FT-IR and ESR Spin-Label Studies of Mesomorphic Phases Formed in

Langmuir 2001, 17, 6915-6922

6915

FT-IR and ESR Spin-Label Studies of Mesomorphic Phases Formed in Aqueous Mixtures of Heptaethylene Glycol Dodecyl Ether Tohru Inoue,*,† Hideo Kawamura,‡ Miyako Matsuda,† Yasuhito Misono,† and Masao Suzuki§ Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan, Department of Applied Chemistry and Biotechnology, Niihama National College of Technology, Yagumo-cho, Niihama 792-8580, Japan, and Advanced Science and Technology Research Center, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Received March 29, 2001. In Final Form: August 23, 2001 The phase behavior of aqueous mixtures of heptaethylene glycol dodecyl ether (C12E7) was studied by the use of Fourier transform infrared spectroscopy and electron spin resonance spin-label techniques, stressing the conformational structure of the surfactant molecules and the dynamic aspects of the molecular assemblies in various phases assumed by this mixture system. When the mixture transforms from solid to mesophases, the hydrogen bonds between terminal OH groups of polyoxyethylene (POE) chains in the surfactant molecules and also between the POE chain and water molecules are mostly broken, whereas the conformational structure of the alkyl and POE chains remains still highly ordered. The order-disorder transformation of the chains is induced by the temperature rise in the mesomorphic phases. The microviscosity of the V1 phase reported by a spin probe was the lowest among the three mesophases assumed by this mixture system, although the bulk viscosity of the V1 phase is higher than those of the other two phases, H1 and LR. This suggests that the surfactant molecules are packed rather loosely in the surfactant bilayer constituting the bicontinuous network structure in the V1 phase. No definite correlation was found between the conformational structure of the surfactant molecule and the order parameter derived from the spin-label study. This implies that the dynamic properties of the surfactant molecular assemblies are determined mostly by the molecular packing in the assemblies and are rather insensitive to the conformational structure of the constituent surfactant molecules.

Introduction Poly(ethylene glycol) alkyl ethers (CnEm) are typical nonionic surfactants and are widely used in the fields of detergents, cosmetics, and many other industrial applications. The aqueous mixtures of this class of surfactants assume various lyotropic mesophases of a liquidcrystalline nature depending on the molecular structure of the surfactants, composition, and temperature.1,2 The phase science of the aqueous mixture of these surfactants has so far been a problem attracting considerable attention in the fields of both fundamental colloid science and practical application of the surfactants, and the molecular arrangements in respective mesomorphic phases have been rather well established;3-19 current advances in this * To whom correspondence should be addressed. E-mail: [email protected]. † Fukuoka University. ‡ Niihama National College of Technology. § Kyushu University. (1) Sjo¨blom, J.; Stenius, P.; Danielsson, I. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; p 369. (2) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994. (3) Mulley, B. A.; Metcalf, A. D. J. Colloid Interface Sci. 1964, 19, 501. (4) Clunie, J. S.; Goodman, J. F.; Symons, P. C. Trans. Faraday Soc. 1969, 65, 287. (5) Ali, A. A.; Mulley, B. A. J. Pharm. Pharmacol. 1978, 30, 205. (6) Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1980, 73, 5849. (7) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (8) Adam, C. D.; Durrant, J. A.; Lowry, M. R.; Tiddy, G. J. T. J. Chem. Soc., Faraday Trans. 1 1984, 80, 789. (9) Andersson, B.; Olofsson, G. Colloid Polym. Sci. 1987, 265, 318.

