Nanomaterials from Ionic Block Copolymers and Single-, Double-, and

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Langmuir 2007, 23, 2838-2842

Nanomaterials from Ionic Block Copolymers and Single-, Double-, and Triple-Tail Surfactants Sergey V. Solomatin,† Tatiana K. Bronich,† Adi Eisenberg,‡ Victor A. Kabanov,§ and Alexander V. Kabanov*,† Department of Pharmaceutical Sciences and Center for Drug DeliVery and Nanomedicine, College of Pharmacy, UniVersity of Nebraska Medical Center, 985830 Nebraska Medical Center, Omaha, Nebraska 68198-5830, Department of Chemistry, McGill UniVersity, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6, and Department of Polymer Sciences, School of Chemistry, M.V. LomonosoV Moscow State UniVersity, Leninskie Gory, Moscow V-234, 119899 Russia ReceiVed September 14, 2006. In Final Form: December 1, 2006 Block ionomer complexes (BICs) are prepared from anionic block copolymers and cationic surfactants of different structure or from their mixtures. Drastic changes in the morphology and stability of BIC nanoparticles caused by changes in the composition of the surfactant mixture are demonstrated. Single-tail and double-tail surfactants appear to mix within the BIC, resulting in the formation of rather uniform BIC particles. Morphologies of the particles of these mixed BICs are intermediate between those prepared from pure single- and double-tail surfactants. Particles of BIC prepared from mixtures of single- and triple-tail surfactants are heterogeneous, and FRET experiments indicate that surfactant components in these systems are strongly segregated. The results of this study provide important insights into the formation and structure of the BIC and have implications for various applications of the BIC (e.g., nanomedicine), in which precise control of the shape, size, and other properties is needed.

Introduction Complexes of oppositely charged polyelectrolytes and surfactants form a variety of phases with different morphologies and physicochemical properties.1-8 Such complexes were studied extensively in the 1980s and 1990s because they are of fundamental interest in colloidal and polymer science and have important applications in lubrication, solubilization, cosmetics, pharmaceutics, and other fields.9 Unfortunately, the tendency of these systems to phase separate from aqueous milieu as a result of neutralization of the polyelectrolyte and surfactant charges has hindered the use of these complexes in nanomedicine.10 Recently, a novel type of polymer-surfactant complexes that form stable aqueous dispersions was described.11-14 Such * Corresponding author. E-mail: [email protected]. Fax: (402) 5599543. † University of Nebraska Medical Center. ‡ McGill University. § M.V. Lomonosov Moscow State University. (1) Thalberg, K.; Lindman, B.; Karlstro¨m, G. J. Phys. Chem. 1990, 94, 4289. (2) Carnali, J. O. Langmuir 1993, 9, 2933. (3) Antonietti, M.; Conrad, J.; Thu¨nemann, A. Macromolecules 1994, 27, 6007. (4) Antonietti, M.; Conrad, J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1869. (5) Zhou, S.; Yeh, F.; Burger, C.; Chu, B. J. Phys. Chem. B 1999, 103, 2107. (6) Zhou, S.; Hu, H.; Burger, C.; Chu, B. Macromolecules 2001, 34, 1772. (7) Huang, J.; Zhu, Y.; Zhu, B.; Li, R.; Fu, H. J. Colloid Interface Sci. 2001, 236, 201. (8) Claesson, P. M.; Bergstro¨m, M.; Dedinaite, A.; Kjellin, M.; Legrand, J.-F.; Grillo, I. J. Phys. Chem. B 2000, 104, 11689. (9) Kurawaki, J.; Hayakawa, K. In Handbook of Polyelectrolytes and Their Applications; Tripathy, S. K., Kumar, J., Nalwa, H.S., Eds.; American Scientific Publishers: Stevenson Ranch, CA, 2002; Vol. 2, p 227. (10) Burger, C.; Zhou, S.; Chu, B. In Handbook of Polyelectrolytes and Their Applications; Tripathy, S. K., Kumar, J., Nalwa, H.S., Eds.; American Scientific Publishers: Stevenson Ranch, CA, 2002; Vol. 3, p 125. (11) Bronich, T. K.; Kabanov, A. V.; Kabanov, V. A.; Yu, K.; Eisenberg, A. Macromolecules 1997, 30, 3519. (12) Kabanov, A. V.; Bronich, T. K.; Kabanov, V. A.; Yu, K.; Eisenberg, A. J. Am. Chem. Soc. 1998, 120, 9941. (13) Bronich, T. K.; Popov, A. M.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 2000, 16, 481. (14) Kabanov, A. V.; Bronich, T. K.; Eisenberg, A.; Kabanov, V. A. Polym. Mater. Sci. Eng. 2000, 82, 303.

