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Langmuir 2006, 22, 4512-4517
Vesicle-Micelle Transition in Mixtures of Dioctadecyldimethylammonium Chloride and Bromide with Nonionic and Zwitterionic Surfactants Eloi Feitosa,*,† Norma M. Bonassi,† and Watson Loh‡ Departmento de Fı´sica, IBILCE/UNESP, Sa˜ o Jose´ do Rio Preto, SP, Brazil, and Instituto de Quı´mica, UniVersidade Estadual de Campinas, Campinas, SP, Brazil ReceiVed October 31, 2005. In Final Form: March 9, 2006 We have investigated the effect of mixing spontaneously formed dispersions of the cationic vesicle-forming dioctadecyldimethylammonium chloride and bromide (DODAX, with X being anions Cl- (C) or Br- (B)) with solutions of the micelle-forming nonionic ethylene oxide surfactants penta-, hepta-, and octaethyleneglycol monon-dodecyl ether, C12En (n ) 5, 7, and 8), and the zwitterionic 3-(N-hexadecyl-N,N-dimethylammonio)propane sulfonate (HPS). We used for this purpose differential scanning calorimetry (DSC), turbidity, and steady-state fluorescence spectroscopy to investigate the vesicle-micelle (V-M) transition yielded by adding C12En and HPS to 1.0 mM vesicle dispersions of DODAC and DODAB. The addition of these surfactants lowers the gel-to-liquid crystalline phase transition temperature (Tm) of DODAC and DODAB, and the transition becomes less cooperative, that is, the thermogram transition peak shifts to lower temperature and broadens to disappear when the V-M transition is complete, the vesicle bilayer becomes less organized, and the Tm decreases, in agreement with measurements of the fluorescence quantum yield of trans-diphenylpolyene (t-DPO) fluorescence molecules incorporated in the vesicle bilayer. Turbidity data indicate that the V-M transition comes about in three stages: first surfactants are solubilized into the vesicle bilayer; after saturation, the vesicles are ruptured, and, finally, the vesicles are completely solubilized and only mixed micelles are formed. The critical points of bilayer saturation and vesicle solubilization were obtained from the turbidity and fluorescence curves, and are reported in this communication. The solubility of DODAX is stronger for C12En than it is for HPS, meaning that C12En solubilizes DODAX more efficiently than does HPS. The surfactant solubilization depends slightly on the counterion, and varies according to the sequence C12E5 > C12E7 > C12E8 > HPS.
1. Introduction There is increasing interest in investigating surfactant aggregates that mimic biological membranes, such as phospholipid liposomes or synthetic amphiphile vesicles, because the architecture of these artificial membranes is considerably simpler than that of cell membranes. In addition, surfactant bilayers exhibit some functional characteristics of the natural membrane, such as permeability and chain melting temperature, and they may accommodate compounds that are usually present in the cellular membrane, including small proteins, natural polymers, or even other lipids.1,2 Vesicles or liposomes with a broad range of size can be obtained by using appropriate amphiphile molecules and preparation method. Since the first preparation of lipid liposomes in the early 1960s3 and synthetic surfactant vesicles in the late 1970s,4 these assembles have been successfully used to understand the complex cell membranes. The gained information on the investigation of synthetic or natural amphiphile vesicles can thus be extended to the understanding of cell membranes. Furthermore, potential application of cationic vesicles includes DNA compaction/ * To whom correspondence should be addressed. Address: Physics Department, IBILCE/UNESP, Rua Cristo´va˜o Colombo, 2265, Sa˜o Jose´ do Rio Preto, SP, Brazil, CEP: 15054-000. E-mail:
[email protected]. Phone: +55 17 3221 22 40. Fax: +55 17 3221 22 47. † IBILCE/UNESP. ‡ Universidade Estadual de Campinas. (1) Lasic, D. D. Liposomes. From Physics to Applications; Elsevier: Amsterdam, 1993. (2) Fendler, J.; H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982. (3) Bangham, A. D.; Horne, R. W. J. Mol. Biol. 1964, 8, 660. (4) (a) Kunitake, T.; Okahata, Y. J. Am. Chem. Soc. 1977, 99, 3860. (b) Kunitake, T.; Okahata, Y.; Tamaki, K.; Kumamaru, F.; Takayanagi, M. Chem. Lett. 1977, 387.
