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J. Phys. Chem. B 2007, 111, 14250-14255
Aggregation of Ionic Surfactants to Block Copolymer Assemblies: A Simple Fluorescence Spectral Study Manoj Kumbhakar* Radiation & Photochemistry DiVision, Bhabha Atomic Research Centre, Mumbai 400 085, India ReceiVed: September 8, 2007; In Final Form: October 15, 2007
A simple and elegant method based on steady-state fluorescence spectral measurement is demonstrated to study the interaction mechanism of copolymers and ionic surfactants with a suitable selection of fluorescent probe and also its general applicability in studying other systems. Three different concentration regions have been indicated from the changes in full width at half-maximum of the emission spectra and fluorescence intensity of coumarin 153 with the molar ratio of ionic surfactant to triblock copolymer (n). At low n values, copolymer-surfactant complexes are basically copolymer-rich micelles with few surfactant molecules, and at very high n values, copolymer-rich micelles are destroyed and surfactant-rich micelles with free copolymer monomers are formed. It has been observed that, in the intermediate surfactant concentration region, the transformation of a dominantly copolymer-rich complex to a mainly surfactant-rich complex can be either gradual incorporation of surfactants into the copolymer-rich micelles with freeing of copolymer units until surfactant-rich micelles are formed (type I) or simultaneous buildup of surfactant-rich micelles together with the destruction of copolymer-rich micelles (type II). The interaction mechanism for nonionic copolymers (P123 and F127) with ionic surfactants (SDS and CTAC) is mainly type II, but at higher copolymer concentrations interaction via the type I mechanism also operates. However, it is dominantly the type I mechanism that operates for common nonionic (TX100) and ionic surfactants.
Introduction Block copolymer supramolecular assemblies continue to receive considerable attention in the areas of nanomaterials and nanotechnology due to their ability to form a wide range of nanoscale morphologies under certain conditions. In particular, water-soluble block copolymers consisting of two dissimilar moieties, that is, hydrophilic ethylene oxide (EO) blocks and hydrophobic propylene oxide (PO) blocks, are a class of nonionic amphiphiles and were recently used to prepare mesoporous material,1-5 organic-inorganic hybrid electrolytes,6 size-controlled nanoparticles,7 ordered nanoparticle arrays,8 etc. Copolymer assemblies, mostly vesicles, are also important for their potential applications as supramolecular nanocarriers in drug delivery and as model systems for biomembranes.9-12 There are many reports where copolymer assemblies are used in the presence of DNA, proteins, and other polyelectrolytes.13-17 Block copolymer nanostructures are often used in conjunction with ionic surfactants to modulate their functional properties.4,5,11,18-24 Due to synergistic interaction25,26 with these surfactants, their performance improves compared to that of individual components. Since the functionality of copolymer assemblies critically depends on the structural integrity and the property of mixed assemblies, clear understanding of the basic aggregation mechanism of block copolymer assemblies with different secondary species, such as ionic surfactants, proteins, etc., is crucial. For simplicity, the aggregation mechanism of copolymer assemblies with ionic surfactants has been studied by many groups as a fundamental step toward achieving this objective. * To whom correspondence should be addressed. Fax: 91-22-25505151 or 91-22-25519613. E-mail:
[email protected] and manojbarc@ yahoo.co.in.