complexes, called block ionomer complexes (BICs), are synthesized by reacting double hydrophilic block copolymers containing a polyion chain and a water-soluble nonionic chain with surfactants of opposite charge. Surfactant-neutralized polyion chains segregate into a separate phase on the nanoscale, forming the hydrophobic core of nanoparticles. Nevertheless, the system remains macroscopically homogeneous as a result of the stabilizing effect of the hydrophilic shell of nonionic chains, such as poly(ethylene oxide) (PEO), that keeps the BIC nanoparticles in an aqueous dispersion (Scheme 1).15 The structure of BIC nanoparticles is largely unexplored. Recent results based on neutron scattering suggest that the BICs prepared from dodecyltrimethylammonuim bromide (DTAB) or sodium dodecylsulfate (SDS) and several different block copolymers form disordered micellar phases.16 These micelles are globular and similar in size to micelles formed by the surfactants alone. In a different study, however, the BIC formed by isothiuroniumhexadecyldimethylammonium bromide, a reactive single-tail surfactant capable of dimerization, produced vesicles with, presumably, lamellar packing of the surfactant tails.17 The difference in the structure of these BICs might arise from differences in the structure of surfactants used for their preparation, in particular, the value of the surfactant packing parameter, P ( ) V/(l × a), where V is the volume of the hydrophobic tail, l is its length, and a is the headgroup area). The relationship between the morphology of surfactant aggregates and P is well-documented and theoretically explained.18 Therefore, one could predict that surfactant molecules with different packing parameters can produce BICs of different morphologies. If this prediction was confirmed, one could purposefully alter the BIC morphology, (15) Solomatin, S. V.; Bronich, T. K.; Eisenberg, A.; Kabanov, V. A.; Kabanov, A. V. Langmuir 2004, 20, 2066. (16) Berret, J.-F.; Herve, P.; Aguerre-Chariol, O.; Oberdisse, J. J. Phys. Chem. B 2003, 107, 8111. (17) Bronich, T. K.; Ouyang, M.; Kabanov, V. A.; Eisenberg, A.; Szoka, Jr., F. C.; Kabanov, A. V. J. Am. Chem. Soc. 2002, 124, 11872. (18) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Biochim. Biophys. Acta 1977, 470, 185.

10.1021/la062693o CCC: $37.00 © 2007 American Chemical Society Published on Web 02/01/2007

Nanomaterials from Ionic Block Copolymers Scheme 1. Core-Shell Structure of the BIC Particlesa

a The inset on the right illustrates the hydrophobic core composed of surfactant-neutralized polyion chains.

