Molecular structure dependence of thermo-diffusion in binary mixture using thermal Lens spectroscopy.

Thermo-diffusion (Soret effect) in binary mixtures arises from the coupling of heat and mass fluxes in an inhomogeneous temperature field. In this work, we investigated how molecular structural isomerism influences thermo-diffusion in binary mixtures of butanol isomers (n-, s-, i-, and t-BuOH) with carbon tetrachloride (CCl4) using femtosecond laser-induced thermal lens spectroscopy (FTLS). The thermal lens (TL) response of the mixtures reflected the interplay between heat conduction and mass transport, which evolved systematically with composition. The magnitude and sign of the Soret coefficient exhibited pronounced nonlinear dependence on composition and showed distinct sign changes at characteristic concentrations, occurring at ∼50% and ∼ 40% CCl4 for n-BuOH and s-BuOH systems and at ∼70% CCl4 for i-BuOH and t-BuOH mixtures. These findings indicate that thermo-diffusion in non-ideal mixtures is governed by molecular structure and intermolecular interactions rather than by bulk transport properties. Furthermore, FTLS is a highly sensitive technique for probing coupled heat-mass transport processes.