Heat Capacity Associated with Phase Transitions in Micellar Cesium Perfluoro-Octanoate Solutions

Abstract

We have carried out a study of the ordering of cesium perfluoro-octanoate (CsPFO) micelles in water using high-resolution ac calorimetric techniques and differential scanning calorimetry (DSC). Aqueous solutions of CsPFO undergo two order-disorder phase transitions: a second-order nematic-smectic A (or lamellar) transition and a very weakly first-order nematic-isotropic transition. Small excess heat capacity peaks associated with each of these transitions have been observed for solutions that are 41 wt% CsPFO and 65 wt% CsPFO. The integrated transition enthalpy \(\int{\varDelta}C_{\text{p}}{\text{d}}T\) for the N-SmA transition is about 0.14 J g -1 (Joule per gram of CsPFO solute) at 65 wt% and about 0.10 J g -1 at 41 wt%. No qualitative difference was observed in the character of these Δ C p (N-SmA) peaks in spite of a reported change in the transition from second order at 41 wt% to tricritical near 65 wt%. The integrated transition enthalpy was larger for the N-I transition: about 0.23 J g -1 at 65 wt% ...