decompaction in gene therapy and serving as a drug vehicle; in this case, the vesicle structure and stability must be wellcontrolled.1 The favorable condition for vesicle application may not be supplied by neat amphiphiles, but rather by a mixture of compounds, which gives more stabilized vesicle dispersions. Micelle-forming surfactants have widely been used in the preparation and stabilization of vesicles of synthetic or natural amphiphiles.1 They affect vesicle properties, such as the fluidity, permeability, curvature, surface potentials, and so on. These characteristics may be relevant for the purpose of vesicle application.5 Dioctadecyldimethylammonium chloride and bromide (DODAC and DODAB), and most probably the other halide-based homologues, self-assemble spontaneously, above their gel-toliquid crystalline phase transition temperature, Tm ( ∼48 and 45 °C, respectively) and at low concentrations (e.g., 1.0 mM), as large unilamellar vesicles.6 When cooled to room temperature, these vesicle structures remain (meta)stable, and most of their physical properties have been reported.6 Smaller DODAX (X ) C or B) vesicles can be prepared by extrusion or sonication of the spontaneously formed dispersions, or by alternative methods, such as chloroform injection.7-13 The advantages of the spontaneous vesicles are that the measured properties are better (5) Rosoff, M., Ed. Vesicles; Marcel Dekker: London, 1996. (6) (a) Feitosa, E.; Brown, W. Langmuir 1997, 13, 4810. (b) Benatti, C. R.; Tiera, M. J.; Feitosa, E.; Olofsson, G. Thermochim. Acta 1999, 328, 137. (c) Feitosa, E.; Barreleiro, P. C. A.; Olofsson, G. Chem. Phys. Lipids 2000, 105, 201. (d) Benatti, C. R.; Feitosa, E.; Fernandez, R. M.; Lamy-Freund, M. T. Chem. Phys. Lipids 2001, 111, 93. (7) Cuccovia, I. M.; Feitosa, E.; Chaimovich, H.; Sepulveda, L.; Reed, W. J. Phys. Chem. 1990, 94, 3722. (8) Cuccovia, I. M.; Sesso, A.; Abuin, E. B.; Okino, P. F.; Tavares, P. G.; Campos, J. F. S.; Florenzano, F. H.; Chaimovich, H. J. Mol. Liq. 1997, 72, 323. (9) Yogev, D.; Guillaume, B. C. R.; Fendler, J. H. Langmuir 1991, 7, 623.
10.1021/la052923j CCC: $33.50 © 2006 American Chemical Society Published on Web 04/12/2006
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reproducible and that the preparation conditions (e.g., temperature, concentration, contamination, etc.) can be well-controlled. For this reason we have chosen DODAB and DODAC spontaneous vesicles to investigate the effect of cosurfactants in the properties of the mixed dispersions. The nonionic C12En (n ) 5, 7, and 8) and the zwitterionic 3-(N-hexadecyl-N,N-dimethylammonio)propane sulfonate (HPS) were used in this study as cosurfactants (C12E8 was included here for comparison since it was already investigated before; see ref 13b). The solubilization effects in these mixed systems resemble those previously reported for the homologue C12E8/DODAB13b and some surfactant/phospholipid systems.14-16 A number of techniques have been used to investigate the effect of mixing micelle- and vesicle-forming surfactants. They include light, X-ray, and neutron scattering, cryogenic transmission electron microscopy (cryo-TEM), electron