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.

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 Lα. 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.