Thermodynamic Study on Lyotropic Liquid Crystal: Octaoxyethylene-n-hexadecyl ether/Water Binary System

Abstract

Heat capacities of the lyotropic liquid crystal formed in the binary system, non-ionic surfactant octaoxyethylene-n-hexadecyl ether (C_<16>E_8) / water, were measured with an adiabatic calorimeter between 290 and 370 K. Samples were prepared in different concentrations of the C_<16>E_8 surfactant in water. In the liquid crystal region, the hexagonal (H_I) - cubic (V_I) and cubic (V_I) - lamellar (L_α) phase transitions were observed, and the phase transition temperatures depended on the concentration. The hexagonal - cubic and cubic - lamellar phase transitions are of first-order, as shown by the existence of biphasic regions. Entropy and enthalpy changes at each phase transition were determined.