subject have been reviewed by Chernik.20 It seems, however, that the structural feature of the surfactant molecule itself in the molecular assemblies has not been well revealed. In a previous work,21 we investigated the phase behavior of an aqueous mixture of heptaethylene glycol dodecyl ether (C12E7) by means of a differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR). The DSC technique was used to construct a binary phase diagram of this mixture system. According to the obtained phase diagram, the mixture with about 30 wt % H2O undergoes successive phase transformation, solid f H1 (normal hexagonal) f V1 (bicontinuous cubic) f LR (lamellar) f liquid, with the increase in temperature. For the C12E7/D2O mixture with this composition, FT-IR (10) Strey, R.; Schoma¨cker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1 1990, 86, 2253. (11) Kratzat, K.; Schmidt, C.; Finkelmann, H. J. Colloid Interface Sci. 1994, 163, 190. (12) Kratzat, K.; Finkelmann, H. J. Colloid Interface Sci. 1996, 181, 542. (13) Kratzat, K.; Guittard, F.; de Givenchy, E. T.; Cambon, A. Langmuir 1996, 12, 6346. (14) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. B 1997, 101, 7952. (15) Shigeta, K.; Suzuki, M.; Kunieda, H. Prog. Colloid Polym. Sci. 1997, 106, 49. (16) Nibu, Y.; Suemori, T.; Inoue, T. J. Colloid Interface Sci. 1997, 191, 256. (17) Nibu, Y.; Inoue, T. J. Colloid Interface Sci. 1998, 205, 231. (18) Nibu, Y.; Inoue, T. J. Colloid Interface Sci. 1998, 205, 305. (19) Huang, K.-L.; Shigeta, K.; Kunieda, H. Prog. Colloid Polym. Sci. 1998, 110, 171. (20) Chernik, G. G. Curr. Opin. Colloid Interface Sci. 2000, 4, 381. (21) Inoue, T.; Matsuda, M.; Nibu, Y.; Misono, Y.; Suzuki, M. Langmuir 2001, 17, 1833.

10.1021/la010488x CCC: $20.00 © 2001 American Chemical Society Published on Web 10/06/2001

6916

Langmuir, Vol. 17, No. 22, 2001

spectroscopy was applied to examine possible changes accompanied by the phase transformation regarding the conformational structure of the surfactant molecules and the interaction between surfactant molecules as well as that between the surfactant and D2O. It was found in this previous work that the order-disorder transformation of both alkyl and polyoxyethylene (POE) chains of the surfactant molecule is induced by the temperature rise within the H1 phase rather than accompanied by the solidto-H1 phase transition. At the solid-to-H1 phase transition, the hydrogen bonds between surfactant molecules and also those between D2O and POE chains are mostly broken, but the conformational structure of the alkyl and POE chains remains almost unaltered. In the case of pure C12E7, both the alkyl and POE chains undergo drastic change from ordered to disordered structure associated with the solid-to-liquid phase transition. In the mixture of C12E7 with D2O, on the other hand, they transform from a transrich ordered structure to a gauche-containing disordered structure rather gradually in the H1 phase. As an extension of the previous study, we have measured IR spectra for C12E7/D2O mixtures with several compositions as a function of temperature in order to examine whether the order-disorder transformation of the alkyl and POE chains, that is, the “chain melting”, of C12E7 molecules occurs commonly within mesomorphic phases. In addition, the dynamic feature of the molecular assemblies in various phases formed in the C12E7/H2O mixture was studied by means of an electron spin resonance (ESR) spin-label technique. The ESR spectra of a spin probe embedded in the molecular assemblies reflect sensitively the motion of the probe and, hence, can report the microviscosity of the environment around the probe molecules. The spin-label technique has frequently been applied to aqueous surfactant systems to characterize the molecular assemblies formed by the surfactants.22-26 It was our interest to examine the correlation between the conformational structure of surfactant molecules and the microviscosity of the surfactant molecular assemblies. Furthermore, how the dynamic aspects of molecular assemblies depend on the type of phase and the water content in the mixture was our other interest. In the present paper, we report the results of FT-IR and ESR studies for phase behavior of aqueous C12E7 mixtures with several compositions. Experimental Section Materials. The sample of heptaethylene glycol dodecyl ether (C12E7) with a homogeneous chain length distribution was obtained from Nikko Chemicals (Tokyo, Japan) and used without further purification. Heavy water (>99.75%) from Wako Pure Chemicals (Tokyo, Japan) and a spin probe, 5-nitroxide stearic acid (5-NS), from Aldrich were used as received. Water was purified by deionization followed by double distillation. FT-IR Measurements. A Bio-Rad model FTS 165 (Cambridge, MA) was used to record the FT-IR spectra for the C12E7/ D2O mixture at various temperatures in the range from -10 to 60 °C. The spectra were recorded with 50 scans at 4 cm-1 resolution. For IR measurements, the C12E7/D2O mixtures with different compositions were sandwiched between two silicone wafers of 3 cm diameter. The silicone wafers sandwiching the mixture sample were sealed by silicone grease in order to prevent the evaporation of D2O during the measurements. The sandwich was placed in a handmade cell holder, which was thermostated (22) Lasic, D. D.; Hauser, H. Mol. Cryst. Liq. Cryst. 1984, 113, 59. (23) Lasic, D. D.; Hauser, H. J. Phys. Chem. 1985, 89, 2648. (24) Lasic, D. D. J. Colloid Interface Sci. 1986, 113, 188. (25) Yamagata, Y.; Senna, M. Langmuir 2000, 16, 6136. (26) Somasundaran, P.; Huang, L.; Fan, A. In Modern Characterization Methods of Surfactant Systems; Binks, B. P., Ed.; Marcel Dekker: New York, 1999; p 229.

