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C NMR Study of Mesomorphic Phases Formed in Aqueous Mixtures of Heptaethylene Glycol Dodecyl Ether LiQiang Zheng,† Masao Suzuki,‡ and Tohru Inoue*,‡ The Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong University, Jinan 250100, China, and Department of Chemistry, Faculty of Science, Fukuoka University, Nanakuma, Jonan-ku, Fukuoka 814-0180, Japan Received June 5, 2001. In Final Form: August 23, 2001 13C NMR was applied to investigate the various phases formed in aqueous mixtures of heptaethylene glycol dodecyl ether (C12E7). On the basis of the analysis of NMR line width, a qualitative picture was drawn concerning the molecular motion of C12E7 in some mesophases as follows. In the H1 and LR phases, the motion of the alkyl chain of the surfactant molecule is rather restricted, whereas the polyoxyethylene (POE) chain can undergo segmental motion to some extent. The molecular motion of the surfactants in the V1 phase, which appears between the H1 and LR phases, is quite rapid and is close to that in the liquid phase. The V1 phase is known to show the highest bulk viscosity in the mesophases due to the network structure constituted by the surfactant bilayer. The rapid motion of C12E7 in the V1 phase suggests that the surfactant molecules are packed rather loosely in the surfactant bilayer. The signal of carbon atoms in the POE chain was shifted upfield by the addition of water in a manner that the extent increased with the increase in water content in the mixture, which reflects the hydration of the POE chain. In addition, the signal of the POE carbons was shifted downfield rather continuously by the temperature rise, suggesting that the dehydration of the POE chain proceeds with the increase in temperature.
Introduction Heptaethylene glycol dodecyl ether (C12E7) belongs to a class of polyoxyethylene (POE)-type nonionic surfactants. These surfactants are widely used in the fields of detergents, cosmetics, and many other industrial applications. It is well-known that aqueous mixtures of POEtype nonionic surfactants assume various lyotropic mesophases of liquid-crystalline nature depending on the molecular structure of the surfactants, composition, and temperature.1,2 The highly viscous mesomorphic phases are unfavorable to treat the surfactant preparations in the industrial production process. Thus, it is of critical importance to avoid the occurrence of the mesomorphic phases in the manufacture of the surfactant preparations. From the viewpoints of both academic interests and practical requirements, the phase science of aqueous mixtures of this class of surfactants has so far been extensively investigated, and the structural aspects of the molecular assemblies formed in various phases have been rather well elucidated.3-20 Besides the structural aspects, * To whom correspondence should be addressed. E-mail:
[email protected]. † Shandong University. ‡ Fukuoka 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; Chapter 5. (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. (11) Kratzat, K.; Schmidt, C.; Finkelmann, H. J. Colloid Interface Sci. 1994, 163, 190.
the elucidation of the dynamics of the molecular assemblies in different phases should be useful for full understanding of the phase behavior of surfactant/water mixtures. Nevertheless, it seems that the dynamic aspects such as the molecular motion of the surfactants in various phases have not been well revealed, except for the case of the micellar phase. In a previous study,21 we applied an electron spin resonance (ESR) technique to estimate the microviscosity of various phases formed in an aqueous mixture of C12E7. This technique allows us to get information regarding the dynamic aspects of the surfactant molecular assemblies through a molecular motion of a spin probe. More direct information about the molecular motion of the surfactant molecule itself can be obtained by a nuclear magnetic resonance (NMR) technique. NMR spectroscopy has been widely applied to study surfactant/ water mixtures including lyotropic mesophases22-28 as well (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.; Kawamura, H.; Matsuda, M.; Misono, Y.; Suzuki, M. Langmuir 2001, 17, 6915. (22) Beyer, K. J. Colloid Interface Sci. 1982, 86, 73. (23) Blackmore, E. S.; Tiddy, G. J. T. Liq. Cryst. 1990, 8, 131. (24) Henriksson, U.; Jonstro¨mer, M.; Olsson, U.; So¨derman, O.; Klose, G. J. Phys. Chem. 1991, 95, 3815. (25) So¨derman, O.; Olsson, U. Curr. Opin. Colloid Interface Sci. 1997, 2, 131. (26) Pampel, A.; Strandberg, E.; Lindblom, G.; Volke, F. Chem. Phys. Lett. 1998, 287, 468. (27) Geil, B.; Feiweier, T.; Pospiech, E.-M.; Eisenbla¨tter, J.; Fujara, F.; Winter, R. Chem. Phys. Lipids 2000, 106, 115.