These systems are also fascinating to study in view of their complex ways of association into nanoscale self-assembled structures. Several methods such as calorimetry, electromotive force measurement, nuclear magnetic resonance (NMR), and scattering techniques such as light, X-ray (SAXS), and neutron (SANS) have been employed to investigate their aggregation and to assess the performance and compatibility of surfactant mixtures for potential applications.25-45 Though the information obtained from all these methods is unparallel, they involve complex data analysis and interpretation procedures and are also time-consuming. Moreover, due to the complexity of these systems only semiquantitative or qualitative information could be drawn from the above-mentioned experimental techniques, and a comprehensive picture for the aggregation mechanism is yet to emerge. In this letter, for the first time, it is demonstrated that a simple and sensitive method based on fluorescence spectral measurement can be employed to study the aggregation mechanism of ionic surfactants with the copolymer and other nonionic surfactant assemblies (i.e., micelles) with a suitable selection of fluorescent probe. It is established that ionic surfactants bind cooperatively with both copolymer monomers and micelles.28-41 At low ionic surfactant to copolymer mole ratios (n), copolymer-surfactant complexes are basically copolymer-rich micelles with few surfactant molecules, and at high n values, copolymer-rich micelles are destroyed and surfactant-rich micelles with free copolymer monomers are formed.28-41 However, there is ambiguity regarding the transformation of dominantly copolymerrich complex to mainly surfactant-rich complex in the intermediate n region (∼2-9).29,30,36,37,39 Either it is a gradual incorporation of surfactants into the copolymer micelles with release of copolymer units until surfactant-rich micelles are formed (type I) or there is simultaneous buildup of surfactant-
10.1021/jp077220k CCC: $37.00 © 2007 American Chemical Society Published on Web 12/04/2007
Copolymer-Surfactant Interaction SCHEME 1: Possible Aggregation Mechanism of Block Copolymer Assemblies with Ionic Surfactant
J. Phys. Chem. B, Vol. 111, No. 51, 2007 14251 the aggregation follows the type II path, there will be two emission peaks, one corresponding to copolymer-rich and the other one to surfactant-rich micelles. However, if they are not resolved properly, an increase in the fwhm is expected in the intermediate region. There are several reports of fluorescence studies employing different probes to comprehend the aggregation behavior of copolymer and ionic surfactants.27,38,56-68 In most of these studies the emission peak wavelength, lifetime, quantum yield, intensity, and/or relative intensities have been correlated to gain insight into these systems. Experimental Section
SCHEME 2: Chemical Structure of C153 (left) and Its Fluorescence Spectra in P123 (s) and CTAC (---) Micelles
rich micelles together with the destruction of copolymer-rich micelles (type II). Scheme 1 depicts a qualitative picture of a possible aggregation mechanism for copolymer assemblies with ionic surfactants. To comprehend the transformation of copolymer-rich assemblies into surfactant-rich systems in the intermediate n region, as well as to design and demonstrate a simple yet elegant spectroscopic method for exploring the aggregation mechanism between copolymer assemblies and ionic surfactants, the present study has been undertaken. For this purpose polarity-sensitive fluorescent probe coumarin 153 (C153; Scheme 2) has been employed in aqueous triblock copolymer P123 (EO20-PO70EO20) and ionic surfactants (sodium dodecyl sulfate, SDS, and hexadecyltrimethylammonium chloride, CTAC). The choice of C153 in the present study is due to its large blue-shifted emission (∼30 nm) in P123 compared to ionic micelles (SDS and CTAC) due to a lower degree of hydration.46-50 Our earlier studies indicate that C153 preferentially resides in the micellar corona region.46-49 The emission maximum of C153 in P123 micellar solution is around 510 nm, while in neat PPO it is around 490 nm. However, in neat PEO the C153 emission spectrum shows a peak around 520 nm,51,52 which is quite red-shifted compared to that in P123 micellar solution. Therefore, comparison of emission maxima indicates that C153 preferentially resides in the PPO-PEO interface region of intermediate polarity.47,48,53-55 A small distribution of probe locations in the copolymer micellar phase has also been inferred in our earlier studies with C153.47 Due to preferential solubilization of C153 inside the micellar phase, any alteration of copolymer micelles in the presence of ionic surfactants can be monitored by following the spectral behavior. Thus, incorporation of ionic surfactants into the copolymer micelles (which increases the degree of hydration39) should lead to a gradual red shift in the fluorescence spectra of C153. In these experiments, n was varied by increasing the surfactant concentration and keeping the copolymer concentration constant. If the transition follows the type I mechanism, a gradual change in the full width at half-maximum (fwhm) is more likely to occur from copolymer-rich micelles to surfactantrich micelles, with only one emission peak. On the contrary, if