which is essential for medical and biological applications of these systems, by varying the structure of the surfactant or by using mixtures of surfactants with different packing parameters. This study describes a variety of morphologies of the BIC prepared from PEO-b-poly(sodium methacrylate) (PEO-b-PMA) and individual or mixed single-, double-, and triple-tail ammonium-based surfactants. The mixing of the surfactants on the molecular level was analyzed by fluorescence resonance energy transfer (FRET) using fluorescently labeled surfactants. This method was previously successfully applied to study the formation of lipid domains in biological membranes19-28 and the redistribution of surfactants in regular polymer-surfactant complexes.29,30 The study suggests that the extent to which BIC morphology can be altered depends on the miscibility of the surfactant components within the BIC particles. When surfactants without a strong tendency to segregate are mixed, such as singleand double-tail surfactants, uniform BIC particles were formed in which the surfactant composition presumably was the same as the overall composition of the mixture. The morphologies of such particles were intermediate between the morphologies of the BIC prepared from individual surfactant components and were varied by changing the composition of the mixture. However, when surfactant mixtures are segregated, such as single- and triple-tail surfactants, heterogeneous mixtures of BIC particles with different morphologies were formed. Materials and Methods Materials. Poly(ethylene oxide)210-b-poly(tert-butyl methacrylate)97 block copolymer was synthesized and characterized as described previously.31 tert-Butyl groups were removed by acid hydrolysis. The block ionomer was converted to the sodium salt form (PEO210-b-PMA97) by precipitation from a tetrahydrofuran/ methanol mixture with an isopropanol solution of sodium hydroxide. The precipitate was thoroughly washed with excess isopropanol, dissolved in water, and then lyophilized. The concentration of the carboxylate groups in the stock solution was determined by potentiometric titration. Surfactants hexadecyltrimethylammonium bromide (HTAB), didodecyldimethylammonium bromide (DDDAB), dioctadecyldimethylammonium bromide (DODAB), trioctylmethy(19) Estep, T. N.; Thompson, T. E. Biophys. J. 1979, 26, 195. (20) Wolber, P. K.; Hudson, B. S. Biophys. J. 1979, 28, 197. (21) Dewey, T. G.; Hammes, G. G. Biophys. J. 1980, 32, 1023. (22) Snyder, B.; Freire, E. Biophys. J. 1982, 40, 137. (23) Fung, B. K. K.; Stryer, L. Biochemistry 1978, 17, 5241. (24) Dewey, T. G.; Datta, M. M. Biophys. J. 1989, 56, 415. (25) Loura, L. M. S.; Fedorov, A.; Prieto, M. Biophys. J. 1996, 71, 1823. (26) Pedersen, S.; Jørgensen, K.; Bækmark, T. R.; Mouritsen, O. G. Biophys. J. 1996, 71, 554. (27) Loura, L. M. S.; Fedorov, A.; Prieto, M. Biochim. Biophys. Acta 2000, 1467, 101. (28) Poveda, J. A.; Encinar, J. A.; Fernandez, A. M.; Mateo, C. R.; Ferragut, J. A.; Gonzalez-Ros, J. M. Biochemistry 2002, 41, 12253. (29) Itaya, T.; Ochai, H.; Ueda, K.; Imamura, A. Polymer 1994, 35, 2004. (30) Hayakawa, K.; Nakano, T.; Satake, I.; Kwak, J. C. T. Langmuir 1996, 12, 269. (31) Wang, J.; Varshney, S. K.; Jerome, R.; Teyssie, P. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 2251.

Langmuir, Vol. 23, No. 5, 2007 2839 lammonium bromide (TMAB), and N-hexadecylpiridinium bromide (HPyB) were purchased from Aldrich Co. and used without further purification. Fluorescent probes, 6-hexadecanoyl-2-((2-(trimethylammonium)-ethyl-)-methylamino)-naphatalene chloride (Patman) and 4-(4-(dihexadecylamino)-styryl)-N-methylpyridinium iodide (DiA) were purchased from Molecular Probes (Invitrogen Corp., New York, NY). Preparation of Surfactant Mixtures and BIC. Stoichiometric amounts ([COO-] ) [NR4+]total) of PEO-b-PMA and of a cationic surfactant mixture were dissolved in 5 mL of methanol to a final concentration of 2 mM of each. Water (0.5 mL)was slowly added to the solution under constant stirring. The solvents were allowed to evaporate at 60 °C. When the residual volume reached ca. 0.8 mL, an additional 0.8 mL of water was added, and evaporation at 60 °C to a 0.5 mL final volume was repeated. This cycle of water addition and evaporation was repeated twice to ensure the elimination of methanol from the mixture. The final volume of the solution was adjusted to 2 mL with water. The BIC dispersions are labeled according to the composition of the surfactant mixture; for example, HTAB/DODAB-BIC refers to the BIC formed from the mixture of HTAB and DODAB. Measurements. Effective hydrodynamic diameters (Deff) of the particles were determined by dynamic light scattering (DLS) using a Zeta-Plus analyzer with a multiangle sizing option (Brookhaven Instrument Co.). Measurements were made in the 0.1-0.5 mM surfactant concentration range. Optical absorbance was measured using a Shimadzu UV-160 spectrophotometer. Fluorescence measurements were carried out using a Shimadzu RF5000U spectrofluorophotometer, typically at 0.5 mM total surfactant concentration. The concentration of the FRET donor, Patman, was 1 µM, and the concentration of the FRET acceptor, DiA, was varied between 1.25 and 5 µM. Fluorescence intensities were corrected for the inner filter effect32 F(corr) ) F × 10(Aex+Aem)/2 where F is the fluorescence intensity measured in the solution with optical densities of Aex at the excitation wavelength and Aem at the emission wavelength. As the concentration of DiA in a mixture increased, the fluorescence intensity of Patman (λem ) 475 nm), illuminated at its excitation maximum λex ) 375 nm, progressively decreased (Figure S1, Supporting Information). Simultaneously, the fluorescence of DiA (λem ) 540 nm) increased, indicating that the energy is nonradiatively transferred from the donor to the acceptor. The FRET efficiency was calculated from the change in the relative fluorescence intensity of the donor FRET ) 1 -

