MICROCALORIMETRIC INVESTIGATION OF THE UPPER PHASE-BOUNDARY OF WATER-IN-OIL MICROEMULSIONS

Experimental data on the thermodynamics of phase separation of an ionic microemulsions [sodium bis(2-ethylhexyl)sulfosuccinate-water-isooctane] are presented. They are obtained by differential scanning microcalorimetric measurements across the upper phase boundary. Sample compositions are chosen in such way that the transition resembles a liquid-gas-like phase transition. Increasing attractive interaction between the droplets is assumed to cause an instability against phase separation into a system with two droplet phases of different droplet density, but equal droplet size on the average, Our data yield first thermodynamic information on the dynamics related to this phase transition. For finite scan speed we observe two peaks in the specific heat. The first peak arises at the phase separation temperature and it is immediately followed by another one. The enthalpy related to the second peak depends quadratically on droplet size and linearly on droplet concentration. We propose that internal degrees of freedom due to hopping possibilities of the surfactant molecules in the contact region of touching droplets form its origin. In the limit of zero scan speed both peaks merge. Together they give rise to the latent heat of the first order phase transition. (C) 1995 American Institute of Physics.