P123 and F127 were obtained from Aldrich and Sigma, respectively, and used without further purification. SDS, CTAC, and Triton X-100 (TX100) samples were obtained from Sigma, Aldrich, and BDH, respectively, and used as received. Laser grade C153 was obtained from Exciton and used as received. Nanopure water, having a conductivity of ∼0.1 µS cm-1, was obtained by passing distilled water through a Barnstead nanopure water system and used for the preparation of the micellar solutions. The appropriate amount of block copolymer was weighed and kept overnight under refrigeration after addition of the requisite amount of water in a sealed container. Samples were prepared by adding C153 in the aqueous solutions of block copolymers at room temperature. The concentration of C153 was kept much lower in comparison to the micelle concentration, only in the range of 1-5 µM. With the above experimental condition, we can assume that almost none of the micelles in the solution can have more than one probe molecule. Steady-state fluorescence spectra were recorded using a Hitachi (Tokyo, Japan) model F-4010 spectrofluorometer. In the present work, the temperature of the solution was varied with the help of a coldfinger arrangement, and the temperature was controlled within (1° C using a microprocessor-based temperature controller. All the measurements involving P123 and F127 were carried out at 40 °C, as structural studies with added surfactants are well reported in the literature. In the case of surfactant pairs (TX100, SDS, and CTAC) experiments were performed at ambient room temperature (25 °C). Results and Discussion Gradual addition of CTAC to P123 micellar solution (1%, w/v) induces the emission spectra of C153 to shift toward lower energy (up to n = 2, Figure 1). This is attributed to an increase in polarity experienced by the probe as a consequence of incorporation of ionic head groups in the corona region, since the P123-CTAC complex is essentially a charged copolymerrich micelle.29,30,36,37,39 It is evident from Figure 1 that around n ≈ 3, another emission peak appears toward the lower energy side, indicating two distinct microenvironments experienced by C153 in this region, one less polar and the other one relatively more polar. Another interesting observation is that, with an increase in the value of n, the higher energy emission peak gradually disappears with a concomitant increase in the lower energy emission peak, which further shifts to the lower energy side. Above an n value of 10, this lower energy emission spectrum nearly matches with that in pure CTAC micelles, which is attributed to the surfactant-rich micelles. On addition of a very small amount of P123 in pure CTAC micellar solution, the fluorescence spectra of C153 show a blue shift, which is quite expected as the copolymer-surfactant aggregates produced contain few copolymer units and are relatively less polar than
14252 J. Phys. Chem. B, Vol. 111, No. 51, 2007
Kumbhakar
Figure 2. Fwhm (a), intensity ratio, I(n)/I(n)0) (b), and emission maximum (c) of C153 emission spectra as a function of n for the P123CTAC system. Experiments were performed with a fixed 1% (w/v) aqueous solution of P123 (1.7 mM).
Figure 1. Normalized fluorescence spectra (a) and enlarged peak portion of the spectra (b) at different n values for the P123-CTAC system. (c) Normalized difference spectrum (I(λ)n)3 - I(λ)n)1, solid line) and spectrum at n ) 12 (dashed line) of C153 for the P123CTAC system. Negative values around 480 nm for the difference spectrum are due to spectral shift toward higher wavelengths at n ) 3 compared to n ) 1. Experiments were performed with a fixed 1% (w/v) aqueous solution of P123 (1.7 mM).
pure surfactant micelles. This further indicates that the lower energy shoulder peak is due to surfactant-rich aggregates. The simplest way to check the possibility of two different probe microenvironments in the intermediate n region is to subtract the emission spectrum for probes dissolved only in the copolymer-rich micelles (0 < n = 2) from that in the intermediate n region. The representative difference spectrum shown in Figure 1c clearly indicates that it closely represents that of dyes in surfactant-rich aggregates (at high n values). However, due to spectral shift toward higher wavelengths with an increase in n, this simple subtraction method can provide only qualitative information unlike line-shape analysis where spectral components for the two aggregates can be distinguished quantitatively (as discussed later). Even then, the difference spectra clearly indicate the existence of surfactant-rich aggregates in the intermediate n region along with copolymer-rich aggregates. Control experiments for the copolymer-surfactant aggregates in the intermediate n region are practically not possible as the actual composition of copolymer-rich and
Figure 3. A few fluorescence spectra of C153 and their log-normal fit for the 1% P123-CTAC system. The red and green line spectra indicate copolymer-rich and surfactant-rich micelles, respectively. The overall spectra are shown by the blue lines. The symbol 9 indicates the experimental data set.
