FT-IR and ESR spin-label studies of mesomorphic phases formed in aqueous mixtures of heptaethylene glycol dodecyl ether

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 V-1 phase reported by a spin probe was the lowest among the three mesophases assumed by this mixture system, although the bulk viscosity of the V-1 phase is higher than those of the other two phases, H-1 and L-alpha. This suggests that the surfactant molecules are packed rather loosely in the surfactant bilayer constituting the bicontinuous network structure in the V-1 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.