THERMODYNAMICAL INVESTIGATIONS ON PHASE-TRANSITIONS OF SURFACTANT WATER-SYSTEMS - THERMODYNAMIC STABILITY OF GEL AND COAGEL PHASES AND THE ROLE OF WATER-MOLECULES IN THEIR APPEARANCE

Phase behaviors of coagel, gel, liquid crystal and micellar solutions of binary systems of water and a homologous series of long hydrocarbon-chain trimethylammonium halides, covering almost all concentrations up to 99.9g% water, were systematically investigated by using differential scanning calorimetry and also a home-made, high precision adiabatic calorimeter. The main findings obtained are summarized as follows: (i) T(c) transition curves decrease rapidly in a stepwise way with increasing water content, particularly in the low water content region; (ii) the gel phases of three binary systems of octadecyltrimethyl-ammonium halides with different counter-ions, i.e., chloride, bromide and iodide, exist in stable, metastable and unstable states, respectively; (iii) in accordance with this difference, the mode of phase transition also differs among these three systems; (iv) in connection with (iii), the thermodynamically stable gel phase is realized only by the existence of a critical amount of water, and the structural water, i.e., intermediate water, is observed in this gel phase; and (v) the critical phenomenon of the transition from the coagel to micellar solution appears in the so-called Krafft point region. These findings will be discussed from the thermodynamic viewpoint on the basis of the energy cycle associated with pseudo-lattice and hydration enthalpies and also from the Gibbs energy versus temperature relationship.