Phase Behavior of Heptaethylene Glycol Dodecyl Ether and Its

Langmuir 2001, 17, 1833-1840

1833

Phase Behavior of Heptaethylene Glycol Dodecyl Ether and Its Aqueous Mixture Revealed by DSC and FT-IR Spectroscopy Tohru Inoue,*,† Miyako Matsuda,† Yoshinori Nibu,† Yasuhito Misono,† and Masao Suzuki‡ Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan, and Advanced Science and Technology Research Center, Kyushu University, Kasuga, Fukuoka 816-8580, Japan Received August 24, 2000. In Final Form: December 27, 2000 The phase behavior of heptaethylene glycol dodecyl ether (C12E7) and its aqueous mixture was investigated by means of differential scanning calorimetry (DSC) and Fourier transform infrared (FT-IR) spectroscopy in the temperature range from -20 to 70 °C. Phase boundaries among various phases including mesomorphic phases were determined on the basis of DSC thermograms, from which the binary phase diagram of this mixture system was constructed. According to the phase diagram, the mixture with the composition of 31 wt % H2O exhibits the following phase sequence with increasing temperature: solid f H1 (normal hexagonal) f V1 (normal bicontinuous-type cubic) f LR (lamellar) f liquid. FT-IR measurements for the mixture with this composition revealed the following features concerning the conformational structure of the C12E7 molecule and the interaction between the surfactant and D2O in each phase. In the solid phase, the polyoxyethylene (POE) chain of the surfactant molecule has a rather extended structure with a trans-rich conformation, in contrast to the case of pure C12E7, in which a helical structure is dominant. When the solid phase transforms to the H1 phase, the hydrogen-bond interaction between C12E7 molecules and between C12E7 and water are both weakened, and the fraction of gauche-conformer in the POE chain increases, whereas the alkyl chain adjacent to the POE chain remains, taking a rather trans-zigzag conformation. The enhanced fraction of gauche-conformer in the POE chain suggests that the POE chain has a helical structure in this phase. The increase in temperature within the H1 region causes a conformational change in both the POE chain and the alkyl chain of the surfactant to a more disordered structure. Among the phases of V1, LR, and liquid, no significant difference is seen in the conformational structure and interaction with water molecules of the surfactant.

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 characteristic feature of this class of surfactants is that their aqueous mixtures assume various lyotropic mesophases of a liquid-crystalline nature depending on the molecular structure of the surfactants, the composition, and the temperature.1,2 The phase science of the mixture systems of these surfactants and water has so far been a problem attracting considerable attention in the fields of both fundamental colloid science and practical applications of the surfactants.3-13 * To whom correspondence should be addressed. E-mail: [email protected]. † Fukuoka University. ‡ 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. (10) Strey, R.; Schoma¨cker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1 1990, 86, 2253.

From the theoretical point of view, the following transformation sequence of the mesophases is potentially possible with increasing water content in the mixture: I2 (reversed discontinuous-type cubic) f H2 (reversed hexagonal) f V2 (reversed bicontinuous-type cubic) f LR (lamellar) f V1 (normal bicontinuous-type cubic) f H1 (normal hexagonal) f I1 (normal discontinuous-type cubic). In these mesophases, the molecular assemblies of the surfactants are different from each other, and the above transformation sequence has been interpreted in terms of order-disorder transition and shape transition.7,11-13 The former is related to the requirement for the arrangement of the surfactant aggregates in the mixture systems arising from the excluded volume effect. The latter is related to the geometrical factor of the surfactant molecules, which determines the shape of molecular assemblies formed in aqueous mixtures by affecting the packing mode of the surfactant molecules into the assemblies. In these possible mesophases, those other than I2 and H2 have been reported to date for polyoxyethylene (POE) type nonionic surfactants.11 In previous papers,14-16 we reported the phase behaviors of a series of aqueous mixtures of poly(ethylene glycol) (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) Nibu, Y.; Suemori, T.; Inoue, T. J. Colloid Interface Sci. 1997, 191, 256. (15) Nibu, Y.; Inoue, T. J. Colloid Interface Sci. 1998, 205, 231. (16) Nibu, Y.; Inoue, T. J. Colloid Interface Sci. 1998, 205, 305.