Inoue et al.

Figure 1. T-X phase diagram of the C12E7/H2O mixture obtained from DSC measurements. by circulating water of constant temperature using a Neslab refrigerated circulation bath RTE-211 (Portsmouth, NH). In the measurements of FT-IR spectra as a function of temperature, the sample was once cooled to -10 °C and then heated stepwise with appropriate temperature intervals. After the sample was kept at the desired temperature for more than 5 min, the spectra were recorded. ESR Measurements. ESR spectra were recorded on a JEOL model RE-1X spectrometer equipped with a temperature controlling accessory. The 5-NS concentration in the sample of the C12E7/H2O mixture was kept at about 1/100 in molar ratio with respect to C12E7. Liquid nitrogen was used to control the temperature below 20 °C. As in the case of FT-IR measurements, the sample was once cooled to -10 °C and then heated stepwise. After the sample was kept at the desired temperature for more than 5 min, the spectra were recorded. The conditions for recording of the ESR spectra were as follows: power, 5 mW; modulation, 0.1 mT; scan range, 327.5 ( 5 mT.

Results and Discussion The binary phase diagram of the C12E7/H2O mixture determined by differential scanning calorimetry has been reported previously.21 The phase diagram is reproduced in Figure 1 for convenience. In this mixture system, there appear three mesomorphic phases, that is, H1 (normal hexagonal), V1 (normal bicontinuous-type cubic), and LR (lamellar), depending on the composition and temperature. The FT-IR and ESR spectra were measured for C12E7/ D2O and C12E7/H2O mixtures, respectively, with four different compositions, increasing the temperature along the dotted lines indicated in Figure 1. FT-IR Study for the C12E7/D2O Mixture. The IR spectra obtained for the C12E7/D2O mixture with 39.6 wt % D2O at various temperatures are shown in Figure 2a-e in different wavenumber regions. The assignment of the absorption bands in each wavenumber region has been described previously21 in some detail. Briefly, the absorption band in the 3000-3800 cm-1 region (Figure 2a) is attributed to the O-H stretching mode (νOH) of the

Mesomorphic Phases of Aqueous C12E7 Mixtures

Langmuir, Vol. 17, No. 22, 2001 6917

Figure 2. IR spectra obtained for the C12E7/D2O mixture with 39.6 wt % D2O at various temperatures in the frequency regions of 3000-3800 cm-1 (a), 2700-3100 cm-1 (b), 2000-2800 cm-1 (c), 1000-1200 cm-1 (d), and 750-950 cm-1 (e). Temperatures in °C and phases of the mixture under those temperatures are shown near each spectrum.

terminal OH group in the polyoxyethylene (POE) chain of the surfactant and is sensitive to the hydrogen-bond interaction of the OH group. The absorption band around 2900 cm-1 (Figure 2b) is assigned to the methylene C-H stretching mode (νCH2). This band reflects the conformation of hydrocarbon chains27 and can be used to monitor the conformational structure of the alkyl chain of the surfactant molecule. The absorption appearing at 2000-2800 cm-1 (Figure 2c) is ascribed to the O-D stretching mode (νOD) and is used to obtain the information about the interaction between the POE chain of the surfactant and D2O molecules. The absorption bands around 1115 cm-1 (Figure 2d) and 850 cm-1 (Figure 2e) which are doublet peaks at low temperatures are attributed to the coupled mode of C-O stretching, C-C stretching, and methylene rocking (νCO + νCC + FCH2) and that of C-O stretching and methylene rocking (νCO + FCH2) of the POE chain, respectively, and reflect sensitively the conformational structure of the POE chain.28 As is seen in Figure 2a, the absorption band due to νOH changes drastically when the solid of this mixture transforms to the H1 phase (compare the spectra at -10.1 °C (solid) and -2.0 °C (H1)). This spectral change demonstrates that the hydrogen bonds between POE (27) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (28) Matsuura, H.; Fukuhara, K. J. Polym. Sci., Part B: Polym. Phys. 1986, 24, 1383.

