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The Soret coefficient of acetone/water mixtures has been experimentally determined by a recently developed thermal lens technique ͓Appl. Phys. Lett. 94, 051103 ͑2009͔͒. The behavior of the Soret coefficient was reproduced, including its sign change with composition. For concentrations around the equimolar ones, we have also confirmed the disagreement between the experimental and simulation data that had previously been reported by Ning and Wiegand ͓J. Chem. Phys. 125, 221102 ͑2006͔͒ using a transient holographic grating technique of thermal diffusion forced Rayleigh scattering. Additionally, we compare our experimental results with the theoretical values predicted by a recently developed viscous energy model ͓J. Chem. Phys. 130, 064506 ͑2009͔͒.

In this letter we generalize the thermal lens model to account for the Soret effect in binary liquid mixtures. This formalism permits the precise determination of the Soret coefficient in a steady-state situation. The theory is experimentally verified using the measured values in the ethanol/water mixtures. The time evolution of the Soret signal has been used to derive mass-diffusion times from which mass-diffusion coefficients were calculated.

In this work we utilize an alternative optical method based on thermal lens spectroscopy for characterizing the thermal diffusion in binary liquid mixtures. In this method, the thermal lens and the Soret signals are separated by a time interval to determine the Soret coefficients. As a demonstrative experiment, the Soret coefficients of isopropanol/water and acetone/water mixtures have been measured using that variant. Our results were compared with the recently published experimental and theoretical calculations and very good agreement was achieved.