FD/A FD°

where FD° is the fluorescence intensity of the donor (Patman) in the absence of the acceptor (DiA) and FD/A is its fluorescence in the presence of the acceptor. A negative staining technique was used for the transmission electron microscopy (TEM) studies. A drop of the sample solution (0.25 mM total surfactant concentration) was allowed to settle on a Formvar-coated copper grid for 1 min. Excess sample was wicked away with filter paper, and a drop of 1% uranyl acetate solution was placed into contact with the sample for 20 s. The samples were air dried and studied using a Hitachi H-7000 microscope.

Results and Discussion Morphologies of the BIC Prepared from Individual Surfactants. This study examined whether the morphology of the BIC formed using one type of block ionomer, PEO210-bPMA97, and different surfactants is dependent on the packing parameter of the surfactant component. Toward this aim, we (32) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, 1983.

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Figure 1. TEM micrographs of the BICs formed by PEO210-bPMA97 and surfactants (a) HTAB, (b) DDDAB, (c) DODAB, and (d) TMAB or surfactant mixtures (e) 87.5/12.5% HTAB/DODAB, (f) 65/35% HTAB/DODAB, (g) 85/15% HTAB/TMAB, (h) 75/ 25% HTAB/TMAB, and (i) 40/60% HTAB/TMAB. All surfactant compositions are expressed in mol %. Arrows in image e point to the lamella-type formations within the BIC. TEM images were obtained using uranyl acetate staining. The bar in each image is 100 nm.

prepared BIC using single-, double-, and triple-tail surfactants having similar chemical natures of their cationic groups (quaternary alkylammonium bromides) with packing parameters ranging from ca. 0.6 for the single-tail HTAB to ca. 2 for the triple-tail TMAB. It was suggested previously that upon formation of BIC the double-tail surfactants may produce nonequilibrium aggregates as a result of slow mixing of the sparingly soluble surfactant components in aqueous solutions.13 Therefore, we prepared all BICs by mixing the components in a common solvent system (methanol-water), followed by a slow and gradual increase in water content and then the evaporation of the residual cosolvent (methanol). The BICs prepared by this method were compared with BICs prepared using other methods, such as direct mixing of components in aqueous solution (with or without sonication) or mixing of components at elevated temperatures followed by a temperature decrease to ambient temperature.13,14 Notably, BICs of the same composition prepared by these different methods exhibited similar Deff values and morphologies. On the basis of this similarity, it is clear that the behavior of the BIC systems studied in this work was independent of the route of preparation. As is seen in Figure 1a-d, the morphology of the BIC particles strongly depended on the packing parameter of the surfactants used for their preparation. The BICs formed by single-tail surfactant HTAB (P < 1) were almost spherical, 50-100 nm in diameter, and had a distinct “spotty” pattern on the surface. In most cases, no aggregation of HTAB-BIC particles was observed. The Deff of the particles measured by DLS was ca. 120 nm. The DLS- and TEM-measured diameters were different probably because DLS measurements provide z-averaged diameter (Dz ) Σ Ni × Di3/Σ Ni × Di2, where Ni is the number of particles with diameter Di), which is more affected by small fractions of very large particles than the number-average diameter (Dz ) Σ Ni × Di/Σ Ni) determined from TEM images. In addition, the PEO “corona” of the BIC particles was probably collapsed under the conditions of TEM sample preparation, resulting in smaller values of the TEM diameter compared to Deff. The Deff