paramagnetic resonance, fluorescence, differential scanning calorimetry (DSC), and isothermal titration calorimetry.13-16 A combination of techniques gives important information about the structure, morphology, and phase behavior of vesicle-micelle (V-M) transitions in mixed surfactant systems. We used DSC, steady-state fluorescence, and turbidity to investigate the effect of C12En and HPS on the thermotropic phase behavior of DODAX spontaneous vesicles and the V-M transition. When these cosurfactants are added to the vesicle dispersion, the critical surfactant concentrations associated with the V-M transition obtained from the turbidity curves indicate that the transition takes place in three stages: first, up to vesicle saturation, cosurfactants are solubilized by the vesicles yielding mixed vesicles (stage I); then mixed micelles are formed, and the DODAX molecules are solubilized by the mixed micelles while the vesicles are destabilized until the complete solubilization of DODAX (stage II); in stage III, all of the vesicles are completely solubilized, and only mixed micelles exist. It is shown that, even though these systems obey the three-stage model,15 the critical concentration of solutilization depends on the surfactant nature according to the sequence C12E5 < C12E7 < C12E8 < HPS, that is, the cationic surfactant solubilization follows the opposite direction of this sequence. 2. Materials and Methods 2.1. Chemicals. DODAC was obtained from DODAB (Sigma) by ion exchange. They were both recrystallized from acetone and a few methanol drops. Recrystallized 1-(N-hexadecyl,N,N′dimethylammonium)-propane,3,sulfonate (HPS) from Sigma, was kindly supplied by Dr. I. M. Cuccovia, and penta-, hepta-, and octaethyleneglycol mono-n-dodecyl ether (C12E5, C12E7, and C12E8) were used as supplied by Nikko Chemicals, Japan. The fluorescence probe trans-trans-1,4-diphenyl-1,3-butadiene (DPB) (Sigma) was used as received. All samples were prepared with high-quality (Milli-Q plus) (10) (a) Carmona-Ribeiro, A. M.; Yoshida, L. S.; Sesso, A.; Chaimovich, H. J. Colloid Interface Sci. 1984, 100, 433. (b) Carmona-Ribeiro, A. M. Chem. Soc. ReV. 1992, 21, 209. (11) Andersson, M.; Hammarstro¨m, L.; Edwards, K. Phys. Chem. 1995, 99, 1431. (12) (a) Pansu, R. B.; Arrio, B.; Roncin, J.; Faure, J. J. Phys. Chem. 1901, 94, 796. (b) Blandamer, M. J.; Briggs, B.; Cullis, P. M.; Last, P.; Engberts, J. B. F. N.; Kacperska, A. J. Therm. Anal. Calorim. 1999, 55, 29. (13) (a) Barreleiro, P. C. A.; Olofsson, G.; Feitosa, E. Prog. Colloid Polym. Sci. 2000, 116, 33. (b) Barreleiro, P. C. A.; Olofsson, G.; Bonasssi, N. M.; Feitosa, E. Langmuir 2002, 18, 1024. (c) Feitosa, E.; Barreleiro, P. C. A. Prog. Colloid Polym. Sci. 2004, 128, 163. (14) (a) Nilsson, K.; Almgren, M.; Brown, W.; Jansson, M. Mol. Cryst. Liq. Cryst. 1987, 152, 181. (b) Edwards, K.; Almgren, M.; Bellare, J.; Brown, W. Langmuir 1989, 5, 473. (c) Edwards, K.; Almgren, M. Prog. Colloid Polym. Sci. 1990, 82, 190. (15) (a) Lichtenberg, D.; Robson, R. J.; Dennis, E. A. Biochim. Biophys. Acta 1983, 737, 285. (b) Lichtenberg, D. Biochim. Biophys. Acta 1985, 821, 470. (16) Bach, D. Chem. Phys. Lipids 1984, 35, 385.