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as surfactant micellar systems.29,30 In the present work, we investigated the phase behavior of the C12E7/H2O mixture stressing the molecular motion of the surfactant molecules by applying 13C NMR spectroscopy. It is well-known that the line width of the NMR signal depends strongly on the molecular motion; when the molecular motion becomes rapid, the line width becomes narrow (motional narrowing).31 Although more quantitative information can be obtained by NMR relaxation studies, the analysis based on the NMR line width would be sufficient for qualitative discussion concerning the difference in molecular motion of surfactants among the different phases. Thus, in the present study, we focused our attention on the change in spectral line width associated with the phase transformation occurring in the C12E7/H2O mixture system. In addition to the dynamic aspects of the surfactant molecular assemblies, we discuss the effect of water on the chemical shifts of carbon atoms in the surfactant molecules, which reflects the interaction between water and surfactant molecules. Experimental Section The sample of C12E7 with a homogeneous chain length distribution was obtained from Nikko Chemicals (Tokyo, Japan) and used without further purification. Water was purified by deionization followed by double distillation. 13C NMR spectra were obtained from samples in standard 5 mm i.d. NMR tubes. NMR measurements were performed with a JEOL GSX-400 spectrometer at 100 MHz under the following conditions: 90° pulse of 9.5 µs; pulse delay, 1.8 s; scan number, 40; sampling points, 8192. Temperature was controlled by the standard JEOL gas flow system. The spectra were recorded about 10 min later after the desired temperature was attained. Dioxane was used as an external reference of chemical shift, and the shift was converted to the tetramethylsilane (TMS) scale by δTMS ) δdioxane + 67.4 ppm.
Results and Discussion The binary phase diagram of the C12E7/H2O mixture determined by differential scanning calorimetry (DSC) has been reported previously,32 which 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 13C NMR spectra were recorded for pure C12E7 and for C12E7/H2O mixtures with four different compositions as a function of temperature. 13 C NMR Spectra Obtained for Pure C12E7 and the C12E7/H2O Mixture in the Liquid Phase. Figure 2 shows the 13C NMR spectra obtained for pure C12E7 and its aqueous mixture with the composition of 31.5 wt % H2O both in the liquid phase. There appear many peaks corresponding to the position of carbon atoms in the surfactant molecule, the assignments of which were made as shown in the figure by referring the results reported for octaethylene glycol dodecyl ether (C12E8).33 As is seen in Figure 2, the chemical shifts for alkyl carbons and POE carbons fall in different regions, which allows us to monitor the behavior of the alkyl chain and (28) Chupin, V.; Boots, J.-W. P.; Killian, J. A.; Demel, R. A.; Kruijff, B. Chem. Phys. Lipids 2001, 109, 15. (29) Ribeiro, A. A.; Dennis, E. A. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; p 971. (30) Lindman, B.; Olsson, U.; So¨derman, O. In Dynamics of Solutions and Fluid Mixtures by NMR; Delpuech, J. J., Ed.; John Wiley & Sons: New York, 1995; p 345. (31) Farrar, T. C.; Becker, E. D. Pulse and Fourier Transform NMR; Academic Press: New York, 1971. (32) Inoue, T.; Matsuda, M.; Nibu, Y.; Misono, Y.; Suzuki, M. Langmuir 2001, 17, 1833. (33) Ribeiro, A. A.; Dennis, E. A. J. Phys. Chem. 1987, 81, 957.
