Hydrostatic Pressure Reveals Bilayer Phase Behavior of

*To whom correspondence should be addressed. Mailing address: Department of Life System, Institute of Technology and Science, The University of Tokush...
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Hydrostatic Pressure Reveals Bilayer Phase Behavior of Dioctadecyldimethylammonium Bromide and Chloride Masaki Goto,† Yuka Ito,‡ Shunsuke Ishida,‡ Nobutake Tamai,† Hitoshi Matsuki,*,† and Shoji Kaneshina† †

Department of Life System, Institute of Technology, and Science and ‡Department of Biological Science and Technology, Faculty of Engineering, The University of Tokushima, 2-1 minamijosanjima-cho, Tokushima 770-8506, Japan Received July 29, 2010. Revised Manuscript Received December 24, 2010

Bilayer phase transitions of dioctadecyldimethylammonium bromide (2C18Br) and chloride (2C18Cl) were observed by differential scanning calorimetry and high-pressure light-transmittance measurements. The 2C18Br bilayer membrane showed different kinds of transitions depending on preparation methods of samples under atmospheric pressure. Under certain conditions, the 2C18Br bilayer underwent three kinds of transitions, the metastable transition from the metastable lamellar crystal (Lc(2)) phase to the metastable lamellar gel (Lβ) phase at 35.4 °C, the metastable main transition from the metastable Lβ phase to the metastable liquid crystalline (LR) phase at 44.5 °C, and the stable transition from the stable lamellar crystal (Lc(1)) phase to the stable LR phase at 52.8 °C. On the contrary, the 2C18Cl bilayer underwent two kinds of transitions, the stable transition from the stable Lc phase to the stable Lβ phase at 19.7 °C and the stable main transition from the stable Lβ phase to the stable LR phase at 39.9 °C. The temperatures of the phase transitions of the 2C18Br and 2C18Cl bilayers were almost linearly elevated by applying pressure. It was found from the temperature (T)-pressure (p) phase diagram of the 2C18Br bilayer that the T-p curves for the main transition and the Lc(1)/LR transition intersect at ca. 130 MPa because of the larger slope of the former transition curve. On the other hand, the T-p phase diagram of the 2C18Cl bilayer took a simple shape. The thermodynamic properties for the main transition of the 2C18Br and 2C18Cl bilayers were comparable to each other, whereas those for the Lc(1)/LR transition of the 2C18Br bilayer showed considerably high values, signifying that the Lc(1) phase of the 2C18Br bilayer is extremely stable. These differences observed in both bilayers are attributable to the difference in interaction between a surfactant and its counterion.

1. Introduction A cationic surfactant with double alkyl chains such as a dialkyldimethylammonium halide forms vesicles, not micelles, in aqueous solutions. The formation of bilayer aggregates by this kind of surfactant was first reported in late 1970s by Kunitake and Okahata.1 Dioctadecyldimethylammonium bromide (2C18Br) is a representative one of such surfactants and can form highly stable vesicles. It is known that the structure and properties are dependent on the surfactant concentration, temperature, ionic strength, method of sample preparation, and so forth.1-10 Especially, the concentration dependence of the morphology is remarkable: the 2C18Br assemblies lose their bilayer nature at concentrations of micromolar order, they form large unilamellar vesicles (LUVs) at ca. 1-10 mM, and complex multilamellar vesicles (MLVs) can be *To whom correspondence should be addressed. Mailing address: Department of Life System, Institute of Technology and Science, The University of Tokushima, 2-1 minamijosanjima-cho, Tokushima 770-8506, Japan. Telephone: þ81-88-6567513. Fax: þ81-88-655-3162. E-mail: [email protected].

(1) Kunitake, T.; Okahata, Y. J. Am. Chem. Soc. 1977, 99, 3860–3861. (2) Carmona-Ribeiro, A. M. Chem. Soc. Rev. 1992, 21, 209–214. (3) 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, 323–336. (4) Feitosa, E.; Brown, W. Langmuir 1997, 13, 4810–4816. (5) Blandamer, M. J.; Briggs, B.; Cullis, P. M.; Last, P.; Engberts, J. B. F. N.; Kacperska, A. J. Therm. Anal. Calorim. 1999, 55, 29–35. (6) Feitosa, E.; Barreleiro, P. C. A.; Olofsson, G. Chem. Phys. Lipids 2000, 105, 201–213. (7) Benatti, C. R.; Feitosa, E.; Fernandez, R. M.; Lamy-Freund, M. T. Chem. Phys. Lipids 2001, 111, 93–104. (8) Feitosa, E.; Barreleiro, P. C. A. Prog. Colloid Polym. Sci. 2004, 128, 163–168. (9) Brito, R. O.; Marques, E. F. Chem. Phys. Lipids 2005, 137, 18–28. (10) Feitosa, E.; Karlsson, G.; Edwards, K. Chem. Phys. Lipids 2006, 140, 66–74. (11) Feitosa, E.; Alves, F. R.; Castanheira, E. M. S.; Oliveira, M. E. C. D. R. Colloid Polym. Sci. 2009, 287, 591–599.

