Microemulsions: Phase transitions and their dynamics
By differential scanning microcalorimetry we investigate temperature-induced phase transitions and their dynamics in mixtures of water, oil and a non-ionic surfactant. Special emphasis is on an investigation of the transition from a lamellar to a microemulsion phase and on the emulsification failure. The first-order phase transition from a lamellar to a microemulsion phase leads to heat changes up to 1k BT per surfactant molecule. These large values for the latent heat are quantitatively described by an interfacial model which takes into account the temperature dependence of the spontaneous curvature.
The dynamics of phase separation due to an emulsification failure leads to oscillations in the specific heat when heated with constant scan speed. The period and amplitude of the oscillations depend on composition and scan speed. A square root dependence on scan speed is found for the period. The values for the heat absorbed during each oscillation are compared with those calculated from the interfacial model, and a mechanism for the origin of the oscillations is proposed.
Key wordsMicroemulsion calorimetry phase transitions and their dynamics oscillations
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