10.1021/la001231m CCC: $20.00 © 2001 American Chemical Society Published on Web 02/08/2001

1834

Langmuir, Vol. 17, No. 6, 2001

decyl ethers (C10Em, m ) 4-8), revealed mainly by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR), stressing the events occurring in the solid phase. As an extension of the previous work, the present paper deals with the phase behavior of an aqueous mixture of heptaethylene glycol dodecyl ether (C12E7) studied by DSC and FT-IR techniques. The vibrational spectra of long-chain compounds are sensitive to the changes in the conformational, packing, and dynamical features of their hydrocarbon chains associated with the phase transformation. Thus, the combination of DSC with vibrational spectroscopy provides a better understanding of the thermotropic behavior of long-chain compounds in terms of molecular properties, and these methods have been widely utilized to study the phase behavior of lipids and lipid mixtures.17-20 In the present work, the phase diagram of the C12E7/H2O mixture was constructed on the basis of DSC measurements, and FT-IR measurements were made in some detail for the C12E7/D2O mixture with a fixed composition where the transformation sequence of solid f H1 f V1 f LR f liquid was observed with the temperature rise. The aim of this work is to clarify the molecular events, that is, the change in the conformational structure of the surfactant and its interaction with water molecules, occurring in the phase transformation sequence in terms of FT-IR spectroscopy. Materials and Methods 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) was used as received. Water was purified by deionization followed by double distillation. DSC Measurements. A Seiko Denshi model SSC 5200 (Tokyo, Japan) was used for DSC measurements. The samples of C12E7/ water mixture for DSC experiments were prepared by weighing the two components directly into the sample pan made from aluminum. For mixtures with compositions below about 65 wt % of water, an appropriate amount of water, up to a maximum of about 10 mg, was added to the preweighed surfactant sample of about 5 mg to give the desired composition. For mixtures with a higher content of water, about 10 mg of water was added to a reduced amount of the surfactant. The pan containing the mixture sample was sealed and was held at 60-70 °C for 30 min in the oven of the DSC apparatus to facilitate the homogeneous mixing of the surfactant and water. Then, a cooling/heating cycle was repeated several times, mostly at the rate of 2 °C/min in the temperature range of, typically, -30 to 70 °C. A good reproducibility in the DSC thermograms was obtained by the repeated scans. In a few experiments, the heating rate was slowed to 1 °C/min. The reduced heating rate somewhat improved the resolution of multiple endothermic peaks, and the peak temperatures were essentially unaffected by the heating rates. FT-IR Measurements. A Bio-Rad model FTS 165 (Cambridge, MA) was used to record the FT-IR spectra for pure C12E7 and 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, pure C12E7 or C12E7/ D2O mixture (31.1 wt % D2O) was sandwiched between two silicone wafers of 3 cm diameter. The silicone wafers sandwiching the mixture sample were sealed by silicone grease to prevent the evaporation of D2O during the measurements. The sandwich was placed in a handmade cell holder, which was thermostated by circulating water of a constant temperature using a Neslab (17) Wegener, M.; Neubert, R.; Rettig, W.; Wartewig, S. Int. J. Pharm. 1996, 128, 203. (18) Neubert, R.; Rettig, W.; Wartewig, S.; Wegener, M.; Wienhold, A. Chem. Phys. Lipids 1997, 89, 3. (19) Wegener, M.; Neubert, R.; Rettig, W.; Wartewig, S. Chem. Phys. Lipids 1997, 89, 73. (20) Wartewig, S.; Neubert, R.; Rettig, W.; Hesse, K. Chem. Phys. Lipids 1998, 91, 145.

Inoue et al. refrigerated circulation bath RTE-211 (Portsmouth, NH). In the measurements of FT-IR spectra as a function of temperature, the sample was first 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.

