Deuterium and potassium-39 N.M.R. and optical microscopy of the dipotassium hexadecanedioate-water mesophases

Abstract

Mixtures of the dipotassium salt of thapsic acid (dipotassium hexadecanedioate, KOOC(CH₂)₁₄COOK) and water exhibit lyomesophases at disoap concentrations between 40 and 65 wt % and temperatures above c. 50°C. The phase diagram of this system has been studied by deuterium and potassium-39 N.M.R., and by optical microscopy. Mixtures of the disoap and water were studied as a function of concentration and temperature, in ordinary bulk samples, and in rectangular cells packed with parallel glass plates to allow measurements on oriented samples. The main features of this system are: between c. 40 and 55 wt % disoap, on cooling from the isotropic liquid, a single uniaxial phase appears with δ_χ > 0, while cooling solutions with disoap concentrations between 55 and 65 wt %, yield two phases separated by a first order transition; a high temperature phase (HTP) with δ_χ lt; 0, and a low temperature phase (LTP) with δ_χ < 0. The latter phase is identified with that observed at lower disoap concentrations. On heating the more concentrated solutions (> 55 wt %) an additional phase appears at the boundary between the LTP and HTP which is optically isotropic and exhibits single deuterium and potassium-39 N.M.R. peaks. It is suggested that the HTP is lamellar and corresponds to the phase studied earlier by Gallot. The LTP appears to have a two dimensional structure similar to the middle phase of monosoaps, while the intermediate isotropic mesophase is most likely cubic. The N.M.R. splitting pattern of the aliphatic deuterons is quite different to that found in monosoaps, and is discussed in terms of possible conformations of the disoap molecules within the mesophases.