chains of two adjacent surfactant molecules formed in the solid phase are mostly broken in the H1 phase. A drastic spectral change associated with the solid-to-H1 transformation is also observed in the νOD band (see Figure 2c). The spectral change indicates that the partial dehydration of D2O bound to the surfactant POE chain through hydrogen-bond interaction occurs accompanied by the solid-to-H1 phase transformation. Contrary to the absorption bands due to νOH and νOD, no significant difference is appreciable in the absorption bands due to νCH2 (Figure 2b), νCO + FCH2 (Figure 2d), and νCO + νCC + FCH2 (Figure 2e) between the spectra at -10.1 °C (solid) and -2.0 °C (H1). This shows that the ordered conformational structure of both alkyl and POE chains of the surfactant molecule in the solid phase remains essentially unaltered even when the phase transforms from solid to H1. The spectral pattern characteristic of the ordered structure of the alkyl and POE chains continues up to 10.3 °C. When the temperature is raised from 10.3 to 15.1 °C, the absorption bands ascribed to νCH2, νCO + FCH2, and νCO + νCC + FCH2 change clearly (Figure 2b,d,e). The spectral change from a rather sharp shape to a broad one occurring associated with this temperature rise demonstrates that the conformational structure of both alkyl and POE chains of the surfactant molecule changes from the trans-rich ordered structure to the gauche-containing disordered structure at the temperature between 10 and 15 °C. In other words, the chain melting of alkyl and POE chains

6918


Inoue et al.

Figure 3. IR spectra obtained for the C12E7/D2O mixture with 20.6 wt % D2O at various temperatures in the frequency regions of 2700-3100 cm-1 (a), 1000-1200 cm-1 (b), and 750-950 cm-1 (c). Temperatures in °C and phases of the mixture under those temperatures are shown near each spectrum.


of the surfactant molecule occurs within the H1 phase when the temperature is increased. The spectral shape showing the ordered structure of the alkyl and POE chains is broadened little by little with the temperature rise even below 10 °C. This suggests that the chain melting does not occur with high cooperativity but proceeds rather gradually. The IR spectra for C12E7/D2O mixtures with other compositions were measured as a function of temperature. Figures 3-5 show the IR spectra obtained for C12E7/D2O mixtures with 20.6, 31.1, and 50.0 wt % D2O, respectively, at various temperatures in the wavenumber regions of 2700-3100, 1000-1200, and 750-950 cm-1. The drastic change in the absorption bands due to νOH and νOD associated with the solid-to-mesophase transformation were also observed for these mixtures similarly to the case of the mixture with 39.6 wt % D2O (data not shown). However, as is shown in Figures 3-5, no significant change is seen in the absorption bands characterizing the conformational structure of the alkyl and POE chains of the surfactant molecule in the course of the solid-to-mesophase transformation, which suggests that the ordered structure of the chains is kept just after the solid-to-mesophase transformation takes place. Instead, the spectral change indicates that the chain melting completes for the mixture

with 20.6 wt % D2O at the temperature between 10.0 and 13.0 °C which corresponds to the LR phase (Figure 3) and also for the mixture with 31.1 wt % D2O between 13.8 and 16.1 °C corresponding to H1 phase (Figure 4). In the case of the mixture with 50.0 wt % D2O, the corresponding spectral change is not so definite, but it is suggested that the chain melting completes at the temperature between 0 and 3 °C (Figure 5). Thus, it may be concluded that the order-disorder transformation of both alkyl and POE chains of the C12E7 molecule in the aqueous mixture occurs within mesomorphic phases with the increase in temperature. In addition, the temperature at which the chain melting completes depends somewhat on the composition of the mixture. It should be noted that the order-disorder transformation of the alkyl chain and that of the POE chain take place not independently but rather simultaneously. Spin-Label Study for the C12E7/H2O Mixture. ESR spectra of 5-NS incorporated in C12E7/H2O mixtures were measured as a function of temperature and composition in order to obtain information regarding the dynamic aspect of each phase assumed by the mixture. Before going to mixture systems, it may be suggestive to discuss the results obtained for pure C12E7 by applying the spin-label technique.