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formed at higher concentrations.6,11 Further, the number of the phase transitions varies with the concentration. At concentrations from 1 to 10 mM, the 2C18Br bilayer undergoes the main transition from the lamellar gel (Lβ) phase to the liquid crystalline (LR) phase at ca. 45 °C. In addition to the main transition, another two transitions were found, namely, pretransition at ca. 33 °C and post-transition at ca. 52 °C.11 Although the mechanism of the pretransition is still unknown, Kodama et al. have shown that the post-transition is the transition from the stable coagel phase to the stable LR phase.12 The coagel phase is induced by hydration of the crystal or dehydration of the Lβ or LR phase and resembles the lamellar crystal (Lc) phase for lipid bilayers. Interestingly, small-angle X-ray scattering (SAXS) measurements of the 2C18Br bilayer have demonstrated that the lamellar repeat distance (D-spacing) of the LR phase is approximately 4 times thicker than that of the Lc or Lβ phase.13,14 Similarly, the structure and properties of dioctadecyldimethylammonium chloride (2C18Cl), of which the counterion is different from that of 2C18Br, are also influenced by the factors described above.6,15 Kodama et al. have also proved that the stable Lc phase is induced in the 2C18Cl bilayer but the transition occurs from the stable Lc phase to the stable Lβ phase15 unlike the stable Lc(1)/LR transition for the 2C18Br bilayer. However, the measurements were carried out in the high concentration region; hence, the detailed (12) Kodama, M.; Kunitake, T.; Seki, S. J. Phys. Chem. 1990, 94, 1550–1554. (13) Jung, M.; German, A. L.; Fischer, H. R. Colloid Polym. Sci. 2001, 279, 105– 113. (14) Schulz, P. C.; Rodriguez, J. L.; Soltero-Martinez, F. A.; Puig, J. E.; Proverbio, Z. E. J. Therm. Anal. 1998, 51, 49–61. (15) Kodama, M.; Kuwabara, M; Seki, S. Thermochim. Acta 1981, 50, 81–91.

Published on Web 01/25/2011

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thermal behavior of the 2C18Br and 2C18Cl bilayers at low concentrations under atmospheric pressure is still unclear. On the other hand, high pressure is one of useful tools for elucidation of bilayer membranes. High pressure studies provide the volume information for bilayer phase transitions and the twodimensional temperature (T)-pressure (p) phase diagram.16 Further, it is possible to identify the kinds of phase transitions by using the pressure dependence of phase transition temperatures. The effect of high pressure on properties of surfactant micelles has been studied.17-20 We have also investigated the bilayer phase behavior of various kinds of phospholipids under high pressure.16,21-23 Although it is known that cationic surfactants with double chains such as 2C18Br and 2C18Cl can form bilayer aggregates, the aggregate formation is still not as clear as that of surfactant micelles and phospholipid vesicles, and opinions for the kinds of phase transitions of the 2C18Br and 2C18Cl bilayers are not consistent among researchers;6,12,15,24 that is, the phase identification, phase stability, and counterion effect on the bilayer aggregates remain quite unclear. In addition, there has been no report, as far as we know, of the pressure effect on bilayers composed of a surfactant with double chains. Therefore, we set about to elucidate the aggregation behavior of the 2C18Br and 2C18Cl bilayers by taking advantage of our knowledge accumulated from high pressure studies on the phospholipid bilayers. In the present study, the bilayer phase transitions of the 2C18Br and 2C18Cl at low concentrations are observed by high-sensitivity differential scanning calorimetry (DSC) under atmospheric pressure and light-transmittance measurements under high pressure. The phase transitions and the phase behavior for the 2C18Br and 2C18Cl bilayers are considered from the T-p phase diagram and the thermodynamic quantities of phase transitions. Further, the thermodynamic quantities are compared with those for bilayer membrane of distearoylphosphatidylcholine (DSPC) which is a phospholid with similar hydrophobic chains. The exact bilayer phase behavior of these surfactants under atmospheric and high pressures is revealed with aid of hydrostatic pressure.

2. Experimental Section 2.1. Materials. Double-chain surfactants dioctadecyldimethylammonium bromide and chloride were purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) and used as received. Water was distilled twice from a dilute alkaline permanganate solution. We adopted the surfactant concentration of 5.0 mmol kg-1 for all samples because the phase transition temperatures of the 2C18Br and 2C18Cl bilayers are virtually constant in the high-water content region (>70%) as reported by Kodama and co-workers,12,15 and the preliminary results obtained by the different concentrations (1, 5, and 50 mmol kg-1) were almost the same as one another. 2.2. Sample Preparations. We employed five methods of sample preparation in order to examine the detailed phase stability of the 2C18Br and 2C18Cl bilayers as follows. (a) The vesicle dispersions of 2C18Br and 2C18Cl were prepared by (16) Ichimori, H.; Hata, T.; Matsuki, H.; Kaneshina, S. Biochim. Biophys. Acta 1998, 1414, 165–174. (17) Tanaka, M.; Kaneshina, S.; Kuramoto, S.; Matuura, R. Bull. Chem. Soc. Jpn. 1975, 48, 432–434. (18) Yamanaka, M.; Iyota, H.; Aratono, M.; Motomura, K.; Matuura, R. J. Colloid Interface Sci. 1983, 94, 451–455. (19) Yamanaka, M.; Aratono, M.; Motomura, K. Bull. Chem. Soc. Jpn. 1986, 59, 2695–2698. (20) Kaneshina, S.; Tanaka, M.; Tomida, T.; Matuura, R. J. Colloid Interface Sci. 1974, 48, 450–460. (21) Matsuki, H.; Miyazaki, E.; Sakano, F.; Tamai, N.; Kaneshina, S. Biochim. Biophys. Acta 2007, 1768, 479–489. (22) Kusube, M.; Matsuki, H.; Kaneshina, S. Biochim. Biophys. Acta 2005, 1668, 25–32. (23) Kusube, M.; Goto, M.; Tamai, N.; Matsuki, H.; Kaneshina, S. Chem. Phys. Lipids 2006, 142, 94–102. (24) Coppola, L.; Youssry, M.; Nicotera, I.; Gentile, L. J. Colloid Interface Sci. 2009, 338, 550–557.

