Binary liquid mixtures near their consolute critical points are shown to possess desirable thermophysical properties for thermal lens absorption measurements. By use of a 2,6-dimethylpyridine/water mixture near critical composition and temperature, sensitivity was found to be enhanced relative to pure liquids by the Soret effect in which a concentration gradient is created between the two solvent components by thermally induced mass diffusion within the laser-excited temperature gradient. A 35-fold increase in sensitivity relative to a thermal expansion lens effect in pure water was obtained. A mechanism of phase separation unique to the critical point region known as spinodol decomposition was observed, as the sample temperature was driven above the critical point by laser excitation. This phenomenon was also found to be a potentially useful spectrophotometric technique for the thermooptical detection of small absorbances.
F. D. Hardcastle and J. M. Harris, "Thermal Lens Absorption Measurements in Binary Liquid Mixtures Near the Consolute Critical Point," Appl. Spectrosc. 40, 606-611 (1986)