surfactant-rich aggregates prevailing in the intermediate n region is not known. Moreover, these compositions vary with n, and they cannot be prepared separately in a controlled manner. The overall spectra of C153 in the intermediate n region are basically determined by the composition of the aggregates and the population of the copolymer-rich and surfactant-rich aggregates in the solution. The appearance of two emission peaks due to two probe microenvironments in the intermediate n region clearly establishes the type II mechanism for the interaction of P123-CTAC micellar systems. More interestingly, in this region copolymerrich micelles are destroyed with the simultaneous formation of surfactant-rich micelles. The increase in the fwhm in the n region of 2-5 is primarily due to the presence of two types of micelles, while the decrease in the fwhm above n ≈ 5 indicates the
Copolymer-Surfactant Interaction
Figure 4. C153 emission maximum (top) and area (normalized with respect to the total area) (bottom) corresponding to the copolymerrich (O) and surfactant-rich (b) aggregates obtained from log-normal line-shape analysis for the 1% P123-CTAC system.
destruction of copolymer-rich micelles (Figure 2a); these are also reflected in the intensity plots at two wavelengths corresponding to the copolymer-rich and surfactant-rich micelles (Figure 2b). Since the maximum of the C153 fluorescence spectrum is related to the polarity of the microenvironment, its correlation with n is also expected to reflect these three regions.27,56,69 Figure 2c indeed shows three distinct regions as observed with fwhm and intensity plots (see the Supporting Information). Line-shape analysis using a log-normal function as reported in the literature has also been carried out for the present spectral results to gain further insight into the transformation mechanism (see the Supporting Information).70-72 It is clearly indicated from Figure 3 that there are two emission peaks for C153 in the intermediate n region. The lower energy emission peak (assigned to the surfactant-rich micelles) shows a gradual red shift with an increase in n along with a rise in fluorescence intensity. The higher energy emission peak (assigned to the mainly copolymerrich micelles) shows a minimum spectral shift but its intensity decreases with n. Correlation of the emission maximum and
J. Phys. Chem. B, Vol. 111, No. 51, 2007 14253 normalized area under the spectra in the intermediate n region is shown in Figure 4. These results clearly establish the type II aggregation mechanism for the P123-CTAC system. A recent scattering study by Jansson et al. also supports the coexistence of two different types of copolymer-surfactant micelles for 1% P123 solution in the intermediate region.36,37 Aggregation of P123 with SDS is found to be similar to that of CTAC (see the Supporting Information). To explore the correlation between the destruction of copolymer-rich complexes and the formation of surfactant-rich complexes with n, an intensity correlation function, In (change in intensity normalized with respect to the total intensity change; see the Supporting Information), has been formulated (Figure 5a). It has been noticed that the extent of increase or decrease in the In value in the intermediate n region (∼5-10) is almost identical, which clearly indicates growth of surfactant-rich complexes at the expense of copolymer-rich complexes for 1% P123 and SDS/CTAC systems. However, at higher copolymer concentrations (5% and 10%, w/v), destruction of copolymerrich complexes gradually becomes slower (Figure 5a,b), which is possibly due to the involvement of type I aggregation along with type II aggregation. As a result, the concentration of copolymer-rich micelles does not decrease to the extent expected from the n values considering only the type II mechanism. The increase in the range of the intermediate region (due to slower destruction of copolymer-rich micelles) with an increase in copolymer concentration indicates changeover from a dominantly type II mechanism to a mixed type I and type II mechanism. Additional support for the type I mechanism has been obtained from SANS studies, where mostly surfactantincorporated copolymer-rich complexes for similar systems at n ) 5 in 10% P123 solution have been reported.39 Therefore, it is to be noted that simple spectral measurements can be utilized to investigate such complex aggregation mechanisms. To be noted, in the literature, changes in intensity have been commonly correlated following relative intensities, i.e., the I(n)/I(n)0) function (cf. Figure 2b), instead of the abovementioned normalized intensity correlation function, In.27,38,57-67 Both the correlations result in a similar trend in intensity changes, although the latter one has the advantage of predicting
Figure 5. In (a) and fwhm (b) as a function of n for P123-CTAC systems at constant P123 concentrations (w/v) of 1% (circles) and 10% (squares). In (c) and fwhm (d) plots for TX100-SDS (circles) and TX100-CTAC (squares) systems at a constant TX100 concentration of 20 mM.