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suspending them in water. After a vortex treatment for about 3 min, the suspension was sonicated for a few minutes by using a bath-type sonifier (Branson model 3510J-DTH with output 130 W) at room temperature and stored in a freezer (-15 °C) for at least 2 h. The suspension was again treated by vortex and sonication for a short period (0.5-1 min) at room temperature just before measurements. (b) Instead of the freezer treatment in (a), the suspension was treated by vortex for a few minutes and kept at 60 °C before measurements. (c) Instead of the freezer treatment in (a), the suspension was stored in a refrigerator (5 °C) for 3 days. (d) After the treatment of (b), the suspension of 2C18Cl was frozen momentarily by use of liquid nitrogen and then melted in the vicinity of 0 °C in a water bath just before measurements. (e) In addition to the treatment of (a), the freeze-thaw cycles as the treatment of (d) were performed for the suspension of 2C18Cl. The suspension was again treated with vortex and sonicated for a few minutes at temperatures below 10 °C just before measurements. The treatment of (a) was made to observe the stable Lc(1)/LR transition of the 2C18Br bilayer by the DSC and light-transmittance measurements, respectively. The treatment of (b) was done to observe the main transition of the 2C18Br bilayer by both the measurements and that of the 2C18Cl bilayer by DSC. The treatment of (c) was done to observe all phase transitions of the 2C18Br bilayers by both the measurements. The treatment of (d) was done to observe the main transition of the muddy suspension of the 2C18Cl bilayer by the transmittance measurements. Finally, the treatment of (e) was done to clearly observe the stable Lc/Lβ transition (and also the main transition) of the suspension of the 2C18Cl bilayer by both the measurements. The most important thing in considering the phase behavior is to discern the phase stability; that is, the phase is stable, unstable, or metastable. The above five preparation methods enabled us to discern the phase stability clearly and to construct exact phase diagrams. Concerning the comparison among data obtained by different preparation methods, we think that it is essentially no problem because the preparation methods affect mainly vesicle sizes and consequently change the cooperativity of surfactant molecules in the vesicles. Hence, thermodynamic quantities related to the cooperativity such as enthalpy and volume changes of the transition are influenced by the preparation methods, but the transition temperatures of the vesicle dispersion do not change except for the case of small unilamellar vesicles (SUVs). This is because the transition temperatures are fundamentally dependent on the surfactant concentration and they can be regarded as virtually constant in the high water content region as in this study. Almost transparent solutions are obtained at temperatures higher than the main-transition temperatures. This indicates that the vesicles are SUVs or LUVs, whereas the cold storage of sample solutions brings about a milky solution, signifying that the aggregation forms the hydrated lamellar crystal state. 2.3. Differential Scanning Calorimetry. The phase transitions of the 2C18Br and 2C18Cl bilayers under atmospheric pressure were observed by using a VP-DSC high-sensitivity differential scanning calorimeter (MicroCal, Northampton, MA). Because the thermal behavior obtained at the low heating rate of 0.1 °C min-1 and high heating rate of 0.75 °C min-1 was almost identical in each case, the heating rate of 0.75 °C min-1 was selected in this study. The enthalpies of phase transitions were determined from the endothermic peak areas as average values over six DSC measurements by use of the software Origin 7.0 (Lightstone Corp., Tokyo, Japan). 2.4. Light-Transmittance Measurements. The light-transmittance measurements under ambient and high pressures were carried out with a U-3010 spectrophotometer (Hitachi HighTechnology Co., Tokyo, Japan) equipped with a model PCI400 high-pressure cell assembly with two sapphire windows (Syn Corporation Ltd., Kyoto, Japan).25,26 The light with a wavelength (25) Goto, M.; Kusube, M.; Tamai, N.; Matsuki, H.; Kaneshina, S. Biochim. Biophys. Acta 2008, 1778, 1067–1078. (26) Goto, M.; Ishida, S.; Tamai, N.; Matsuki, H.; Kaneshina, S. Chem. Phys. Lipids 2009, 161, 65–76.

