THERMAL-DIFFUSIVITY IN SUPERCRITICAL FLUIDS MEASURED BY THERMAL LENSING

Thermal diffusivities of supercritical CO2 and C2H6 were determined ov er a wide density range with a photothermal technique. The thermal len s, formed by the degradation of the absorbed light energy as heat by t he sample, allows the employment of a nonequilibrium method in the cri tical region. Controlling the refractive-index gradient, i.e., a densi ty gradient, perturbations can be maintained at levels where convectio n is negligible. An easy-to-operate setup allowed us to measure therma l diffusivities in the density ranges 5 to 20 mol.dm(-3) for CO2 at 30 8 and 313 K and 2 to 12 mol.dm(-3) for C2H6 at 308 K with a standard p recision of 15%.