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Figure 1. Molecular structures of DODAB, DODAC, C12En, DPB, HPS, and B4A. water, and trans-trans-1,4,phenyl-p-carboxypropyl-1,3,butadiene (B4A) was synthesized as reported.17 Figure 1 shows the molecular structures of the surfactants and fluorescence probes used in this investigation. 2.2. Sample Preparation. Spontaneous DODAX vesicle dispersions (stock solutions) were prepared by a simple dilution of the surfactant (5.0 mM) in water at 60 °C, that is, above the surfactant Tm.6 After preparation, the dispersions were kept standing and cooled to room temperature, at which they were stored for at least 24 h before the experiments. In the presence of the fluorescence probe, 1.0 mM DODAX dispersions were added to a glass flask containing a thin film of the probe previously prepared by placing a 7 × 10-6 M chloroform solution of the probe followed by evaporation of the organic solvent using a N2 flux. At 1.0 mM DODAX, the dispersions are clear (giving a low turbidity signal) above Tm and slightly bluish (high turbidity signal) at room temperature (below Tm). All measurements were made at 1.0 mM DODAX obtained by dilution of the 5.0 mM stock solution. Concentrated micelle solutions (typically 20 mM) of C12En and HPS were prepared by simple dilution of the surfactants in water. The mixed surfactant/DODAX mixtures were obtained by adding aliquots of the 20 mM micelle solution into the vesicle dispersion to the desired final concentrations of the compounds. The mixture of vesicle and micelle solutions was stirred for at least 5 min before the measurements. Mixing time trials were also made to verify mixture stability. 2.3. Turbidity Measurements. Turbidity was measured at selected wavelengths (λ) using a spectrophotometer (Hitachi model U-2001). The experiments were made at 22 °C, controlled by a thermostatic bath. The temperature in the cell was measured using a thermocouple. A 2.0 mL portion of the DODAX dispersion was placed in a quartz cell (optical length 1.0 cm), and the reference cell was filled with water for baseline control. Microvolumes of the micelle solution were added to the vesicle dispersion, and the mixture was stirred (17) (a) Allen, M. T.; Miola, L.; Whitten, D. G. J. Am. Chem. Soc. 1988, 110, 3198. (b) Allen, M. T.; Miola, L.; Shin, D. M.; Sudabby, B. R.; Whitten, D. G. J. Membr. Sci. 1987, 33, 201.
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Figure 2. Turbidity as a function of the cosurfactant/DODAB molar ratio R. Measurements made at (a) 5 and (b) 15 min after adding the micelle solution to the vesicle dispersion (25 °C). for 5 or 15 min before the measurements. The results were plotted as a turbidity versus cosurfactant/DODAX molar concentration ratio (R). 2.4. DSC Measurements. The DSC thermograms were obtained using a high-sensitivity microcalorimeter (MicroCal, model VPDSC), equipped with 0.5 mL twin cells for the sample and reference. The solutions were degassed using a Nueva II stirrer (Thermolyne) before being transferred to the cells to avoid bubble formation. The instrument measures the power to keep the temperature of the sample and reference equal while the temperature is raised or lowered at a constant rate (1 °C/min, in this work). The baseline thermogram was obtained under the same conditions for both cells filled with water, and the difference between the sample and baseline thermograms gives the excess heat capacity, ∆Cp. The experiments were repeated at least two times, and reproducibility better than 1% was observed. The maximum of the main peak corresponds to the gelto-liquid-crystalline phase transition temperature (Tm), the area under the peak gives the transition enthalpy, and the peak width at halfheight (∆T1/2) is inversely proportional to the transition cooperativity.13,16 2.5. Fluorescence Quantum Yield Measurements. The fluorescence quantum yield, Φf, of the fluorescence probes DPB and B4A was determined using eq 1:17 Φf ) [(IfΦfo)/Ifo][(1 - 10-Afo)/(1 - 10-Af)]
(1)
where Φf, Φfo, If, and Ifo are the quantum yields and fluorescence
Figure 3. Same as Figure 2, but for DODAC instead of DODAB. intensities of the probes in the vesicle and reference solutions, respectively. Af and Afo are the absorbance of the probe in the vesicle and reference solutions, respectively, measured at λmax ) 340 nm. The excitation wavelength was taken as the same as λmax ) 340 nm. In this work, we used DPB/cyclohexane as the reference solution, for which Φfo ≈ 0.44 at 25 °C.17 Further details on the calculation of Φf can be found in previous communications.13b,17
3. Results and Discussion The mechanism of the V-M transition for the mixed surfactants investigated here is shown to obey the three-stage model proposed by Lichtenberg for lipid/surfactant mixtures.15 This conclusion is based on the turbidity, DSC, and steady-state fluorescence results reported here, and the cryo-TEM, DSC, and isothermal titration calorimetry (ITC) results reported previously for similar systems.13b 3.1. Turbidity Results. Figures 2 and 3 show the changes in turbidity of mixed surfactant solutions by adding 20 mM HPS or C12En (n ) 5, 7, and 8) micelles to 1.0 mM DODAB or DODAC spontaneous vesicles. At this concentration, the DODAB vesicles are large and unilamellar in the gel state, as reported.6,18 All turbidity curves exhibit the same profile, typical of the Lichtenberg three-stage model15 for the V-M transition already reported for the C12E8/DODAX/water13 and C12E8/lipid/water14 systems. For these systems, there are two critical surfactant (18) Feitosa, E.; Karlsson, G.; Edwards, K. Chem. Phys. Lipids, in press.