Results and Discussion DSC Results for C12E7/H2O Mixture. Typical DSC thermograms obtained by heating scans for C12E7/H2O mixtures of different compositions are shown in Figure 1. The endothermic peak observed for pure C12E7 at about 25 °C is ascribed to the melting of solid C12E7. The addition of water at, say, 6.1 wt % leads to the appearance of another endotherm at about -5 °C concomitant with the lowering of the temperature of the higher temperature peak. In addition, small endotherms are superimposed between the two large endothermic peaks as indicated by asterisks and arrows in Figure 1a, which are seen more clearly in the DSC curves drawn in a more expanded scale (Figure 1b). When the water content is increased, the temperature of the highest temperature peak decreases progressively, whereas those of the lowest temperature peak and the superimposed small endothermic peaks remain essentially unaltered. With the increase in water content, the relative intensity of the lowest temperature peak increases more and more at the expense of the intensity of higher temperature peaks, and a single endothermic peak is observed when the water content in the mixture reaches about 40 wt %. The endothermic peak observed in the DSC curve means that some phase transition occurs at the corresponding temperature. Four isothermal phase changes appear in the composition range up to 40 wt % H2O (see Figure 1b). According to the phase rule, the isothermal phase change in the binary mixture system corresponds to the coexistence of three phases at that temperature. The three-phase coexisting line should touch a one-phase region at both ends. The three-phase coexisting line at the lowest temperature should touch two solid phases at its ends, and hence the appearance of three additional three-phase coexisting lines at higher temperatures predicts the existence of three kinds of mesomorphic phases in this mixture system. When the water content in the mixture is increased beyond about 40 wt %, another endothermic peak appears on the higher temperature side. This peak moves toward a higher temperature with the increase in the water content, accompanied by an increase in its relative intensity, and finally approaches an endothermic peak due to the melting of pure water, that is, ice. On the other hand, the temperature of the lower temperature peak remains almost constant, although the relative intensity becomes progressively smaller. For the mixtures containing water above 80 wt %, an additional small endotherm is appreciable as a shoulder of the higher temperature peak (black arrow in Figure 1a). In addition to the rather distinct endotherms described above, various peaks with small heat absorption were observed at higher temperatures for the mixtures in the composition range 20-60 wt % of H2O as shown in Figure 1c, in which the vertical scale is much more expanded than that in Figure 1a. The transition enthalpies for these small peaks are in the range of 0.14-0.69 kJ mol-1 of C12E7, which are much smaller compared with the heat of fusion of pure C12E7, 87.0 kJ mol-1. These endothermic peaks should be ascribed to the phase transition between mesophases or to that from a mesophase to a liquid phase. Binary Phase Diagram of C12E7/Water Mixture. The temperatures of the endothermic peaks of DSC thermo-

Phase Behavior of C12E7/H2O Mixture

Langmuir, Vol. 17, No. 6, 2001 1835

Figure 1. DSC thermograms obtained for C12E7/H2O mixtures with various compositions (a and b) and those drawn in an expanded scale for a higher temperature range (c). The compositions of the mixtures expressed in terms of wt % of H2O are indicated in the figure. The heating rate is 2 °C/min.

Figure 2. Binary phase diagram for the C12E7/H2O mixture constructed by plotting the peak temperature of the DSC curves against composition. Open and filled circles are peak temperatures in the DSC curves obtained by the heating scans at the rate of 2 and 1 °C/min, respectively.

grams define the boundaries among different phases assumed by the surfactant/water mixture. Thus, the binary T-X phase diagram of the mixture can be constructed by plotting the DSC peak temperatures against the composition. The phase diagram thus obtained for