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and scattering intensity did not change significantly over the course of several weeks, indicating that the HTAB-BIC dispersions did not aggregate on this time scale. The BIC particles formed by double-tail surfactant DDDAB (P ≈ 1) were, in general, of irregular elongated shape and ca. 150-250 nm in the largest dimension. The Deff was ca. 190 nm, and it did not change significantly over time. The BIC particles formed by another double-tail surfactant, DODAB (P ≈ 1), had a pancake shape with a base diameter of ca. 100 nm. These particles often appeared “stacked” upon each other, forming larger aggregates of 300-500 nm in length. The Deff value was ca. 300 nm, and it gradually increased over time, suggesting that a slow aggregation processes was taking place. Apparently, the neutralized surfactant-polyion sites in these BICs were sufficiently hydrophobic as a result of the effect of long aliphatic chains of DODAB so that the cohesion forces between the particles exceeded the steric repulsion of the PEO chains, resulting in aggregation. The BIC particles formed by triple-tail TMAB (P ≈ 2) were the smallest for all of the studied systems (30-40 nm) and mostly spherical. In some cases, the particles appeared to be tightly overlapping, resembling vesicles undergoing fusion or fission. However, the Deff for these dispersions (70 nm) did not change over the course of many weeks, which is especially remarkable considering that TMAB alone does not form stable or metastable dispersions in water. The above results confirm the prediction that surfactants with different packing parameters produce BICs with different morphologies. Although a detailed understanding of such relationships will require high-resolution studies of the structure of BICs formed by different surfactants (e.g., by neutron and X-ray scattering), this initial characterization already enables one to alter the dispersion properties of BICs in a controlled way. The level of control over these properties can, in principle, be greatly extended by producing BICs from mixtures of surfactants with different packing parameters and controlling the average packing parameter simply by varying the composition of the mixture. Morphologies of the BICs Prepared from Surfactant Mixtures. The morphology of the BIC particles prepared from surfactant mixtures changed as the composition of the mixture was varied. As an example, Figure 1e,f presents micrographs of the BIC formed in the mixture of single- and double-tail surfactants, HTAB/DODAB-BIC. At 12.5% DODAB content (Figure 1e), the particles still retained the size, shape, and spotty appearance characteristic of the HTAB-BIC (Figure 1a). However, within many of these particles small regions with different structures were observed. These regions (identified by arrows in Figure 1e) seemed to consist of stacked flat, lamellatype formations. At higher DODAB content (35%, Figure 1f) each particle assumed a “layered” structure resembling a small stack of pancakes. This morphology appeared to be transitional to the DODAB-BIC morphologyslong stacks of similar sized pancakes (Figure 1c). The particles observed in the TEM micrographs were quite similar to each other, suggesting that each particle contained a mixture of surfactant of the same bulk composition (i.e., the surfactants did not segregate). If they did, one could expect to observe two different types of BIC particles. This hypothesis was further tested by FRET as discussed below. A different situation was observed in the BIC produced using the mixtures of HTAB and a triple-tail TMAB. In this case, the particle population in the mixture containing 15% TMAB (Figure 1g) already appeared to be heterogeneous. As the TMAB content increased to 25%, the system became very obviously hetero-