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Table 1. Values of Rsat and Rsol for Different Times of Mixing the Surfactant Solutions (5 and 15 min) before the Turbidity Measurements, Obtained from Figures 2 and 3 for C12En/ DODAX and HPS/DODAX in Water surfactant/DODAX
mixing time (min)
Rsat
Rsol
C12E5/DODAC C12E7/DODAC C12E8/DODAC HPS/DODAC C12E5/DODAC C12E7/DODAC C12E8/DODAC HPS/DODAC C12E5/DODAB C12E7/DODAB C12E8/DODAB HPS/DODAB C12E5/DODAB C12E7/DODAB C12E8/DODAB HPS/DODAB
5 5 5 5 15 15 15 15 5 5 5 5 15 15 15 15
0.1 0.6 0.3 0.2 0.3 0.3 0.2 0.2 0.3 0.6 0.5 0.5 0.1 0.3 0.3 0.2
3.2 5.5 5.1 ∼12 3.2 3.8 5.1 ∼12 3.5 7.1 6.1 ∼12 3.1 6.3 6.2 ∼12
concentration ratios, Rsat and Rsol, at which structural changes occur in the surfactant aggregates. When the nonionic or zwitterionic surfactant is added to a DODAX vesicle dispersion, the turbidity initially increases slightly until Rsat is reached; within this first stage, the surfactants are accommodated in the vesicle bilayer, resulting in vesicle swelling, as suggested by the initial increase in the turbidity curve. At Rsat, the bilayer is saturated by the surfactant, and, above it, the exceeding surfactants form micelles and a process of DODAX solubilization in the micelles starts, resulting in vesicle degradation (second stage) and thus a decrease in turbidity until Rsol is reached, at which the V-M transition is complete. Above Rsol, there are only mixed micelles in solution (third stage). All these cosurfactant/DODAX systems investigated exhibit an initial increase in turbidity when the molar concentration ratio R is increased up to Rsat. It results from the vesicle swelling due to the surfactant solubilization into the vesicle bilayer. According to the turbidity curves, Rsat ranges from 0.1 to 0.6, and there is no clear relation between Rsat and the surfactant nature. Table 1 summarizes the values of Rsat and Rsol obtained from the turbidity experiments. These parameters are system-dependent and exhibit the following general behavior: The cosurfactant/ DODAX systems investigated here demand a certain amount of time to attain equilibrium after adding the micelle solution to the vesicle dispersion. According to Figures 2 and 3 and Table 1, some systems demand more time than others. For all systems, however, 15 min is enough for equilibrium. The figures also indicate that different surfactants may demand different times for equilibrium, thus affecting Rsat and Rsol differently. Relative to the other surfactants, C12E5 seems to be less sensitive to the mixing time, indicating that it is more quickly solubilized in the vesicle bilayer and solubilizes the DODAX molecules more quickly, resulting in smaller Rsat and Rsol. Thus, the less polar the surfactant, the more quickly it solubilizes or is solubilized into or by the vesicles. In addition, the V-M transition is slower for HPS than it is for C12En, meaning that C12En are better solubilizers for DODAX. The V-M transition exhibits a small dependence on the surfactant counterion. In general, Rsol is larger for DODAB than it is for DODAC because of the stronger affinity of Br- for the vesicle interfaces,6c which also reflects in the lower Tm of DODAB relative to that of DODAC, that is, the surfactant chains in the
Figure 4. DSC thermograms for 1.0 mM DODAB (a) and DODAC (b) aqueous dispersions in the absence and presence of C12E5. The DSC traces were obtained in the upscan mode at the rate of 1 °C/ min.
