stable (do not phase separate) as the C16TA+:pVB- monomer group is increased above 1:1, as was observed previously using other methods.32 Conclusions
Figure 6. Surfactant binding fraction, β as a function of excess surfactant (C16TA+:pVB- monomer group molar ratio). The filled circles represent binding data from a titration of 0.1 mg/mL pC16TVB with excess surfactant. The open circles represent binding data from a titration of 1.0 mg/mL pC16TVB with excess surfactant.
shown in Figure 6. As the concentration of added C16TAB is increased, the surfactant to PE charge group ratio (C16TA+: pVB- monomer group) in the system increases. Therefore, the bound fraction of surfactant for these experiments is presented as a function of this ratio (C16TA+:pVB- monomer group). In both cases, the binding fraction initially decreases with added surfactant (increased C16TA+:pVB- monomer group). This is a result of the increase of Cbulk with added surfactant seen in Figure 5. However, in the binding fraction curves, a minimum is observed as added surfactant is increased. The observed minimum values for the 1.0 and 0.1 mg/mL pC16TVB cases occur at C16TA+:pVB- monomer group values of roughly 1.5 and 4.0, respectively. Here, as Cbulk (Figure 5) reaches a plateau, a minimum followed by an increasing trend is observed in the binding fraction curve with increasing C16TA+:pVB- monomer group. The resulting inflection point in the binding curve marks a critical value of the ratio C16TA+:pVB- monomer group. Above this point, the binding fraction increases, indicating the appearance of an additional binding mode in the system as the
We have characterized the surfactant partitioning behavior of an oppositely charged polyelectrolyte-surfactant system. Since the aggregates are formed from a purified equimolar mixture of the charge groups in the PE and S components, saltfree solutions provide the starting point for this work and the electrolyte concentration can be accurately controlled. The results show that the partitioning can be described by a competition of driving forces: a dissociation force and a binding force. The balance of these forces leads to a competition for surfactant in dilute conditions. However, when electrostatic interactions are screened by added electrolyte, the surfactant partitioning is less complicated and agrees with the simple model presented. A discussion of surfactant-only micelle formation is presented including evidence for the point of surfactant-micelle formation. Acknowledgment We would like to acknowledge James Hoggard, Paul Sides, and Dennis Prieve for their help and useful comments associated with construction and implementation of the electrode. Literature Cited (1) Antonietti, M.; Conrad, J. Synthesis of very highly ordered liquidcrystalline phases by complex-formation of polyacrylic-acid with cationic surfactants. Angew. Chem., Int. Ed. Engl. 1994, 33, 1869–1870. (2) Antonietti, M.; Conrad, J.; Thunemann, A. Polyelectrolyte-surfactant complexes: A new-type of solid, mesomorphous material. Macromolecules 1994, 27, 6007–6011. (3) Faul, C. F. J.; Antonietti, M. Ionic self-assembly: Facile synthesis of supramolecular materials. AdV. Mater. 2003, 15, 673–683. (4) Goddard, E. D.; Ananthapadmanabhan, K. P. Interactions of Surfactants with Polymers and Proteins.; CRC Press: Boca Raton, FL, 1993. (5) La Mesa, C. Polymer-surfactant and protein-surfactant interactions. J. Colloid Interface Sci. 2005, 286, 148–157. (6) Wang, C.; Tam, K. C.; Jenkins, R. D.; Tan, C. B. Interactions between methacrylic acid/ethyl acrylate copolymers and dodecyltrimethylammonium bromide. J. Phys. Chem. B 2003, 107, 4667–4675. (7) Bai, G. Y.; Nichifor, M.; Lopes, A.; Bastos, M. Thermodynamic characterization of the interaction behavior of a hydrophobically modified
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ReceiVed for reView March 20, 2008 ReVised manuscript receiVed April 25, 2008 Accepted April 28, 2008 IE8004572
