SIMULTANEOUS MEASUREMENTS OF REFRACTIVE-INDEX AND DENSITY IN A MICROEMULSION NEAR A CRITICAL END-POINT

The behavior of the density and the refractive index of a microemulsion of sodium dodecyl sulfate, water, n-pentanol, and n-dodecane is investigated near its critical end point. Measurements are made simultaneously and both are to an accuracy of a few ppm. In contrast to the density data which can be fitted to a regular, linear behavior in the whole temperature range (28.3-30.5-degrees-C), the refractive index exhibits a pronounced anomaly near the critical point. The temperature dependence of such an anomaly cannot be described by a power law behavior. However, it can be understood as a chemical change connected to morphological modifications in the dispersed phase, a phenomenon connected to the close vicinity of sponge-like phases. This approach accounts well for the data.