vesicle bilayer are more densely packed for DODAC than they are for DODAB, which somehow obstructs the cosurfactant solubilization in the DODAC bilayer. 3.2. DSC Results. Figures 4-6 show the DSC thermograms of 1.0 mM DODAX vesicles in water and in the presence of increasing amounts of C12E5, C12E7, and HPS, and Tables 2 and 3 summarize the corresponding typical parameters obtained from these thermograms for C12E5/DODADAB and HPS/DODAB systems. The DSC results are also compared with those previously reported for C12E8/DODAX/water.13b Accordingly, C12En (n ) 5, 7, and 8) and HPS decrease the Tm of DODAB and DODAC, and this transition becomes less cooperative as the C12En/DODAX or HPS/DODAX molar ratio (R) is increased, that is, the main transition peak broadens to vanish around Rsol when the V-M is complete. The V-M mechanism is clearly system-dependent, meaning that a transition from a vesicle, which exhibits a well-defined Tm, to another structure without Tm (most possibly a micelle) takes place at Rsol, which depends on the surfactant nature. Cryo-TEM and dynamic light scattering data reported previously for the C12E8/DODAX/water systems indicate that this transition is from vesicle to micelle.13b The very weak turbidity of the mixed solution for R > Rsol is characteristic of micelles, since DODAX vesicle dispersions exhibit high turbidity. Like C12En, HPS yields V-M transition, but it does so more gradually, that is, at a higher and less defined Rsol, indicating that
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Figure 5. Same as Figure 4, except for C12E7 instead of C12E5. Table 2. Values of ∆H, ∆T1/2, and Tm Associated with the Gel-Liquid Crystalline Transition for C12E5/DODAB/Water, Obtained from the Thermograms in Figure 4a C12E5/DODAB
∆H (kcal/mol)
∆T1/2 (°C)
Tm (°C)
0 0.1 0.2 0.4 0.5 1.2 2.0 3.0 3.5 4.0
10.7 10.3 6.6 6.3 6.2 6.4 3.6 4.7 5.1 4.4
1.0 1.4 2.2 2.6 2.4 1.5 1.5 1.5 1.5 1.5
45.0 43.2 41.4 41.4 40.0 37.3 36.1 35.6 35.1 35.0
the nonionics interact more with the cationic vesicles than the zwitterionic ones. The sharper V-M transition for C12En/DODAX relative to the HPS/DODAX is a consequence of a more intense solubilization of the nonionic surfactants relative to that of the zwitterionic ones. For all these mixed systems, the transition enthalpy ∆H decreases with R, meaning that as R becomes larger, the energy associated with the chain melting becomes smaller. Also, the peak width tends to broaden with R, but, again, at a lower rate for HPS, indicating a less cooperative transition (lower solubilization). Overall, when the surfactant is added to the vesicle dispersion, the transition peak becomes broader and asymmetric. The overlapping peaks indicate the coexistence of populations of bilayer structures with different Tm in stage II.