Langmuir, Vol. 17, No. 22, 2001 6919


distribution and rather isotropic motion of the probe molecules even though C12E7 is still solid phase. The ESR spectra obtained above 21.2 °C were analyzed in terms of the rotational correlation time, τc, of the spin probe by applying the following equation:

(x x

τc ) (6.5 × 10-10)W0

Figure 6. ESR spectra of 5-NS incorporated in pure C12E7 at various temperatures. Temperatures in °C are shown near each spectrum.

ESR Spectra of 5-NS Incorporated in Pure C12E7. Figure 6 shows the ESR spectra of 5-NS incorporated in pure C12E7 at various temperatures. The spectra obtained in the solid phase below 16.2 °C exhibit a quite broad pattern, in which the hyperfine splitting is not well resolved. This spectral pattern is probably caused by a spin exchange of the probe molecules (exchange broadening).29 It is likely that the fatty acid spin probe, 5-NS, and C12E7 are mutually immiscible or only partly miscible in a solid phase. Then, the 5-NS molecules are forced to exist as a separate phase or in a concentrated domain, which would facilitate the spin exchange between the probe molecules. When temperature rises to 21.2 °C, the spectral pattern becomes close to that characteristic of the isotropic motion of the probe molecules. This is well correlated with the previous observation by FT-IR spectroscopy,21 where it was demonstrated that the hydrogen bonds between C12E7 molecules in the solid phase begin to be broken at the temperature below the melting point (25.8 °C) by about 5 °C. It is likely that the breaking of the hydrogen bonds between C12E7 molecules facilitates the lateral movement of the probe molecules and results in rather homogeneous (29) Marsh, D. In Biological Magnetic Resonance. 8. Spin Labeling; Berliner L. J., Reuben, J., Ed.; Plenum Press: New York, 1989; p 270.

h0 + h-1

h0 -2 h+1

)

(1)

where W0 is the peak-to-peak width of the central line measured in Gauss, and h0, h+1, and h-1 are the heights of the center, low, and high field spectral lines, respectively. This expression has been frequently used to evaluate the motional behavior of probe molecules solubilized in the surfactant micelles.30,31 The values of τc estimated from eq 1 are plotted in Figure 7a against temperature. It can be seen in this figure that τc decreases steeply with the increase of temperature in a solid phase, and the slope of the τc versus temperature plot changes discretely at the melting temperature, reflecting the solid-to-liquid phase transition. Figure 7b shows a plot of ln(1/τc) against 1/T (the Arrhenius plot). From the slopes of the straight lines drawn above and below the melting temperature, the activation energies for the rotational motion of the nitroxide moiety of the probe molecule are estimated to be 16.8 ( 1.6 kJ mol-1 and 27.7 ( 1.4 kJ mol-1 for a liquid phase and a solid phase in the temperature range from 21 °C to the melting temperature, respectively. These activation energies are regarded to correspond to those of microviscosity around the probe molecules. The activation energy in the solid phase is only 1.6 times larger than that in the liquid phase. Thus, the solid of C12E7 at the temperature just below the melting point exhibits the dynamic property rather close to that of the liquid as long as being observed through the rotational motion of the probe molecules. This enhanced molecular motion must be attributed to the breaking of the hydrogen bonds between terminal OH groups in the POE chain of the surfactant molecules. ESR Spectra of 5-NS Incorporated in the C12E7/ H2O Mixture with 31.1 wt % H2O. The ESR spectra obtained for 5-NS incorporated in the C12E7/H2O mixture (30) Waggoner, A. S.; Griffith, O. H.; Christensen, G. R. Proc. Natl. Acad. Sci. U.S.A. 1967, 57, 1198. (31) Schreier, S.; Ernandes, J. R.; Cuccovia, I.; Chaimovich, H. J. Magn. Reson. 1978, 30, 283.