14254 J. Phys. Chem. B, Vol. 111, No. 51, 2007 the extent of reaction visually from the figure itself as the initial and final states are normalized to 1 and 0, respectively.47,48,54,72 Moreover, different systems where the final states are different can also be visually compared from the plots of In. Additionally, the extent of the decrease and increase in the intensity at any n value can be judged from these plots with ease. Hence, the In function has been preferred over the intensity ratio in the present study. Such correlation has been extensively used in solvation dynamics to calculate the normalized spectral shift correlation function.47,48,54,55,72 To elucidate the role of polymer composition (PO/EO) on the transformation of copolymer-rich to surfactant-rich micelles, similar studies were performed with F127 (EO100-PO70-EO100). The aggregation mechanism of F127 with SDS and CTAC is found to be quite similar to that of P123 (see the Supporting Information), except that the region for only copolymer-rich assembly is very narrow (0 < n e 1). There are several reports on the aggregation behavior of copolymer and ionic surfactants where C153, coumarin 480 (C480), pyrene, anthracene, fluorine, etc. have been used as fluorescent probes.27,38,54-68 Among them, only coumarins show large spectral shifts in copolymer and ionic surfactant micelles, which is the basic criterion for the present study (cf. Scheme 2). Depending on the extent of spectral shift in these two micelles, either only an increase in fwhm is observed or also the appearance of two emission peaks in the intermediate n region is determined. Due to the relatively smaller spectral shift between ionic micelles and F127 (∼20 nm) compared to that with P123, only an increase in fwhm is observed in F127CTAC/SDS systems and spectral features as observed for P123CTAC/SDS (Figure 1) are absent (see the Supporting Information). Moreover, the probe location also seems to be important, as probes preferentially residing in the micellar core54 (C480) do not adequately reflect the changes observed by probes solubilized in the corona region47,48,55 (C153). To assess the potential of this simple spectroscopic method in predicting the course of aggregation as well as its general applicability, this method has been tested with other mixed surfactant systems, such as TX100 with SDS and CTAC, where coexistence of two different surfactant complexes has not been observed; i.e., the type I mechanism is followed (Figure 5c,d). Destruction of TX100-rich micelles is almost complete at n ≈ 1, while this value is ∼5 and ∼10 for copolymer-rich micelles of F127 and P123, respectively. Thus, the observed destruction efficiency of these nonionic micelles by the ionic surfactants increases with an enhancement in the degree of hydration in the micellar corona region (P123 < F127 , TX10047,48). Conclusion In essence, the present steady-state fluorescence spectral measurements ascertain a comprehensive picture for the aggregation of copolymer assemblies with ionic surfactants and also clearly establish the transformation mechanism of copolymerrich assemblies into surfactant-rich micelles, which are extremely difficult to accomplish with other experimental techniques. Additionally, this method can be applied for in-depth understanding of the interaction mechanism not only for polymer-surfactant systems but also in principle for a range of mixed systems such as surfactant-surfactant, polymerprotein, polymer-DNA, etc. with a proper selection of fluorescent probe. Understanding the influence of various amphiphiles on the structure of proteins and DNAs and the denaturation mechanism is now in progress using this simple fluorescence spectroscopic method.
