Unraveling molecular interactions in binary liquid mixtures with time-resolved thermal-lens-spectroscopy

Abstract

Non-contact localized laser heating-based thermal lensing has emerged as a technique for probing the heat transport in liquids. A mode-mismatched dual-beam pump–probe spectroscopic technique was employed to investigate the photothermal response and modes of heat dissipation in methanol and binary mixtures of methanol with polar (water, methanol (MeOH)) and nonpolar (CCl4) solvents. We recorded the time-resolved thermal lens (TL) signal of a probe beam at 780 nm after heat deposition by a 1560 nm pump beam. For pure solvents, the TL signal was found to be approximately one order of magnitude larger for methanol than for water, DMSO, or CCl4, implying that the energy deposition is larger for methanol than for any of the other solvents. Subsequently, binary mixtures were studied where the TL signal increased with an increase in the volume fraction of methanol. All TL signals are shown to have a physical interpretation in terms of heat conduction and convection. In the case of methanol–water, the observed trend can be rationalized in terms of a strong intermolecular interaction. Convective heat transfer is shown to dominate the overall heat transfer in pure methanol and in binary mixtures for all volume fractions where MeOH is in excess of 50%. No convection is observed for very dilute mixtures with a small amount of methanol; in this case, heat conduction is sufficient to reach equilibration. Interestingly, for binary mixtures of methanol with DMSO or water, a decreasing trend is observed in the concentration range between 90% and 100% volume fraction of methanol. We observe also that the TL signal is modified in case of intermolecular interactions forming large clusters of methanol with the cosolvent. In such cases, heat diffusion is affected. Thus, TL can be seen as a sensitive probe for intermolecular interactions as well.