6920


Figure 7. Plots of rotational correlation time, τc, of 5-NS in pure C12E7 against temperature (a) and ln(1/τc) against 1/T (b). τc values were calculated according to eq 1.

Inoue et al.

the H1 phase to the V1 phase by the temperature rise from 20 to 26 °C, the spectrum begins to show rather isotropic molecular motion of the probe. The spectra obtained for the LR phase demonstrate that the molecular motion of the probe in this phase becomes anisotropic again although the LR phase appears at a higher temperature than the V1 phase. The spectrum observed for the liquid phase exhibits the pattern characteristic of isotropic motion of the probe molecules. The above ESR observation indicates that the microviscosity around probe molecules is the lowest in the V1 phase among the three mesomorphic phases formed in the C12E7/H2O mixture. On the other hand, it is generally known that the bulk viscosity of the V1 phase is higher than that of the other two mesophases.32 The high bulk viscosity of the V1 phase may be attributed to the network structure being formed by the surfactant bilayer in a bicontinuous manner throughout the whole system. The microviscosity reported by the spin probe may be determined by the packing of the surfactant molecules in molecular assemblies in each phase. The lowest microviscosity of the V1 phase suggests that the surfactant molecules are packed more loosely in the surfactant bilayer constituting the V1 phase than in the molecular assemblies formed in the H1 phase (rod-micellar-like) and the LR phase (lamellar). The motion of the spin probe was analyzed in terms of order parameter, S. S is a parameter representing the degree of inclination of the principal axis of the nitroxide radical against the rotational axis of the long-chained probe molecule; S ) 1 when the two axes are parallel, and S ) 0 for completely isotropic rotation of the nitroxide radical.26 Thus, the S values become a measure of a dynamic feature or microviscosity of the medium in which the probe molecules are incorporated. The values of S were determined from the ESR spectra according to the following relation.26

S)

Figure 8. ESR spectra of 5-NS incorporated in the C12E7/H2O mixture with 31.1 wt % H2O at various temperatures. Temperatures in °C and phases of the mixture under those temperatures are shown near each spectrum.

with the composition of 31.1 wt % H2O are shown in Figure 8 at various temperatures. The spectrum observed in the solid phase exhibits a typical pattern expected for nitroxide spin probes randomly and rigidly oriented in frozen solution.26 This spectrum is quite different from that obtained for pure C12E7 in the solid phase (see Figure 6), and the effect of water is appreciable even in the solid phase. When the mixture transforms from the solid to the H1 phase, the restriction of molecular motion is somewhat released, but the rotation of the probe molecules is still highly anisotropic. When the mixture is transformed from

A| - A⊥ Azz - (Axx + Ayy)/2

(2)

where Axx, Ayy, and Azz are the principal values of the A tensor, and the numerical value of the denominator of the right-hand side of eq 2 is 27.55 G for 5-NS. The values of A| and A⊥ were determined from the ESR spectra according to the conventional procedure. The above analysis was limited to the case in which A| and A⊥ can be determined definitely. The values of S thus obtained for the H1 phase of C12E7/H2O mixture with 31.1 wt % H2O are plotted in Figure 9 as a function of temperature. As is seen in this figure, the value of S decreases with the increase in temperature. This is readily understood, because the microviscosity of the molecular assemblies is expected to decrease with the temperature rise. The temperature corresponding to the completion of the chain melting of the alkyl and POE chains of the surfactant molecule revealed by FT-IR measurements is indicated in the figure by the white arrow. No discrete change is observed in the variation of S at this temperature, although a subtle change in the slope is appreciable. A similar tendency was also observed for C12E7/H2O mixtures with other compositions. At first glance, it is expected that the microviscosity of the mixture would depend strongly on the conformational structure of the surfactant molecule. Actually, however, this is not the case. This result suggests that the dynamic feature of the (32) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994; Chapter 8.


Figure 9. Plot of the order parameter, S, calculated from eq 2 for the C12E7/H2O mixture with 31.1 wt % H2O against temperature. The vertical lines indicate the temperatures corresponding to phase boundaries.