Figure 1. T-X phase diagram of the C12E7/H2O mixture obtained from DSC measurements.
the POE chain separately. By comparison of the two spectra in this figure, it can be seen that the chemical shifts for alkyl carbons are not essentially changed by the addition of water, whereas those for POE carbons suffer a somewhat upfield shift. This suggests that water molecules are mostly existing around the surfactant POE chains and interacting with them in a liquid phase. This is quite reasonable considering the hydrophilic nature of the POE chain and the hydrophobic nature of the alkyl chain. 13C NMR Spectral Change Associated with the Melting of Pure C12E7. Before going to mixture systems, it may be suggestive to discuss the melting behavior of pure C12E7 revealed by 13C NMR spectra. 13C NMR spectra observed for pure C12E7 at various temperatures are depicted in Figure 3, where the two chemical shift regions are shown separately. At temperatures well below the melting point (25.8 °C), no NMR signal is observed, which indicates that the molecular motion of the surfactant is highly restricted in a solid phase. When temperature increases up to about 23 °C, there appear broad peaks around 71 and 30 ppm, demonstrating that both the POE chain and the alkyl chain of the surfactant molecule begin to move at this temperature. This is well correlated to the previous observation by IR spectroscopy,32 in which it was shown that the hydrogen bonds between terminal OH groups of the surfactant molecules begin to be broken at a temperature below the melting point by about 5 °C. It is likely that the POE and alkyl chains of the surfactant molecules become able to undergo segmental motion to some extent even in the solid phase as a result of the release from the restriction due to the hydrogen bonding. Similar events are also seen in the results of an ESR spinlabel study,21 according to which the ESR spectra of the spin probe incorporated in pure C12E7 become close to that expected for the isotropic motion of the probe molecule at a temperature below the melting point by about 5 °C.
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Figure 2. 13C NMR spectra obtained for the liquid phase of pure C12E7 and the C12E7/H2O mixture with 31.5 wt % H2O. The assignment of the NMR signals is indicated in the figure.
Figure 3. Variation of the
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C NMR spectra for pure C12E7 with temperature. Temperatures in °C are indicated in the figure.
The broad NMR signal observed in the solid phase becomes bigger with the increase in temperature, reflecting the increase in the segmental mobility of the chains with temperature. At 27.8 °C, which is above the melting point, sharp NMR signals are observed corresponding to a rapid motion of the surfactant molecules in the liquid phase. With the increase in temperature in the liquid phase, the NMR signal becomes sharper, indicating that the molecular motion becomes more and more rapid with the increase in temperature. 13 C NMR Spectral Change Associated with the Phase Transformation of the C12E7/H2O Mixture. As can be seen in Figure 1, the mixture with the composition of about 30 wt % H2O undergoes a series of phase transformations, solid f H1 phase f V1 phase f LR phase f liquid, with the increase in temperature, say, from -10
to 60 °C. Thus, the 13C NMR spectra for the C12E7/H2O mixture with this composition were measured as a function of temperature. The spectra obtained for each phase are compared in Figure 4. In the solid phase, no NMR signal is observed, indicating that the molecular motion of the surfactant is highly restricted in the solid phase of the mixture just as in the case of the pure C12E7 solid. When the mixture transforms to the H1 phase, broad NMR signals appear in both the POE chain and alkyl chain regions, in which the signals due to alkyl carbons are much weaker than those due to POE carbons. This demonstrates that the POE chain of the surfactant molecule can undergo segmental motion to some extent in the H1 phase. In particular, it can be seen that the motion of the two carbons adjacent to the terminal OH group is relatively rapid,
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Figure 4. 13C NMR spectra obtained for various phases of the C12E7/H2O mixture with 31.5 wt % H2O: (a) POE carbons and (b) alkyl carbons. Temperatures in °C and phases of the mixture under those temperatures are shown near each spectrum.