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Figure 1. DSC thermograms of (A) 2C18Br and (B) 2C18Cl bilayer membranes prepared by different methods under atmospheric pressure. Surfactant concentrations were 5.0 mmol kg-1. Bilayer phases are assigned as the liquid crystalline (LR), lamellar gel (Lβ), stable lamellar crystal (Lc(1)), metastable lamellar crystal (Lc(2)), and lamellar crystal (Lc) phases. Bilayer phases in parentheses refer to metastable phases. Curve 1 in (A) was obtained by the treatment of (a) in the Experimental Section, curve 2 by the treatment of (b), and curve 3 by the treatment of (c). Curve 1 in (B) was obtained by the treatment of (e), and curve 2 by the treatment of (d).

of 560 nm was adopted as the incident beam. The temperature of the high-pressure cell was controlled within (0.1 °C by circulating thermostated water from a water bath through the jacket enclosing the measurement cell. Pressure was generated by a handoperated KP-3B hydraulic pump (Hikari High Pressure Instruments, Hiroshima, Japan) and monitored within an accuracy of 0.2 MPa by using a Heise gauge (Heise Co., Newtown, CT). The heating rate at a given pressure was 0.33 °C min-1. The measurements were carried out at least three times, and the transition temperature was determined as a peak temperature in the temperature-derivative curve of the transmittance versus temperature profile that exhibits an abrupt change of the transmittance.

3. Results and Discussion 3.1. Phase Transitions under Atmospheric Pressure. DSC thermograms in the heating scan of the 2C18Br and 2C18Cl bilayers are presented in Figure 1. Curve 1 in Figure 1A illustrates the thermogram of the 2C18Br bilayer obtained by the treatment of (a). The thermogram showed only one endothermic transition at 52.9 °C. In that condition, the 2C18Br bilayer undergoes a transition from the stable lamellar crystal (Lc(1)) phase to the stable LR phase.12 The suspension of the bilayer forming the Lc(1) phase became completely turbid. On the other hand, the thermogram of bilayer prepared by the treatment of (b) showed also one endothermic peak at 44.5 °C (curve 2 in Figure 1A), which corresponds to the main transition from the metastable Lβ phase to the metastable LR phase. This thermal behavior was in good agreement with that in the previous reports.12 SAXS measurements of the 2C18Br bilayer as a function of temperature have demonstrated that the D-spacings of the LR, Lc(1), and Lβ phases are 17.5, 3.7, and 3.7 nm, respectively,13 signifying that the 2C18Br molecules in the Lc(1) and Lβ phases considerably tilt with respect to the bilayer normal or 1594 DOI: 10.1021/la104552z

these two phases occur the bilayer interdigitation. Taking into account the repulsive interaction between head groups and spontaneous formation of the unilamellar structure, the 2C18Br bilayer probably forms an interdigitated structure like the bilayer interdigitation of diacylphosphatidylcholines (diacyl-PCs).16,25,26 Therefore, it may be rigorously appropriate to express the Lc(1) and Lβ phases to be Lc(1)I and LβI phases, respectively. Interestingly, the thermogram of the 2C18Br bilayer obtained by the treatment of (c) exhibited three kinds of endothermic transitions at temperatures of 35.4, 44.5, and 52.9 °C (curve 3 in Figure 1A). The temperatures of the upper two phase transitions are comparable with the main transition and the Lc(1)/LR transition; however, the ΔH values of these transitions were smaller than those of transitions observed independently. This indicates that the metastable Lβ phase and the stable Lc(1) phase can coexist at temperatures below the maintransition temperature. Concerning the transition at the lowest temperature, unfortunately, we could not observe only this transition independently. The ΔH values of this transition and the Lc(1)/LR transition increased with an increase in the period of cold storage, while that of the main transition conversely decreased. The storagetime dependence of the ΔH value suggests that the transition is related to hydration/dehydration similar to the stable Lc(1) phase. Accordingly, we judged the phase transition at 35.4 °C as the metastable transition from a metastable lamellar crystal (Lc(2)) phase to the metastable Lβ phase. It can be said from the fact that the thermal behavior of the 2C18Br bilayer shows the complicated three phase transition under the condition that three (Lc(1), Lc(2), and Lβ) phases coexist at temperatures below 35.4 °C. Further, we noticed that the formation of the Lc phase is much faster than that of the diacyl-PC bilayers,25 which requires the treatments of successive long-time thermal annealings. On the other hand, the DSC thermogram for the 2C18Cl bilayer obtained by treatment of (e) showed two kinds of endothermic transitions at 19.7 and 39.9 °C (curve 1 in Figure 1B). These transitions can be identified as the stable transition from the Lc phase to the stable Lβ phase and the stable main transition from the stable Lβ phase to the stable LR phase, respectively.15 The main-transition temperature was roughly in accord with that in the previous paper,6,27,28 although there is a scattering among the values to some extent and there is no thermal transition around 20 °C in these papers. The thermogram for the 2C18Cl bilayer prepared by treatment of (d) exhibited an endothermic peak at 39.9 °C (curve 2 in Figure 1B). This transition clearly corresponds to the main transition, and the Lβ phase becomes stable phase unlike the case of the 2C18Br bilayer. It should be noted that the Lβ/LR and Lc/Lβ transition temperatures of the 2C18Cl bilayer are both lower than those of the 2C18Br bilayer, which is closely related to the difference in interaction between surfactant ions and counterions as discussed later. The results of light-transmittance measurements, which were obtained from the samples prepared by the same procedure as the DSC measurements for the 2C18Br and 2C18Cl bilayers, are drawn in Figure 2. Curve 1 in Figure 2A presents the transmittance-temperature profile for the 2C18Br bilayer obtained by the treatment of (a). The transmittance increased markedly at 52.0 °C corresponding to the temperature of the Lc(1)/LR transition resulting from the transformation of turbid suspension into transparent one. This indicates that the Lc(1) phase of the 2C18Br bilayer is highly stable. Curve 2 in Figure 2A is the transmittance profile obtained by the treatment of (b). The transmittance increased (27) Feitosa, E.; Alves, F. R. Chem. Phys. Lipids 2008, 156, 13–16. (28) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994.