Figure 10. ESR spectra of 5-NS incorporated in C12E7/H2O mixtures with various compositions at 35 °C. The compositions in wt % of H2O and the phases of the mixture under those compositions are shown near each spectrum.

molecular assemblies in the surfactant/water mixture is mainly determined by the molecular arrangements or molecular packing within the assemblies rather than the conformational structure of the surfactant molecules. Composition Dependence at Constant Temperature and Temperature Dependence at Fixed Composition of ESR Spectra. The dynamic feature of the molecular assemblies formed in the surfactant/water mixture would depend on the water content in the mixture as well as on the temperature even within the same phase. Thus, the effects of water content and temperature on the ESR spectra of 5-NS in the C12E7/H2O mixture were examined. The ESR spectra of 5-NS incorporated in C12E7/H2O mixtures with various compositions obtained at 35 °C are compared in Figure 10. This figure shows that the molecular motion of the probe is rather isotropic in the V1 phase and the liquid phase, whereas it is anisotropic in the LR phase and the H1 phase, being consistent with the above results obtained for the mixture with 31.1 wt % H2O. It can be recognized, however, that the extent of motional anisotropy of the probe in the LR and H1 phases

Langmuir, Vol. 17, No. 22, 2001 6921

Figure 11. ESR spectra of 5-NS incorporated in the C12E7/ H2O mixture with 49.7 wt % H2O at various temperatures. Temperatures in °C are shown near each spectrum. The phase at each temperature is H1, except for 55.4 °C.

somewhat decreases with the increase in the water content in the mixture. This demonstrates that the microviscosity in the molecular assemblies formed in the surfactant/ water mixture decreases, or in other words, the molecular packing becomes looser, with the increase in water content in the mixture even within the same phase. According to the phase diagram of the C12E7/H2O mixture, the H1 phase appears over a wide temperature range for the mixtures of the composition range 40-56 wt % H2O. Thus, the ESR spectra were measured for the mixture with 49.7 wt % H2O varying the temperature over a wide range in order to examine the temperature effect on the molecular motion of the spin probe. The results are shown in Figure 11. With the increase in temperature, the extent of the rotational anisotropy of the probe molecules decreases. However, the double minimum in the higher field remains up to 49.1 °C; this can be recognized clearly in the spectra drawn in a more expanded scale than those in Figure 11. This suggests that the molecular motion of the probe in thte H1 phase is essentially anisotropic even up to such high temperatures. When the mixture transforms to the liquid phase, the molecular motion of the probe becomes isotropic, as can be seen from the spectra obtained at 55.4 °C. Conclusion In the present work, the phase behavior of aqueous C12E7 mixtures was investigated by means of FT-IR and ESR spin-label techniques, stressing the conformational structure of the surfactant molecules and the dynamic aspects of the molecular assemblies in various phases assumed by this mixture system. It was found that when the mixture transforms from solid to mesophases, the hydrogen bonds between terminal OH groups of the surfactant molecules and also between the surfactant POE chain and water molecules are mostly broken, whereas the conformational structure of both the alkyl and POE chains of the surfactant molecules remains still highly ordered. The order-disorder transformation of the chains occurs within the mesomorphic phases with the increase in temperature. This conformational transformation takes place rather simultaneously for both the alkyl chain and the POE chain of the surfactant molecule. The microviscosity of the surfactant molecular assemblies formed in the aqueous C12E7 mixture is the lowest in the V1 phase among the three mesophases assumed by this mixture system,

6922


although the bulk viscosity of the V1 phase is higher than those of the other two phases, H1 and LR. This suggests that the surfactant molecules are packed rather loosely in the surfactant bilayer constituting the bicontinuous network structure in the V1 phase. No definite correlation was found between the conformational structure of the surfactant molecule and the order parameter derived from the spin-label study. This fact may be interpreted as that the dynamic properties of the surfactant molecular assemblies are determined mostly by the molecular packing in the assemblies regardless of the conformational structure of the constituent surfactant molecules. Finally,

Inoue et al.

it was shown that the microviscosity of the surfactant molecular assemblies decreases with the increase in water content in the mixture as well as with the increase in temperature even in the same phase. Acknowledgment. We are grateful to Professor Takeo Yamaguchi of Fukuoka University for his useful discussion in the present ESR work. This work was supported in part by funds (No. 965012) from the Central Research Institute of Fukuoka University. LA010488X

FT-IR and ESR Spin-Label Studies of Mesomorphic Phases Formed in

Recommend Documents