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

Abstract

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 99.9 g% 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 ofoctadecyltrimethylammonium 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.