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Figure 2. Light-transmittance curves of (A) 2C18Br and (B) 2C18Cl bilayer membranes prepared by different methods under atmospheric pressure. Bilayer phases are assigned as the liquid crystalline (LR), lamellar gel (Lβ), stable lamellar crystal (Lc(1)), metastable lamellar crystal (Lc(2)), and lamellar crystal (Lc) phases. Bilayer phases in parentheses refer to metastable phases. Curve 1 in (A) was obtained by the treatment of (a) in the Experimental Section, curve 2 by the treatment of (b), and curve 3 by the treatment of (c). Curve 1 in (B) was obtained by the treatment of (e), and curve 2 by the treatment of (d).

at 44.2 °C corresponding to the main transition. In the treatment of (c), we observed three stepwise transmittance changes at 34.4, 44.5, and 51.4 °C (curve 3 in Figure 2 A). This sequence of the phase transitions agrees with that obtained from the DSC measurements, meaning that the turbidity of the bilayer suspension definitely changes depending on the phase state. Therefore, we can say that light-transmittance measurements are very effective to observe phase transitions of surfactant bilayers as well as DSC measurements. On the other hand, the transmittance profile for the 2C18Cl bilayer prepared by the treatment of (e) is depicted as curve 1 in Figure 2B. The transmittance increased steeply at a temperature corresponding to the Lc/Lβ transition and was followed by a slight increase at a temperature corresponding to the main transition, of which temperatures were in good agreement with those observed in the DSC measurements. Although the change in transmittance for the main transition was much smaller than that for the Lc/Lβ transition, we could definitely observe the main transition. Curve 2 in Figure 2B indicates the transmittance profile for the 2C18Cl bilayer obtained by the treatment of (d). In this condition, we could observe only the main transition independently as well as the case of the DSC measurements. The transmittance profile of the main transition for the 2C18Cl bilayer was broader than that for the 2C18Br bilayer. This is attributable to the difference in packing state between the 2C18Cl and 2C18Br bilayers arising from the difference of the counterion, which will be described in a later section. Therefore, all phases observed in the 2C18Cl bilayer are stable phases in contrast with several metastable phases in the 2C18Br bilayer. The difference in phase stability is clearly caused by the difference in counterion. Langmuir 2011, 27(5), 1592–1598

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Figure 3. Light-transmittance curves of 2C18Br bilayer membrane at (A) 80 MPa (curve 1) and (B) 180 MPa. Curve 2 of a broken line in (A) indicates the light-transmittance curve of 2C18Cl bilayer membrane at 150 MPa. Bilayer phases are assigned as the liquid crystalline (LR), lamellar gel (Lβ), stable lamellar crystal (Lc(1)), metastable lamellar crystal (Lc(2)), and lamellar crystal (Lc) phases. Bilayer phases in parentheses refer to metastable phases. Curve 1 in (A) was obtained by the treatment (c) in the Experimental Section, and curve 2 by the treatment of (e). Curve in (B) was obtained by the treatment (c).

3.2. Phase Transitions under High Pressure. As is seen from the transmittance-temperature profiles under atmospheric pressure for the 2C18Br bilayer prepared by the treatment of (c), it enables us to observe three transitions at the same time. Hence, we adopted the treatment of (c) to inquire the phase transitions under high pressure. The light-transmittance profiles for the 2C18Br bilayer under high pressure are shown in Figure 3. The temperatures of three kinds of transitions increased with an increase in pressure. At ca. 80 MPa, the transmittance increased abruptly at three temperatures corresponding to the Lc(2)/Lβ, Lβ/ LR, and Lc(1)/LR transitions in the same phase sequence as that under atmospheric pressure, which is shown in Figure 3A. On the other hand, we observed different transmittance profiles at pressures higher than ca. 130 MPa; that is to say, the transmittance conversely decreased at the third transition as shown in Figure 3B. Since we have confirmed that a decrease in transmittance is only observed at the transition for the bilayer interdigitation in isobaric thermotropic measurements,25 the transition may be closely related to an nonbilayer interdigitated structure into a bilayer lamellar structure. As for the 2C18Cl bilayer, the transmittance profiles under high pressure were similar to those under atmospheric pressure (curve 2 in Figure 3A), and the transition temperatures simply increased linearly with an increase in pressure. Figure 4A depicts the T-p phase diagram of the 2C18Br bilayer, which was constructed from the phase-transition temperatures and pressures determined by the DSC and light-transmittance measurements. The temperatures of all three transitions, metastable Lc(2)/Lβ transition, main transition, and stable Lc(1)/ LR transition, increased with increasing pressure. Interestingly, the temperatures of the main transition and the Lc(1)/LR transition DOI: 10.1021/la104552z

