X-ray diffraction and deuterium N. M. R. of the lyomesophases of dipotassium hexadecanedioate–water mixtures

Abstract

Mixtures of water and the dipotassium thapsate form hexagonal and lamellar lyomesophases with structures similar to those found for monosoaps. X-ray diffraction from a high intensity synchrotron radiation source exhibits the first four reflections of the hexagonal phase and the first two from the lamellar phase yielding lattice parameters of about 28 Å and 22 Å, respectively. Within the stability range of the mesophases these parameters do not vary significantly with the disoap concentration and the temperature. An intermediate phase, found previously at about 120°C between the hexagonal and the lamellar phase was shown to be an isotropic liquid rather than a cubic mesophase. Combined X-ray diffraction and N. M. R. experiments on magnetically aligned samples showed that the lamellar and hexagonal phases have respectively negative and positive anisotropic magnetic susceptibilities. Samples which are allowed to equilibrate in the isotropic-mesophase boundary region for several hours, in the absence of a magnetic field, undergo surface-induced alignment. The lamellar phase aligns homeotropically while the hexagonal phase aligns homogeneously, but with the directors perpendicular to the capillary axis and with the hexagonal lattice vectors inclined at a constant orientation to the surface. Deuterium N. M. R. measurements on perdeuteriated disoap samples indicate that the amphiphile molecules in both mesophases are highly disordered, partly due to isomerization equilibria and partly due to orientational disorder. Based on the X-ray and N. M. R. results molecular models for both the lamellar and hexagonal phases are proposed. These models resemble very closely the corresponding mesophases of the monosoaps.