the C12E7/H2O mixture is shown in Figure 2, where the open and filled circles correspond to the peak temperatures in the DSC curves obtained with the heating rates of 2 and 1 °C/min, respectively. The T-X phase diagram for the same mixture system is not available in the literature, as far as we know, although those for aqueous mixtures of C12Em (m ) 4, 5, 6, and 8) have been reported by Mitchell et al.7 and Clunie et al.4 Huang et al.21 have shown a phase diagram expressed by plotting m against composition at a fixed temperature of 25 °C for aqueous mixtures of a series of C12Em compounds. The mesomorphic phases in Figure 2 were identified by consulting the phase diagrams for aqueous mixtures of C12E6 and C12E8 presented by Mitchell et al.7 and also the m-X phase diagram reported by Huang et al.21 According to Figure 2, the composition ranges for the LR, V1, and H1 phases at 25 °C are approximately 16-26, 26-31, and 31-60 wt % H2O, respectively, which are in good agreement with those presented in the m-X phase diagram.21 The alternation rule to construct a phase diagram2 leads to the requirement that a two-phase region should exist between the neighboring one-phase regions. However, the phase boundaries for the two-phase region intervening between the mesophase and liquid phase or between two different mesophases could not be detected definitely by the present DSC experiments. Therefore, the phase boundaries drawn in Figure 2 for these two-phase regions should be regarded as assumed ones. Chernik and coworkers22,23 have demonstrated that these phase boundaries can be successfully detected for a dimethyldecylphosphine oxide/water system by DSC measurements employing a scanning rate slower than 0.5 °C/min. (21) Huang, K.-L.; Shigeta, K.; Kunieda, H. Prog. Colloid Polym. Sci. 1998, 110, 171. (22) Chernik, G. G. J. Colloid Interface Sci. 1991, 141, 400. (23) Chernik, G. G.; Sokolova, E. P. J. Colloid Interface Sci. 1991, 141, 409.

1836

Langmuir, Vol. 17, No. 6, 2001

Inoue et al.

Figure 3. IR spectrum obtained for the C12E7/D2O mixture with 31.1 wt % D2O at -10.0 °C. The symbols marked on several absorption bands are explained in the text.

IR Spectrum of the C12E7/D2O Mixture. According to the phase diagram presented in Figure 2, the mixture with the composition of 31 wt % H2O undergoes a series of phase transitions, solid f H1 f V1 f LR f liquid, when the temperature is increased, say, from -10 to 60 °C. Between each two successive phases, there exist coexisting regions of the two phases. The FT-IR measurements were carried out by varying the temperature along the line of this composition. Figure 3 shows the IR spectrum obtained for the mixture with 31.1 wt % D2O at -10.0 °C. We focused our attention on five absorption bands indicated in the figure in order to analyze the conformational structure of the surfactant molecule and the interaction with D2O in the respective phases. The absorption band around 2900 cm-1 is assigned to the methylene C-H stretching mode (νCH2) and is sensitive to the conformation of hydrocarbon chains.24 The IR band of the νCH2 mode shifts toward a higher wavenumber with the increase in the fraction of gaucheconformer in the alkyl chains. Thus, this band was used to monitor the conformational structure of the alkyl chain adjacent to the POE chain of the surfactant molecule. The conformational structure of the POE chain was examined in terms of the absorption bands around 1100 and 850 cm-1 which are ascribed 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.25 These absorption bands shift toward higher wavenumbers with the increase in the fraction of gauche-conformer in the POE chain.25 The absorption bands around 3300 and 2450 cm-1, which are attributed to O-H and O-D stretching modes (νOH and νOD), respectively, were used to obtain the information about the interaction associated with the terminal OH groups of POE chains in the surfactant molecules and that between the POE chain and D2O molecules. The stronger the hydrogen-bond interaction between the POE chain and D2O, the lower the frequency of νOD. Similarly, the frequency of νOH decreases when the (24) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (25) Matsuura, H.; Fukuhara, K. J. Polym. Sci., Part B 1986, 24, 1383.