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Figure 4. Temperature-pressure phase diagrams for (A) 2C18Br and (B) 2C18Cl bilayer membranes. Bilayer phases are assigned as the liquid crystalline (LR), lamellar gel (Lβ), stable lamellar crystal (Lc(1)), metastable lamellar crystal (Lc(2)), and lamellar crystal (Lc) phases. Bilayer phases in parentheses refer to metastable phases. Phase transitions in (A): (O) Lβ/LR, (b) Lc(1)/LR (at pressures < 130 MPa); Lc(1)/Lβ (at pressures > 130 MPa) and (2) Lc(2)/Lβ. Phase transitions in (B): (O) Lβ/LR and (b) Lc/Lβ. Table 1. Thermodynamic Properties of Phase Transitions of 2C18Br, 2C18Cl, and DSPC Bilayers Obtained from DSC and Light-Transmittance Measurements sample

transition

T (°C)

T (K)

dT/dp (K MPa-1)

ΔH (kJ mol-1)

ΔS (J K-1 mol-1)

ΔV (cm3 mol-1)

2C18Br

Lc(1)/LR Lβ/LR Lc(2)/Lβ

52.9 ( 0.25a 44.5 ( 0.01a 35.4 ( 0.30a

326.1 317.7 308.6

0.18 0.26 0.11

101.2 ( 1.97a 45.2 ( 0.73a

310 142

55.8 37.0

2C18Cl

Lβ/LR Lc/Lβ

39.9 ( 0.03a 19.7 ( 0.23a

313.1 292.9

0.23 0.13

41.9 ( 0.52a 32.7 ( 0.23a

134 112

30.8 14.5

55.6b 328.8 0.23b 45.2b 137 31.6 Pβ0 /LR b b 0 0 50.9 324.1 0.14 5.0b 15 2.2 Lβ /Pβ 28.2c 301.4 0.22d 28.1c 93 20.6 Lc/Lβ0 a b 16 c 30 d Each value is denoted as (average ( standard deviation) over six measurements. Data from Ichimori et al. Data from Lewis et al. Data from Goto et al.25 DSPC

coincided with each other at ca. 130 MPa, and a part of the T-p curve for the transition arising from the decreased transmittance at pressures higher than ca. 130 MPa is in accord with that of the main transition. We judged from the appearance patterns of both transitions that the T-p curves for the main transition and the Lc(1)/LR transition intersect at ca. 130 MPa and the high-pressure transition appeared at higher temperatures than the Lc(1)/LR phase transition is the main transition. This fact means that the stability of the Lβ phase changes from a metastable state to a stable state at ca. 130 MPa. The corresponding T-p phase diagram of the 2C18Cl bilayer is shown in Figure 4B. The diagram took a simple shape as compared with that for the 2C18Br bilayer: the temperatures of the Lc/Lβ transition and main transition only increased with an increase in pressure. The slope of the phase boundary (dT/dp) for the main transition of the 2C18Cl bilayer under atmospheric pressure was comparable to that of the 2C18Br bilayers. Because the dT/dp values have similar ones if the kinds of phase transitions are the same,16,21,22 this result also supports the view that the transition observed at 39.9 °C in the 2C18Cl bilayer under atmospheric pressure is the main transition. Therefore, the Lc, Lβ, and LR phases of the 2C18Cl bilayer all exist as stable phases in the pressure range studied in this study. 3.3. Thermodynamic Quantities of Phase Transitions. The thermodynamic quantities, namely, enthalpy (ΔH), entropy (ΔS = ΔH/T), and volume (ΔV) changes of respective phase transi-

tions of the 2C18Br and 2C18Cl bilayers, were obtained from the DSC data and by the application of the dT/dp value to the Claypeyron equation (dT/dp = ΔV/ΔS).29 Here we could not determine the exact ΔH value of the metastable Lc(2)/Lβ phase transition of the 2C18Br bilayer because the transition was not obtained independently, so we do not discuss the thermodynamic properties of the Lc(2) phase in this study. In Table 1 are summarized the thermodynamic properties of the phase transitions for the 2C18Br and 2C18Cl bilayers together with those of the DSPC bilayer membranes.16,30 The dT/dp values of the main transition for the 2C18Br and 2C18Cl bilayers (0.26 and 0.23 K MPa-1) under atmospheric pressure are comparable to the main transitions (Pβ0 /LR) for the DSPC bilayers (0.23 K MPa-1).16 We can say that the transitions related to the formation of gauche conformers in the hydrocarbon chains are invariant irrespective of molecular structures under the condition of similar hydrophobic chains. In the case of the stable transition related to the Lc phase, the dT/dp values change among the 2C18Br, 2C18Cl and DSPC bilayers (0.18, 0.13, and 0.22 K MPa-1, respectively). Since the transition is closely related to the hydration/dehydration process in polar head groups of the molecule, the difference in the dT/dp value may be caused by that in the structure of polar head groups. The ΔH and ΔV values of the main transition for the 2C18Br bilayer were 45.2 kJ mol-1 and 37.0 cm3 mol-1, respectively, while those for the 2C18Cl bilayer were 41.9 kJ mol-1 and 30.8 cm3 mol-1, respectively. Although

(29) Ichimori, H.; Hata, T.; Yoshioka, T.; Matsuki, H.; Kaneshina, S. Chem. Phys. Lipids 1997, 89, 97–105.