because the peaks due to these carbons (peaks j and k in Figure 2) are fairly sharp. On the other hand, the segmental motion of the alkyl chain is rather restricted. The signal corresponding to peak g in Figure 2 is not observed in the spectra of the H1 phase. This suggests that the motion of the carbon atom just adjacent to the POE chain is most restricted in the carbons of the surfactant molecule. The spectrum observed for the V1 phase exhibits quite sharp peaks and is very close to that observed for the liquid phase. Thus, it can be seen that the surfactant molecules in the V1 phase undergo rapid motion with respect to both the POE chain and the alkyl chain. When the V1 phase is transformed to the LR phase by the temperature rise, the spectra begin to show rather broad peaks again, which is close to the spectra observed for the H1 phase. The peak around 71 ppm is somewhat resolved, and this demonstrates that the motion of the POE chain in the LR phase is faster than that in the H1 phase. The signals in the alkyl chain region for the LR phase are somewhat clearer than those for the H1 phase, but they are still much smaller and broader compared with those in the POE chain region. In addition, no signal corresponding to peaks g and f in Figure 2 appears also in the LR phase. Thus, it can be regarded that the motion of the surfactant alkyl chain, especially the portion adjacent to the POE chain, is rather restricted in the LR phase just as in the H1 phase. The spectrum obtained for the liquid phase exhibits quite sharp signals in both the POE and alkyl chain regions, reflecting a rapid motion of these chains in the liquid phase. The above aspects for the dynamic property of surfactant molecules in different phases of the C12E7/H2O mixture revealed by 13C NMR measurements correspond well to those obtained by the ESR spin-label study.21 In the ESR study, it was found that the ESR spectra observed for the H1 phase and the LR phase exhibit the spectral pattern characteristic of highly anisotropic motion of probe molecules, while the spectra for the V1 phase is close to those expected for isotropic molecular motion. Thus, the molecular motion of the spin probe embedded in different phases of the C12E7/H2O mixture is well correlated to the motion of the surfactant molecule itself.
The most interesting feature in the results of the present study is that the surfactant molecules in the V1 phase undergo such a rapid motion as comparable to the motion in the liquid phase. The mesophases formed in aqueous surfactant mixtures are highly viscous.34 Among the three mesophases appearing in the present mixture system, the bulk viscosity increases in the order LR < H1 < V1;34 that is, the V1 phase has the highest bulk viscosity. Nevertheless, the motion of surfactant molecules in this phase is much more rapid than that in the H1 and LR phases. The high bulk viscosity may be attributed to a network structure formed throughout the whole system by surfactant bilayers in a bicontinuous manner. The present NMR results suggest that the surfactant molecules themselves are rather loosely packed in the bilayers constituting the network structure and probably undergo somewhat free tumbling motion just like in the liquid phase. As described above, the signal corresponding to peak g in Figure 2 disappears completely in the spectra of the H1 phase and the LR phase. This means that the motion of the carbon atom just adjacent to the POE chain is almost completely restricted. This greatest immobilization of the carbons constituting the hydrophobic/hydrophilic interface in the molecular assemblies of surfactant/ water mixtures has been observed for a micellar system33 and some mesomorphic phases.22 However, this is not the case for the V1 phase probably due to the loose packing of the surfactant molecules in the surfactant bilayer formed in this phase. Composition Dependence at Constant Temperature and Temperature Dependence at Fixed Composition of 13C NMR 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 13 C NMR spectra of the C12E7/H2O mixture were examined. The spectra for the mixtures with various compositions obtained at 34.8 °C are compared in Figure 5. The phase diagram presented in Figure 1 predicts that the mixtures take the LR phase (19.9 wt % H2O), the V1 phase (31.5 wt (34) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994; Chapter 8.
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Figure 5. 13C NMR spectra obtained for C12E7/H2O mixtures with different compositions at 34.8 °C: (a) POE carbons and (b) alkyl carbons. The compositions in wt % of H2O are shown near each spectrum.
Figure 6. 13C NMR spectra obtained for the C12E7/H2O mixture with 40.4 wt % H2O at various temperatures: (a) POE carbons and (b) alkyl carbons. Temperatures in °C are shown near each spectrum.
% H2O), and the H1 phase (40.4 and 49.9 wt % H2O) at this temperature. The spectra shown in Figure 5 exhibit the characteristic patterns of the respective phases and coincide well with the phase diagram. By comparison of the spectra for 40.4% and 49.9% H2O, it can be seen that the main peak of the POE carbons is more resolved for the mixture with 49.9% H2O than in the case of 40.4% H2O, although no essential difference is seen in the signals of the alkyl carbons. This demonstrates that the segmental motion of the POE chain becomes fast with the increase in water content in the mixture even within the same phase. This effect of water enhancing the segmental motion of the surfactant POE chain may be attributed to the release from the crowdedness of the POE chains on or between the surfactant aggregate entities, which would result from the increase in the separation distance between
them caused by the increase in the amount of intervening water molecules. 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 13C NMR spectra were measured for the mixture with 40.4 wt % H2O varying the temperature over a wide range in order to examine the temperature effect on the surfactant molecular motion in the H1 phase. The results are shown in Figure 6. With the increase in temperature, the main peak of the POE carbons is gradually resolved and the signals of the alkyl carbons become somewhat bigger, although the spectral pattern remains characteristic of the H1 phase. Thus, as expected, the molecular motion is somewhat enhanced in both the POE chain and the alkyl chain by the temperature rise.