Kumbhakar Acknowledgment. We thank Dr. G. Bhaskar Dutt, Dr. Rajib Ganguly, and Dr. Haridas Pal for critically reviewing this manuscript. Help in log-normal line-shape analysis from Dr. Niherendu Chowdhury is acknowledged. Encouragement and support from Dr. Sisir K. Sarkar and Dr. Tulsi Mukherjee is gratefully acknowledged. Supporting Information Available: Analysis procedures, spectral results, fwhm and intensity plots as a function of n for other relevant systems. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Huang, Y.; Cai, H.; Yu, T.; Zhang, F.; Zhang, F.; Meng, Y.; Gu, D.; Wan, Y.; Sun, X.; Tu, B.; Zhao, D. Angew. Chem. 2007, 119, 1107; Angew. Chem., Int. Ed. 2007, 46, 1089. (2) Zao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (3) Zhou, N.; Bates, F. S.; Lodge, T. P. Nano Lett. 2006, 6, 2354. (4) Tan, Y.; Srinivasan, S.; Choi, K.-S. J. Am. Chem. Soc. 2005, 127, 3596. (5) Suzuki, K.; Ikari, K.; Imai, H. J. Am. Chem. Soc. 2004, 126, 462. (6) Kao, H.-M.; Chen, C.-L. Angew. Chem. 2004, 116, 998; Angew. Chem., Int. Ed. 2004, 43, 980. (7) Niesz, K.; Grass, M.; Somorjai, G. A. Nano Lett. 2005, 5, 2238. (8) Watanable, S.; Fujiwara, R.; Hada, M.; Okazaki, Y.; Iyoda, T. Angew. Chem. 2007, 119, 1138; Angew. Chem., Int. Ed. 2007, 46, 1120. (9) Broz, P.; driamov, S.; Ziegler, J.; Ben-Haim, N.; Marsch, S.; Meier, W.; Hunziker, P. Nano Lett. 2006, 6, 2349. (10) Ranquin, A.; Versees, W.; Meier, W.; Steyaert, J.; Gelder, P. V. Nano Lett. 2005, 5, 2220. (11) Discher, D. E.; Eisenberg, A. Science 2002, 297, 967. (12) Fukushima, S.; Miyata, K.; Nishiyama, N.; Kanayama, N.; Yamasaki, Y.; Kataoka, K. J. Am. Chem. Soc. 2005, 127, 2810. (13) Koch, S. J.; Renner, C.; Xie, X.; Schrader, T. Angew. Chem. 2006, 118, 6500; Angew. Chem., Int. Ed. 2006, 45, 6352. (14) Osada, K.; Yamasaki, Y.; Katayose, A.; Kataoka, K. Angew. Chem. 2005, 117, 3610; Angew. Chem., Int. Ed. 2005, 44, 3544. (15) Li, Z.; Zhang, Y.; Fullhart, P.; Mirkin, C. A. Nano Lett. 2004, 4, 1055. (16) Ding, K.; Alemdaroglu, F. E.; Borsch, M.; Berger, R.; Herrmann, A. Angew. Chem. 2007, 119, 1191; Angew. Chem., Int. Ed. 2007, 46, 1172. (17) Fan, C. Y.; Kurabayashi, K.; Meyhofer, E. Nano Lett. 2006, 6, 2763. (18) Hu, Z.; Jonas, A. M.; Varshney, S. K.; Gohy, J.-F. J. Am. Chem. Soc. 2005, 127, 6526. (19) Chen, D.; Li, Z.; Yu, C.; Shi, Y.; Zhang, Z.; Tu, B.; Zhao, D. Chem. Mater. 2005, 17, 3228. (20) Ramos, L.; Ligoure, C. Macromolecules 2007, 40, 1248. (21) Ikari, K.; Suzuki, K.; Imai, H. Langmuir 2006, 22, 802. (22) Lu, Q.; Bazuin, C. G. Nano Lett. 2005, 5, 1309. (23) Yeh, Y.-Q.; Chen, B.-C.; Lin, H.-P.; Tang, C.-Y. Langmuir 2006, 22, 6. (24) Bronich, T. K.; Ouyang, M.; Kabanov, V. A.; Eisenberg, A., Szoka, F. C., Jr.; Kabanov, A. V. J. Am. Chem. Soc. 2002, 124, 11872. (25) Hecht, E.; Hoffman, H. Langmuir 1994, 10, 86. (26) Thurn, T.; Couderc, S.; Sidhu, J.; Bloor, D. M.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2002, 18, 9267. (27) Tapia, M. J.; Burrows, H. D.; Knaapila, M.; Monkman, A. P.; Arroyo, A.; Pradhan, S.; Scherf, U.; Pinazo, A.; Perez, L.; Moran, C. Langmuir 2006, 22, 10170. (28) Almgren, M.; Stam, J. V.; Lindblad, C.; Li, P.; Stilbs, P.; Bahadur, P. J. Phys. Chem. 1991, 95, 5677. (29) Hecht, E.; Mortensen, K.; Gradzielski, M.; Hoffman, H. J. Phys. Chem. 1995, 99, 4866. (30) Li, Y.; Xu, R.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 10515. (31) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 2001, 17, 5742. (32) Silva, R. C. D.; Olofsson, G.; Schillen, K.; Loh, W. J. Phys. Chem. B 2002, 106, 1239. (33) Proietti, N.; Amato, M. E.; Masci, G.; Segre, A. L. Macromolecules 2002, 35, 4365. (34) Wang, C.; Tam, K. C. Langmuir 2002, 18, 6484. (35) Lisi, R. D.; Lazzara, G.; Milioto, S.; Muratore, N. J. Phys. Chem. B 2004, 108, 18214. (36) Jansson, J.; Schillen, K.; Nilsson, M.; Soderman, O.; Fritz, G.; Bergmann, A.; Glatter, O. J. Phys. Chem. B 2005, 109, 7073.
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