(30) Lewis, R. N. A. H.; Mak, N.; McElhaney, R. N. Biochemistry 1987, 26, 6118–6126.

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Figure 5. Schematic diagrams for chemical potential-temperature profiles for 2C18Br bilayer among liquid crystalline (LR), lamellar gel (Lβ), stable lamellar crystal (Lc(1)), and metastable lamellar crystal (Lc(2)) phases at (A) 0.1 MPa and (B) 200 MPa. Solid and broken lines refer to stable and metastable states, respectively. The slope of curve reflects the partial molar entropy of 2C18Br in each state. Break points on curves refer to phase-transition points.

the values for the 2C18Br bilayer are slightly larger than those for the 2C18Cl bilayer (the difference is discussed later), they are also comparable to those for the DSPC bilayer (45.2 kJ mol-1 and 31.6 cm3 mol-1). This fact signifies that the stability of the gel phase among these bilayers is quite similar. On the contrary, a conclusive difference among three bilayers was found in the transition related to the Lc phase. First of all, the Lc(1)/LR transition temperature of the 2C18Br bilayer is higher than the main-transition temperature, whereas the situation is reversed for the 2C18Cl and DSPC bilayers, indicating that in the Lc(1) phase the 2C18Br bilayer can be packed the tightest in the bilayers among them and, thus, has considerably high stability. Moreover, the ΔH value of the Lc(1)/LR transition is exceedingly high (101.2 kJ mol-1). Taking into account that the Lc(1)/LR transition of the 2C18Br bilayer includes the chain melting (i.e., the transformation from the Lβ phase to the LR phase), therefore, the net ΔH value for the transformation from the Lc(1) phase to the Lβ phase is obtained as 56 kJ mol-1 by subtracting the ΔH value of the main transition and the value is almost twice the values for the 2C18Cl and DSPC bilayers. Since the enthalpy change of the Lc(1)/Lβ transition does not contain the rotation energy between trans and gauche conformers, the value of ΔH includes only the contribution of the van der Waals force between hydrocarbon chains and the Coulomb force between head groups of surfactant bilayers. The above estimation for ΔH seems to include not only the stronger electrostatic stabilization by the Brion between head groups in bilayer but also the stabilization due to the formation of the interdigitated structure. Consequently, the stability of the Lc(1) phase for the 2C18Br bilayer is quite different from that for the 2C18Cl and DSPC bilayers although the properties of the gel phases are almost the same. Furthermore, we consider the stability of the bilayer phases and the phase transitions for the 2C18Br bilayer membrane given in Figure 4 thermodynamically.22 On the basis of the entropy changes associated with the phase transitions of the 2C18Br bilayer, we can draw chemical potential (μ)-temperature (T) profiles among four (Lc(1), Lc(2), Lβ, and LR) states of the 2C18Br bilayer, and the profiles are schematically shown in Figure 5. The intersection point of chemical potential curves corresponds to the phase-transition point. The slopes of the curves indicate the partial molar entropies of the 2C18Br molecule in the four phases, which increase in that order by taking account of the ΔS values Langmuir 2011, 27(5), 1592–1598

in Table 1. The slope for the Lc(2) phase is anticipated because we could not obtain the ΔH value. The chemical potentials of the 2C18Br molecule in the Lβ and Lc(2) phases are larger than that in the Lc(1) phase at ambient pressure, which is shown in Figure 5A. By the sample treatment of (b), the 2C18Br bilayer undergoes the transition from the metastable Lβ phase to the metastable LR phase because the transformation into the Lc(1) phase requires cold storage. Further, the metastable Lc(2)/metastable Lβ transition occurs by treatment of (c). On the other hand, since the stable Lβ phase appears at pressure higher than ca. 130 MPa, two kinds of transitions between stable phases, namely, the Lc(1)/Lβ and Lβ/LR transitions, are observed, which are shown in Figure 5B. 3.4. Effect of Counterion on Phase Transitions. It should be noted that the transition temperatures and the thermodynamic quantities of the stable transition related to the Lc phase and main transition are dependent on the counterions of surfactants. When substituting the counterion from Cl- ion to Br- ion, both transition temperatures shift about 5 and 30 K to a higher temperature region, respectively. Feitosa and co-workers have reported6,27 the main transition temperatures and enthalpies of the 2C18Br and 2C18Cl bilayers from various preparation methods such as nonsonication (only hydration), sonication, and extrusion. They showed that the main-transition temperature of the 2C18Br bilayer is lower by ca. 3 K than that of the 2C18Cl bilayer and the corresponding enthalpy of the 2C18Br bilayer is smaller by ca. 2.3 kcal mol-1. They explained the reason by the different affinity and binding specificity of the counterions to the vesicle interfaces, and thus, the 2C18Br bilayer exhibits stronger effect on turning the bilayer more fluid and complex state, producing the smaller maintransition temperature of the 2C18Br bilayer than that of the 2C18Cl bilayer and only slightly larger vesicles.27 The results are quite opposite to the present results: the main-transition temperature and enthalpy of the 2C18Br bilayer are higher and larger than those of the 2C18Cl bilayer as seen in Table 1. Although the reason for the discrepancy between the values has not yet been clear, we consider that the elevation of the main-transition temperature and enhancement of the main-transition enthalpy of the 2C18Br bilayer as compared with those of the 2C18Cl bilayer are originated from the difference in interaction between surfactant cation and halide counterion in bilayer phases. DOI: 10.1021/la104552z

