The study of liquid-liquid interfaces is crucial for understanding many important processes in broad areas of science. Developing both a quantitative understanding of ion adsorption to these interfaces, as well as a molecular level picture of the solvation environment are therefore of great interest. In this work, we continue our investigation of the behavior of simple ions at aqueous interfaces, employing the combination of two surface-sensitive nonlinear spectroscopy tools, broadband deep UV electronic sum-frequency generation and UV second harmonic generation, to characterize the adsorption of thiocyanate (SCN<sup>-</sup>) to the interface of water with prototypical liquid hydrophobes. We find that both the interfacial spectra and the Gibbs-free energies of adsorption closely match results obtained for the air-water interface. We observe no relative spectral shift in the higher-energy CTTS transition of thiocyanate, implying similar solvation environments for these seemingly very different interfaces. Furthermore, the Gibbs free energies of adsorption agree within error; however, we expect the respective enthalpic and entropic contributions to differ between the interfaces, as for our earlier findings for the air-water vs. graphene-water interfaces. Nevertheless, the similarities indicate a remarkably general behavior of water-hydrophobe interfaces.

Investigating adsorption to water-hydrophobe interfaces

ACS Spring 2022

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