hydrogen-bond interaction related to the terminal OH group of the POE chain becomes stronger. Melting Behavior of Pure C12E7 Revealed by IR Spectra. Before considering the aqueous mixture of C12E7, we discuss here the melting behavior of the surfactant revealed by IR spectral changes associated with the temperature rise. The IR spectra for pure C12E7 obtained at various temperatures by heating the sample from -5 to 45 °C are shown in Figure 4 at different frequency regions. It can be seen in the spectrum at -5.2 °C presented in Figure 4a that the absorption bands due to νOH appear at 3220 and 3320 cm-1. From this, it may be interpreted that the POE chain in the surfactant molecule has two types of structure in the solid phase, in which the strength of the hydrogen-bond interaction with the partner is different. This difference in the hydrogen-bond interaction may arise from the difference in the conformational structure of the POE chain. This is suggested by νCO + νCC + FCH2 band presented in Figure 4c, which is not a single band but is composed of two components with different frequencies; the doublet pattern of the IR band in the νCO + νCC + FCH2 mode would be attributed to a fractional difference in the trans- and gauche-conformers in the POE chain.25 It has been reported for polyoxyethylene that the absorption band due to the νCO + νCC + FCH2 mode is observed at 1107 cm-1 in the molten state, whereas the absorption band appears at 1102 cm-1 in the solid state.25 Thus, the high-wavenumber component of the 1110 cm-1 (νCO + νCC + FCH2) band observed for C12E7 would be assigned to the POE chain containing the gaucheconformer with a larger fraction relative to the POE chain responsible for the low-wavenumber component of the absorption band. It is noticeable that the absorption band at 3320 cm-1 becomes weak with the increase in temperature and disappears at about 20 °C. Concomitant with this change, the lower frequency component in the νCO + νCC + FCH2 band (black arrow in Figure 4c) reduces its intensity with the growth of the higher frequency component (white arrow in Figure 4c). This indicates that the POE chain providing the absorption band due to νOH at 3320 cm-1 has a larger fraction of trans-conformer compared with another POE chain responsible for the

Phase Behavior of C12E7/H2O Mixture

Figure 4. IR spectra obtained for pure C12E7 at various temperatures in the frequency regions of 3000-3800 cm-1 (a), 2700-3100 cm-1 (b), and 1000-1200 cm-1 (c). Temperatures in °C are indicated in the figure.

absorption band at 3220 cm-1. It has been reported that the POE chain of C12E7 has a helical structure in the solid phase assuming the trans-gauche-trans conformation about successive O-CH2-CH2-O bonds.26 The major absorption at 3220 cm-1 may be attributed to the OH group attached to the POE chain with this helical structure. The POE chain responsible for another OH absorption band at 3320 cm-1 is supposed to have a rather extended structure because of its trans-rich conformation. It is likely that the extended POE chain transforms to the helical structure which is more stable and, hence, is dominant in solid phase, when the temperature is raised to allow the POE chain to undergo a conformational rearrangement. (26) Matsuura, H.; Fukuhara, K. J. Phys. Chem. 1987, 91, 6139.