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The above observations may be explained by the differences in the degree of hydration of negative halide counterions in the bilayer membrane and the difference of electrostatic interaction between surfactant cations and counterions in the bilayer. The difference in thermodynamic properties between them seems to be attributable to the differences in the degree of the following three factors: (i) hydration interaction between water molecules and counterions, (ii) counterion distribution in the bilayer, and (iii) ion-pair formation in the bilayer between surfactant cations and counterions. The halide ions have a weak interaction with water molecules compared with the alkaline metal ions, such as Liþ and Naþ ions, because of their large crystallographic radii;31,32 they are known as structure-breaking or negative hydration ions. However, the hydration interaction between water molecules and Br- ions is weaker than that of Cl- ions because the Stokes radius of the Cl- ion is slightly larger than that of the Br- ion33 and the hydration Gibbs energy of the Cl- ion is more negative than that of the Br- ion.34 This may contribute to the degree of attractive interaction between a surfactant cation and its counterion, namely, Br- ions lesser hydrate and greater distribute around surfactant ions than Cl- ions. Similar behavior is also observed in structural changes of surfactants such as monolayer film and micelle formation and coagel-micelle transition; the surface adsorption,35,36 the degree of counterion binding of cationic surfactant micelles,37 and the coagel-micelle transition temperature38 of the Br- ion are greater than those of the Cl- ion. In addition, ion-pair formation between the tetraalkylammonium ion and the halide ion was reported by Lindenbaum and Boyd.39 They found (31) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, 2nd ed.; Butterworths: London, 1959. (32) Shannon, R. D. Acta Crystallogr., Sect. A: Found. Crystallogr. 1976, A32, 751–767. (33) Nightingale, E. R., Jr. J. Phys. Chem. 1959, 63, 1381–1387. (34) Rosseinsky, D. R. Chem. Rev. 1965, 65, 467–490. (35) Matsuki, H.; Yamanaka, M.; Kaneshina, S. Bull. Chem. Soc. Jpn. 1995, 68, 1833–1138. (36) Okuda, H.; Ozeki, S.; Ikeda, S. J. Colloid Interface Sci. 1987, 115, 155–166. (37) Underwood, A. L.; Anacker, E. W. J. Colloid Interface Sci. 1987, 117, 242– 250. (38) Matsuki, H.; Ichikawa, R.; Kaneshina, S.; Kamaya, H.; Ueda, I. J. Colloid Interface Sci. 1996, 181, 362–369. (39) Lindenbaum, S.; Boyd, G. J. Phys. Chem. 1964, 68, 911–917.

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that the bromide salts of tetraalkylammonium form slightly an ion-pair in the solution, while the chloride salts do not at all. It is considerable that the above-mentioned factors are all closely related to the degree of electrostatic repulsion between cationic head groups, and that they reduce the repulsion in bromide salts effectively rather than chloride salts. Therefore, we may say that the packing of surfactant molecules in the bilayer becomes tighter in bromide salts, which causes high main-transition temperature and ΔH value. Then, the difference in the main transition properties between the 2C18Br and 2C18Cl bilayers can be explained from the difference in interaction between a surfactant and its counterion. Regarding the Lc(1)/LR transition of 2C18Br, the larger thermodynamic properties and the greater stabilization of the Lc(1) phase might be partially attributable to the ion-pair formation between surfactant cation and bromide ion in the Lc(1) phase.

4. Conclusions We examined the phase transitions of the 2C18Br and 2C18Cl bilayers prepared by different methods under atmospheric pressure and revealed the bilayer phase behavior by the T-p phase diagrams and the thermodynamic quantities of the phase transitions. Regarding the phase transitions of the 2C18Br bilayer, first, the main transition is a transition between metastable phases. Second, there are two kinds of transitions related to the Lc phase caused by the hydration or dehydration; one is the stable Lc(1)/LR transition in the high temperature region and the other is the metastable Lc(2)/Lβ transition in the low-temperature region. Third, the curves of the main transition and Lc(1)/LR transition intersect each other at ca. 130 MPa, and the stable Lβ phase appeared at pressures higher than ca. 130 MPa. On the other hand, all the Lc, Lβ, and LR phases of the 2C18Cl bilayer exist as stable phase and the T-p phase diagram takes a simple shape as compared with the 2C18Br bilayer. Moreover, the thermodynamic quantities of the Lc(1)/LR transition of the 2C18Br bilayer have extremely high values although the thermodynamic quantities of the main transitions are comparable to each other between them. All differences between the 2C18Br and 2C18Cl bilayers are attributable to the difference in interaction between a surfactant and its counterion.

Langmuir 2011, 27(5), 1592–1598