Figure 6. Same as Figure 4, except for HPS instead of C12E5. Table 3. Values of ∆H, ∆T1/2, and Tm Associated with the Gel-Liquid Crystalline Transition for HPS/DODAB/Water, Obtained from the Thermograms in Figure 6a HPS/DODAB
∆H (kcal/mol)
∆T1/2 (°C)
Tm (°C)
0 0.6 1.0 2.6 4.7 8.0 9.0 11.0
10.7 10.4 9.5 9.6 9.2 7.6 2.4 1.7
1.0 1.0 1.0 1.3 1.3 1.3 3.1 2.3
45.0 44.8 44.3 43.3 42.5 41.5 39.0 36.6
According to Tables 2 and 3, we can notice that the transition enthalpy (∆H) decreases with an increasing amount of added cosurfactant, while ∆T1/2 tends to increase slightly within the range of 1-3 °C, indicating that mixed surfactant bilayers are less densely packed, thus the transition becomes slightly less cooperative. 3.3. Fluorescence Results. The three-stage model for the V-M transition in C12En/DODAB/water and HPS/DODAB/water is also confirmed by fluorescence investigation. Figure 7 shows typical fluorescence emission spectra of DPB solubilized in HPS/ DODAB/water at selected cosurfactant/DODAB molar ratios (R). The addition of HPS to DODAB vesicles changes the fluorescence intensity, but does not shift the characteristic bands of the emission spectra (Figure 7), indicating that the polarity of the aggregate microenvironment varies with R.
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Figure 7. Typical fluorescence emission spectra for DODAB/HPS/ DPB/water for selected cosurfactant/DODAB molar ratios R (30 °C). Table 4. Estimated Values of Rsat and Rsol for the Mixed Surfactant/Probe Solutions Obtained from the Fluorescence Quantum Yield Curves for B4A and DPB (Figure 8)a system
Rsat
Rsolt
C12E5/DODAC C12E8/DODAC HPS/DODAC C12E5/DODAB C12E8/DODAB HPS/DODAB
1.0 0.7 1.0 1.0 0.9 0.8
6 7 9 6 7 8
a Surfactants investigated: C E (n ) 5 and 8), HPS, DODAC, and 12 n DODAB.
Typical curves of the fluorescence quantum yield, Φf, of DPB and B4A, as a function of R are shown in Figure 8, and Table 4 summarizes the critical Rsat and Rsol obtained from these curves. Overall, these parameters are larger than those obtained by turbidity, but they resemble those for C12E8/DODAX/water reported previously13b and follow the Lichtenberg three-stage model:15 Φf initially increases with R until saturation Rsat; then it decreases until DODAX molecules are completely solubilized by the cosurfactants at Rsol; above Rsol, Φf is roughly constant, indicating no structure change. In the presence of C12E8, up to Rsat ≈ 1, the increase in Φf indicates that a more ordered bilayer is formed, owing to the shorter chain of the cosurfactants that yields smaller Tm. The disorder caused by the surfactant solubilization is probably compensated by the decrease in curvature and increase in the local polarity. Above Rsat, the decrease in Φf can be related to the formation of an increasing amount of mixed micelles, and the probes are solubilized in both the bilayer and micelle interior, with the more isotropic micellar domain being increasingly dominant. Above Rsol, the V-M is complete, and Φf remains constant.
4. Concluding Remarks The results reported here not only agree with the previous ones for the C12E8/DODAX/water systems, but also stress that the V-M transition for the homologue C12En/DODAX (n ) 5
Figure 8. Fluorescence quantum yield (Φf) of DPB or B4A 7.0 × 10-6 M, as a function of the (a) cosurfactant/DODAB or (b) cosurfactant/DODAC molar ratio in water for C12E5, C12E8, and HPS (30 °C).
and 7) systems also obeys the Lichtenberg three-stage model. They also show that this model is followed as well by the HPS/ DODAX/water system, although with a lower solubilization effect. Furthermore, it is shown in this communication that the extent of surfactant solubilization in C12En/DODAX/water decreases with the length of the ethylene oxide (EO) group in C12En. HPS, on the other hand, induces a less defined and less cooperative V-M transition, typically for R > 10. The less pronounced V-M transition for HPS relative to that for C12En is probably due to an existing net electrostatic repulsion between the quaternary ammonia group in DODAX and the dipole group in HPS. Acknowledgment. E.F. and W.L. express their gratitude to FAPESP and CNPq, and N.M.B. wishes to thank FAPESP for a scholarship (Grant 98/09772-4). LA052923J