Langmuir, Vol. 17, No. 6, 2001 1837

The melting point of C12E7 determined from the peak temperature of the DSC thermogram is 25.9 °C. Corresponding to this melting temperature, the spectral structures in the frequency regions of the νCH2 and νCO + νCC + FCH2 modes change drastically on going from 24.2 to 26.0 °C, as shown in Figure 4b,c. The νCH2 bands shift toward higher wavenumbers together with the broadening of the bandwidth, indicating that the melting of the alkyl chain of the surfactant occurs cooperatively at the melting point. Similarly, the shape of the absorption bands due to νCO + νCC + FCH2 modes becomes wider when the temperature rises above the melting point. This demonstrates that the abrupt change from an ordered helical structure to a disordered structure occurs in the POE chain of the surfactant at the melting point. On the other hand, the spectrum in the region of 3000-3800 cm-1 changes rather gradually below the melting point (Figure 4a). That is, the absorption band around 3480 cm-1 begins to appear at about 20 °C and grows more and more in intensity with the increase in temperature at the expense of the absorption intensity at 3220 cm-1; the former absorption band is clearly attributed to νOH for liquid C12E7. This means that the hydrogen bond between terminal OH groups of the POE chains begins to be broken at a temperature about 5 °C below the melting temperature of the alkyl and POE chains. It can be seen in the DSC thermogram for pure C12E7 that the endothermic peak is highly asymmetrical and heat absorption begins at a temperature considerably below the melting point. This endotherm occurring below the melting point may be attributed to the breaking of the hydrogen bond between terminal OH groups of the POE chains in the surfactant molecules. IR Spectra of the C12E7/D2O Mixture in Various Phases. Typical IR spectra obtained for the C12E7/D2O mixture with the composition of 31.1 wt % D2O are shown in Figure 5a-e at various temperatures corresponding to different phases. There is a possibility that the hydrogen of the OH group in the surfactant molecule is replaced by deuterium as a result of an exchange reaction with D2O. If the deuterium exchange occurs, there exist H2O and HOD in the mixture system. The IR spectra obtained for pure H2O and the C12E7/D2O mixture at 55 °C are compared in Figure 5f. As is seen in this figure, the wavenumbers due to the νOH mode (black arrows in Figure 5f) are different from each other for pure H2O (around 3380 cm-1) and the C12E7/D2O mixture (around 3480 cm-1), and the wavenumber for the C12E7/D2O mixture is close to that obtained for liquid C12E7 (see Figure 4a). This implies that the absorption around 3500 cm-1 observed for the C12E7/D2O mixture is attributed to the OH group in the surfactant molecules instead of water molecules formed by the deuterium exchange. In addition, the absorption band around 1650 cm-1 that is characteristic to H2O (OH deformation band, δOH; white arrow in Figure 5f) does not appear in the spectrum for the C12E7/D2O mixture. These facts suggest that the deuterium exchange on the OH group of the surfactant molecule causes no serious problem, at least within the time range of the present experiments. The absorption band due to the νOH mode for the C12E7/ D2O mixture in the solid phase appears at 3310 cm-1 (Figure 5a), which is close to the high-wavenumber component of the two absorption bands observed for pure C12E7 in the solid phase (Figure 4a). This spectral feature provides the following knowledge concerning the solid phase of the aqueous mixture of C12E7. First, if the horizontal phase boundary extending over the whole composition range (see Figure 2) were regarded as an

1838

Langmuir, Vol. 17, No. 6, 2001

Inoue et al.

Figure 5. IR spectra obtained for the C12E7/D2O mixture 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) and comparison of the IR spectra obtained for the C12E7/D2O mixture and pure H2O at 55 °C (f). Temperatures and phases of the mixture under those temperatures are indicated in the figure.

eutectic line, the complete phase separation into the surfactant and water would be expected in a solid phase. When the surfactant and water exist separately as different phases, the superimposed spectra of the two components would be observed. However, the spectral pattern for the solid C12E7/D2O mixture at the νOH region is quite different from that for solid C12E7. Thus, the actual case is not phase separation. This can be also seen in the absorption band resulting from D2O (Figure 5c); in the absorption band of the solid C12E7/D2O mixture, the high-wavenumber component is much reduced compared with solid D2O. In previous papers dealing with the mixtures of poly(ethylene glycol) decyl ethers with water,14-16 we demonstrated that the molecular compounds are formed in a solid phase, in which water molecules are bound to POE chains of the surfactants with a definite stoichiometry. This may also be the case for aqueous mixtures of C12E7.

Second, the absorption band at 3310 cm-1 due to the νOH mode suggests that the POE chain of the surfactant has a rather extended structure instead of a helical structure in the solid C12E7/D2O mixture, because the frequency is close to that of the absorption band characteristic to the OH group attached to the extended POE chain of pure C12E7 in the solid phase, as mentioned above. This is supported by the spectral patterns in the frequency regions of the νCO + νCC + FCH2 (∼1110 cm-1) and νCO + FCH2 (∼850 cm-1) modes (parts d and e of Figure 5, respectively), where the low-wavenumber component is dominant for the solid C12E7/D2O mixture, indicating that the POE chain has a trans-rich conformational structure.25 The fact that an extended structure is favorable rather than a helical structure for the POE chain of the surfactant in the solid C12E7/D2O mixture may be attributed to the effect of D2O bound to the POE chain. The binding of D2O to the POE