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Scheme 2. Structures of the Surfactant-Mimicking FRET Dyes

geneous, revealing two sets of particles with diameters of 30-50 and 100-150 nm (Figure 1h). As the TMAB content further increased to 60%, the system remained heterogeneous with two major populations of particles, one having diameters of 10-20 nm and another having diameters of 60-70 nm (Figure 1i). Overall, the TEM data suggested that HTAB/TMAB-BICs were composed of two different populations of particles. General Consideration of FRET in BIC Systems. Results of the TEM study described above suggest that the BIC prepared from a mixture of surfactants can form either a single type of particles or a heterogeneous population that contains two different types of particles. The surfactant mixing behavior in such BIC systems was analyzed by FRET. For this purpose, small amounts of two hydrophobic fluorescent dyes were added to the surfactant mixture. Dyes were chosen by the following criteria: (1) They formed a FRET pair (i.e., the emission spectrum of the donor overlapped sufficiently with the excitation spectrum of the acceptor to allow efficient energy transfer).32 (2) The structure of the donor dye closely resembled the structure of a single-tail cationic surfactant, whereas the structure of the acceptor dye resembled that of a double-tail cationic surfactant (Scheme 2). A simple model relating the surfactant mixing behavior and structure formation in BIC was proposed. (See the detailed discussion in Supporting Information.) If two surfactants are fully miscible with each other within a BIC, then a single population of particles is formed. However, two populations of particles are observed in cases when surfactants segregate so that one type of particle is enriched with the first surfactant component and the other one is enriched with the second component. Segregation of surfactants can be detected by measuring FRET efficiency in mixed systems containing such probes as follows. For every FRET donor and acceptor pair, the efficiency of energy transfer is inversely proportional to the sixth power of the distance between this donor and acceptor. Because there are multiple acceptors randomly distributed at various distances around each donor, the overall FRET efficiency is defined by the concentration of acceptors within certain distance of a donor, typically on the order of 10 nm.19-24,32 This distance is smaller than the size of individual BIC particles and much smaller than the distance between different BIC particles in the dispersion. When there is no segregation of surfactants, all BIC particles have the same composition. As a result, all donor molecules are equivalent, and the FRET efficiency is determined simply by the

Figure 2. FRET efficiency in BIC dispersions with various compositions of the surfactant mixture. BICs were prepared using stoichiometric amounts ([COO-] ) [NR4+]total ) 0.5 mM) of PEOb-PMA and a mixture of HTAB with (a) DDDAB or (b) DODAB. At each composition of the mixture FRET was measured at three different acceptor concentrations: 1.25 µM (open bars), 2.5 µM (filled bars), and 5 µM (cross-hatched bars), while donor concentration was constant. CPatman ) 1 µM. Error bars correspond to inter-sample variability (s.d., n ) 3); instrumental errors were negligible. Asterisks indicate FRET values in mixed BIC that were significantly different (p < 0.05) from FRET values in BIC containing only HTAB.

average concentration of the acceptors. Therefore, FRET should not change when the composition of the surfactant mixture is varied as long as the concentration of acceptors remains constant. The situation is different, however, if (1) surfactants segregate into two different types of BIC particles (R and β) and (2) either the acceptor or both the donor and the acceptor are not uniformly distributed between these particle types. In this case, the FRET efficiency for donors residing in the BIC particles enriched with one surfactant is different from the FRET efficiency for donors in particle enriched with the other surfactant because the acceptor concentrations in these particles are different. The average FRET efficiency in such case is equal to

FRET ) φRFRETR + φβFRETβ where φR and φβ designate the molar fractions of the donor molecules in particles of type R and β and FRETR and FRETβ are the partial FRET efficiencies for these particle types, respectively. Therefore, the average FRET efficiency value is dependent on the partitioning of donors and acceptors between the two types of particles and should be affected by varying the composition of the mixture. If donors and acceptors preferentially partition into different types of particles (segregation of donor and acceptor), then the average FRET efficiency should be lower than that for a single type of particle. If they partition into the

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Figure 3. FRET efficiency in mixed BIC dispersions with various compositions of the surfactant mixture. BICs were prepared using stoichiometric amounts ([COO-] ) [NR4+]total ) 0.5 mM) of PEOb-PMA and a mixture of HTAB with TMAB. At each composition of the mixture, FRET was measured at three different acceptor concentrations: 1.25 µM (open bars), 2.5 µM (filled bars), and 5 µM (cross-hatched bars) while the donor concentration was constant. CPatman ) 1 µM. Error bars correspond to intersample variability (s.d., n ) 3); instrumental errors were negligible. Asterisks indicate FRET values in mixed BICs that were significantly different (p < 0.05) from FRET values in BICs containing only HTAB.