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Figure 7. Plot of chemical shifts against temperature for peaks j (a), g (b), and b (c). The compositions in wt % H2O are 0 (square), 19.9 (diamond), 31.5 (circle), 40.4 (reversed triangle), and 49.9 (triangle). The phases are liquid (open), LR (right half-filled), V1 (left half-filled), and H1 (filled).
More definite enhancement of the segmental motion in the POE chain compared with that in the alkyl chain may be caused in part by the progress of dehydration in the POE chain associated with the temperature rise (see below). Chemical Shift Analysis. Temperature dependence of chemical shifts was examined for several carbons representing sharp singlet peaks. Typical examples are shown in Figure 7, where the chemical shifts of peaks j, g, and b obtained for the C12E7/H2O mixture with different compositions are plotted against temperature. As can be seen in Figure 7a, the chemical shift of carbon in the POE chain is remarkably shifted upfield by the addition of water, and the upfield shift is enhanced by the increase in the water content. This observation suggests that the upfield shift of the POE carbon resulting from the water addition may be caused by the hydration of the POE chain. The chemical shift for pure C12E7 undergoes a slight downfield shift with the increase in temperature, whereas the downfield shift caused by the temperature rise is much more significant for the mixture with water. This suggests that the dehydration of the POE chain occurs progressively with the increase in temperature. In addition, the variation of the chemical shift caused by temperature rise is rather continuous; that is, no discrete change is appreciable at the temperature corresponding to the phase transformations appearing in this mixture system. Thus, it may be concluded that the degree of hydration of the POE chain is determined by temperature rather independently of the type of molecular assemblies formed in the C12E7/H2O mixture. The behavior of the chemical shift of another POE carbon (peak k) was essentially the same as that of peak j. Contrary to the case of POE carbons, the effect of water on the chemical shifts of the carbon atom connecting the POE chain and the alkyl chain (peak g) and also of the alkyl carbons (peak b) is quite small. This means that water molecules are essentially excluded from the hydrophobic region constituted by the surfactant alkyl chains in the C12E7/H2O mixture.
Conclusion In the present work, 13C NMR spectra were measured for the C12E7/H2O mixture in order to investigate the dynamic feature of the surfactant molecule in various phases formed in the mixture. The analysis based on NMR line width provided a qualitative picture concerning the molecular motion of the surfactant in various phases, which is summarized as follows. In the H1 and LR phases, the POE chain of the surfactant molecule can undergo segmental motion to some extent; especially, the motion of the two carbons adjacent to the terminal OH group is rather rapid. On the other hand, the motion of the alkyl chain is rather restricted; in particular, the carbon adjacent to the POE chain is highly immobilized. The segmental motion of the surfactant molecules in these two phases depends somewhat on the water content in the mixture and on temperature. The molecular motion of the surfactants in the V1 phase is quite rapid and is close to that in the liquid phase. It is known that the V1 phase exhibits the highest bulk viscosity in the three mesophases. The high bulk viscosity of the V1 phase may be attributed to the network structure constituted by the surfactant bilayer in this phase. The rapid motion of C12E7 in the V1 phase suggests that the surfactant molecules are packed rather loosely in the surfactant bilayer. It is of interest that the molecular motion of the surfactant itself is quite rapid in the V1 phase although the bulk viscosity of this phase is quite high. Acknowledgment. We are grateful to Professor Takeo Yamaguchi of Fukuoka University for his kind cooperation and useful discussion for the present NMR work. This work was supported in part by funds (No. 965012) from the Central Research Institute of Fukuoka University. LA010836V