Phase Behavior of C12E7/H2O Mixture

Langmuir, Vol. 17, No. 6, 2001 1839

Figure 6. Schematic representation of the events occurring with the temperature rise for pure C12E7 from solid to melt (upper) and for the C12E7/H2O mixture from the solid to the H1 phase (lower). Black and shaded bold lines represent alkyl and POE chains of the surfactant, respectively. Straight lines represent the extended structure of the chains, and zigzag lines represent the helical structure of the POE chain. Irregularly bent lines are disordered structure of the chains. Small circles represent water molecules. Hydrogen bonds formed between the surfactants are indicated by dotted lines. The upper right panel shows the speculated model for the hydrogen bonds formed between POE chains of the surfactant molecules.

chain through hydrogen-bond interaction is demonstrated by comparing the absorption spectra due to the νOD mode between the C12E7/D2O mixture and pure D2O (Figure 5c). The absorption band for the mixture is relatively rich in low-wavenumber components compared with that for pure D2O. This means that D2O forms stronger hydrogen bonds in the mixture than in bulk D2O. Thus, it is suggested that D2O molecules in the solid C12E7/D2O mixture are bound to POE chains of the surfactant because of the hydrogen-bond interaction with the oxygen atom in the POE chain. It is likely that it is difficult for the POE chain with bound D2O molecules to take a helical structure because of some steric restriction, and hence it is forced to have an extended structure. The IR spectral pattern observed for the C12E7/D2O mixture below -5 °C was the same as that obtained at -10 °C, being consistent with the phase diagram (Figure 2), whereas it changed remarkably when the temperature was raised beyond -5 °C as shown in Figure 5. In the first place, we discuss here the spectrum obtained at 6.0 °C where the mixture is in the H1 phase. It can be seen in the frequency region ascribed to the νOH mode that the absorption around 3440 cm-1 appears and its intensity exceeds that of the absorption at 3310 cm-1 (Figure 5a). This indicates that the hydrogen bonds between OH groups of the surfactant molecules are broken at this temperature, although they remain partly. The POE chain of the surfactant is supposed to take a helixlike structure, because the absorption bands in the regions of ∼1110 and ∼850 cm-1 exhibit the feature that the high-wavenumber components are dominant (Figure 5d,e), which in turn demonstrates the enhanced fraction of gauche-conformer in the POE chain.25 As for the absorption band arising from D2O (Figure 5c), the absorption around 2500 cm-1 emerges in addition to that around 2420 cm-1. This indicates the appearance of weakly hydrogen-bonded D2O, which suggests that partial dehydration of the POE chain

of the surfactant takes place at this temperature. The partial dehydration of the POE chain may facilitate the chain to take a helical structure owing to the release from a steric restriction forced on the POE chain by the bound D2O molecules. Contrary to the POE chain, the alkyl chain is regarded to have a trans-zigzag structure, because the frequencies of the absorption ascribed to νCH2 modes are almost the same as those of the solid C12E7/D2O mixture (Figure 5b). The above spectral features observed for the C12E7/D2O mixture at 6.0 °C remained essentially unchanged until the temperature was raised to about 14 °C (see Figure 5). When the temperature was raised from 14 to 16 °C, the spectral change occurred again even though the mixture was in the H1 phase at both temperatures according to the phase diagram (Figure 2). As shown in Figure 5, the most pronounced change in the spectral pattern is seen in the frequency regions corresponding to the absorption due to the νCH2, νCO + νCC + FCH2, and νCO + FCH2 modes (parts b, d, and e of Figure 5, respectively). The absorption band due to the νCH2 mode shifts toward a higher wavenumber together with the broadening of the bandwidth. This indicates that the alkyl chain of the surfactant undergoes a transformation from an ordered trans-zigzag structure to a disordered structure containing the gaucheconformer or, in other words, the melting of the alkyl chain occurs at about 15 °C. Similarly, the spectral patterns in the νCO + νCC + FCH2 and νCO + FCH2 mode regions become wider when the temperature is raised across this temperature, which suggests that the POE chain also transforms from an ordered helixlike structure to a disordered structure at this temperature. Spectral changes are also seen in the νOH and νOD regions, although not so pronounced as in other frequency regions. In the absorption bands due to the νOH mode, the component at 3310 cm-1 disappears completely when the temperature rises from 14 to 16 °C (Figure 5a). It may be regarded from this

