Thermodynamic Study of Phase Transitions in Lyotropic Systems: Adiabatic Calorimetry on Non-Ionic Surfactant C16E8-Water System

Kazuya SAITO, Noriaki KITAMURA and Michio SORAI

Heat capacities of the binary systems consisting of a non-ionic surfactant C16E8 and water were precisely measured as a function of temperature by adiabatic calorimetry over the temperature and the concentration ranges where lyotropic liquid crystals are formed.  The enthalpy and entropy of transitions were determined for all known phase transitions observed.  Comparison of the present result and the previous one on the C12E6–water system suggests that the higher-order structure in the liquid crystalline phases in these systems be mainly constructed by surfactant molecules with a fixed amount of water.  The excess heat capacities, as estimated by measuring the heat capacity of neat C16E8, are positive over the whole temperature and concentration ranges.  Properties of lyotropic and thermotropic systems are compared briefly, while paying attention on the geometries of surfaces characterizing the aggregation (triply periodic minimal surface for cubic phases and flat surfaces for lamellar and smectic phases).
(J. Phys. Chem. B, 107, 7854-7860 (2003))


Thermodynamic Study of Phase Transitions in Lyotropic Liquid Crystals: Adiabatic Calorimetry on Non-Ionic Surfactant C12E6-Water System

Makoto NISHIZAWA, Kazuya SAITO and Michio SORAI

The heat capacities of the binary system consisting of a non-ionic surfactant C12E6 and water were precisely measured as a function of temperature by adiabatic calorimetry over the temperature and the concentration ranges where lyotropic liquid crystals are formed. The enthalpy and entropy of transitions were determined for all transitions observed. The enthalpy and entropy of transition between liquid crystalline phases suggest that the higher order structure in the liquid crystalline phases in this system is mainly constructed by C12E6 molecules with a fixed amount of water. The excess heat capacities, as estimated by measuring the heat capacity of neat C12E6, is positive over the whole temperature and concentration ranges. The excess heat capacities also support the suggestion given above concerning the role of C12E6 molecules on the structure building.
(J. Phys. Chem. B, 105, 2987-2992 (2001))


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