same type of particle (concentration of donor and acceptor), then the average FRET efficiency should be higher than that for a single type of particle. (For a detailed discussion and simulation of the segregation and concentration cases, see Supporting Information.) Experimental Observations of FRET in BIC Systems. To examine the miscibility of the single- and double-tail surfactants in BICs, we compared the FRET efficiency at different compositions of surfactant mixtures HTAB/DDDAB and HTAB/ DODAB (Figure 2). These compositions are shown in the Figure as the mol % of the corresponding double-tail surfactant. The BICs comprising only the single-tail surfactant (HTAB) are also shown for comparison. (The measurements of FRET in BICs containing only the double-tail surfactants (DDDAB or DODAB) were not possible because of the relatively high turbidity of these samples.) For each composition of the mixture, the FRET was measured using three different concentrations of the acceptor to ensure that the acceptor did not perturb the BIC structure. On the basis of the TEM images suggesting the formation of only one population of particles for each single- and double-tail surfactant ratio, one could expect that for each pair of surfactants the FRET would remain constant as the composition of the mixture changes. However, surprisingly, the FRET efficiency was higher in the BIC containing the HTAB/DDDAB mixture than that in the BIC containing only the single-tail surfactant, HTAB (Figure 2a). Similar, although less pronounced, FRET changes were observed in the BIC containing the HTAB/DODAB mixtures (Figure 2b). As discussed above, one possible explanation is that (1) several types of particles were formed in the mixed surfactant systems and (2) the donor and acceptor dyes were concentrated into one type of these particles. This explanation seems unlikely considering the results of TEM that suggested that the BIC particles were uniform in these systems. However, TEM also

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suggested that some heterogeneous regions were formed within these mixed BIC particles. Local segregation of the surfactants within one BIC particle (formation of surfactant “domains”) and the concentration of donors and acceptors within these domains would also explain the increase in FRET efficiency. An alternative explanation is that there were some morphological changes in the mixed BIC (also observed by TEM) accompanied by an increase in the surfactant volume density (relative to that of HTAB-BIC) and a decrease in the average distance between the donor and acceptor. The same approach was also applied to an examination of the BICs comprising mixtures of single- and triple-tail surfactants, HTAB/TMAB. In this case, the FRET efficiency was higher in the mixed BIC than in the BIC containing only one surfactant, either HTAB or TMAB (Figure 3). As discussed above, at least two different BIC particle types were formed in this system, which were presumably enriched by the single- and triple-tail surfactants. Therefore, the most plausible explanation for the pronounced increase in the FRET efficiency in these systems was a strong segregation of the single- and triple-tail surfactants between different BIC particles accompanied by the concentrations of the donor and acceptor in one of the particle types. In this case, it is most likely that both the donor (single-tail) and acceptor (double-tail) were preferentially partitioned into the BIC particles enriched by the single-tail surfactant.

Conclusions This work demonstrates that particle morphology in the BIC dispersions strongly depends on the architecture of the surfactant component. The preparation of the BIC from mixtures of surfactants of different molecular architectures is proven to be a valid route for controlling the morphological properties of the BIC dispersions. Electron microscopy indicates that the mixed HTAB/DDDAB-BIC and HTAB/DODAB-BIC formed homogeneous dispersions of particles. On the contrary, the BICs from HTAB and TMAB mixtures appear to form two different types of particles, which is likely due to the segregation of the surfactant components. This was confirmed by the FRET study using hydrophobic donor and acceptor fluorescence dyes, which revealed preferential partitioning of these dyes in only one type of BIC particle. On the basis of these observations, by using components that are miscible in the BIC, such as the single- and double-tail surfactants, one can gradually fine tune BIC properties such as size, particle morphology, and perhaps solubilization capacity and other properties. This can greatly expand the potential for the future use of BICs comprising surfactant mixtures in research and various industrial applications. Acknowledgment. The financial support of the National Science Foundation (DMR 0071682 and DMR 0513699) and the NSERC, Canada (STR-0181003) is gratefully acknowledged. Supporting Information Available: Detailed discussion of the model relating the surfactant mixing behavior to the changes in the FRET efficiency. This material is available free of charge via the Internet at http://pubs.acs.org. LA062693O