1840

Langmuir, Vol. 17, No. 6, 2001

spectral change that the hydrogen bond formed between terminal OH groups of the surfactant molecules is completely broken at about 15 °C. In addition, the absorption band around 3440 cm-1, which is probably due to OH groups of the POE chain interacting with D2O through hydrogen bonding, shifts slightly toward a higher wavenumber. It seems that the interaction of the OH group with D2O is weakened somewhat discretely at this temperature. The change in the absorption band of D2O at this temperature is small, although the relative decrease in the intensity at the 2420 cm-1 band is appreciable, indicating that the hydrogen-bond interaction between the POE chain of the surfactant and D2O is weakened. It is quite interesting that the structural change occurs in the alkyl chain and the POE chain of the surfactant within the same phase, the H1 phase, of the C12E7/D2O mixture. The IR spectra obtained for the C12E7/D2O mixture of the H1 phase above 16 °C, the V1 phase, the LR phase, and the liquid phase are similar to each other, particularly in the frequency regions corresponding to the absorption bands ascribed to the νCH2, νCO + νCC + FCH2, and νCO + FCH2 modes. Thus, it can be regarded that there is no essential difference in the conformational structure of the surfactant molecule among these phases. As for the absorption band due to the νOH and νOD modes, slight shifts toward higher wavenumbers are appreciable with the increase in temperature. This suggests that the hydrogen-bond interaction between the POE chain of the surfactant and D2O becomes somewhat weak on going from the lower temperature phase to the higher temperature phase. Conclusions In the present work, the binary phase diagram of the C12E7/H2O mixture was constructed on the basis of DSC measurements, and IR spectra were measured for pure C12E7 and the C12E7/D2O mixture with 31.1 wt % D2O as a function of temperature. The spectroscopic study revealed the molecular events associated with the phase changes of the surfactant and the surfactant/water mixture, as shown schematically in Figure 6. As for pure C12E7, the surfactant molecules have extended alkyl chains with a trans-zigzag conformation in the solid phase. The POE chains of the surfactant at low temperatures in the solid phase are mostly helical,

Inoue et al.

although there exists a small amount of POE chains with an extended structure. These extended POE chains undergo a transformation to a helical structure with the increase in temperature and disappear at about 15 °C. The hydrogen bonds formed in the solid phase between the surfactant molecules, the probable fashion of which is depicted in Figure 6, begin to be broken at the temperature below the melting point by 5 °C. This premelting event might be characteristic to the POE-type nonionic surfactants. At the melting point, a cooperative transformation to a disordered structure occurs in both the alkyl chain and the POE chain. In the solid phase of the C12E7/H2O mixture, both the alkyl chain and the POE chain assume an extended structure with a trans-zigzag conformation. Water molecules are bound to the POE chain, and hydrogen bonds are formed between POE chains of the surfactant molecules. When the solid phase changes to the H1 phase, the surfactant molecules undergo a structural transformation in the POE chains to a helical structure keeping the alkyl chains unaltered. The hydrogen bonds between surfactant molecules are mostly broken, and water molecules bound to the POE chains are partly released. Within the H1 phase, an additional event occurs in the surfactant molecules at about 15 °C. That is, the alkyl chain and the POE chain transform to a disordered structure from an ordered transzigzag structure and a helical structure, respectively, and the hydrogen bonds between the surfactant molecules are completely broken. Among the phases of H1 above 15 °C, V1, LR, and liquid, no significant difference is appreciable in the spectral features and, hence, in the conformational structure of the surfactant molecules. It is of interest that the “melting” of the alkyl chain and the POE chain is induced by the temperature rise within the mesomorphic H1 phase rather than in the course of a solid-to-mesophase transition. A study on the dependence of this phenomenon on the composition of the C12E7/H2O mixture is now in progress. Acknowledgment. This work was supported in part by funds (No. 965012) from the Central Research Institute of Fukuoka University. LA001231M