Thermal diffusion measurements near a critical end point in a microemulsion system

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Abstract
A new optical technique has been developed for the study of thermal conductivity Λth and thermal diffusivity Dth in critical fluids. The new method combines the advantages of two traditional methods (induced thermal lens and thermal dynamic grating) while eliminating difficulties associated with them. This method has been applied to the investigation of dynamic behavior in a microemulsion composed of n-dodecane, water, n-pentanol, and sodium dodecyl sulfate. This system exhibits critical phase separation (into two microemulsions with different dispersed phases concentrations) that can be considered as analogous to binary liquid phase separation near a consolute point. Over a range of microemulsion compositions there exists a line of such critical points terminating at a critical end point. Near this critical end point both Λth and Dth exhibit an anomalous increase as the temperature approaches the critical temperature. The ratio Λth/Dth is found to be essentially constant near Tc, in agreement with the behavior of independently measured heat capacity values. The results are discussed in the context of current theoretical predictions for the critical transport properties of binary liquids.