Characterizing Anion Adsorption to Aqueous Interfaces: Toluene- Water versus Air-Water Shane W. Devlin, Debra L. McCaffrey, and Richard J. Saykally* Cite This: J. Phys. Chem. Lett. 2022, 13, 222-228 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: 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 to the interface of water with toluenea prototypical hydrophobe. We find that both the interfacial spectrum and the Gibbs free energy of adsorption closely match results previously reported for the air-water interface. We observe no relative spectral shift in the higher- energy CTTS transition of thiocyanate, implying similar solvation environments for the two interfaces. Similarly, the Gibbs free energies of adsorption agree within error; however, we expect the respective enthalpic and entropic contributions to differ between the two interfaces, similar to our earlier findings for the air-water versus graphene-water interfaces. Further experiments and theoretical modeling are necessary to quantify the mechanistic differences. T he study of ions at interfaces has revealed a wealth of vital information on many fundamental chemical systems and processes, 1 for example, surface catalysis, 2-4 atmospheric aerosol chemistry, 5,6 and electrochemistry. 7,8 Many of these studies have focused on the air-water interface, wherein detailed advances have been made in elucidating the mechanism of ion adsorption 9,10 and a molecular level picture of the solvation environment at the interface. 11-13 Equally important for developing a more detailed understanding of how small, inorganic ions behave at interfaces, the liquid- liquid interface warrants further examination. Specifically, the oil-water interface can serve as a model system to untangle water-hydrophobe interactions in complex environments, such as proteins in solution and biological membranes. 14,15 Both theory 16-18 and surface selective spectroscopies have been used to study liquid-liquid interfaces. For example, vibrational sum-frequency generation (VSFG) has revealed details on surface structure by monitoring changes in the OH- stretching region of water. 19-22 The Richmond group has published an extensive study using VSFG, of the CCl 4 -water interface in the presence of various surfactants, biomolecules, and ions. They report an increase in the overall interfacial ordering of water molecules at the hydrophobic, CCl 4 -water surface, attributed to weak hydrogen bonding interactions between the two condensed phases. 23 This weak hydrogen bonding is an important characteristic of specific water- hydrophobe interfaces, manifested in distinct surface proper- ties (for example, the fact that the water-toluene interface has a lower surface tension than water-alkane or water-alcohol interfaces is attributed to hydrogen bonding interactions between the water hydrogen and the π-system of toluene 24 ). The Saykally group has investigated a number of water- hydrophobe interfaces with deep UV electronic second harmonic generation (DUV-SHG), determining the Gibbs free energies of adsorption (Table 1). Onorato et al. investigated the behavior of bromide 25 and thiocyanate 26 anions at the dodecanol-water interface and showed that the free-energy of adsorption was equal within experimental uncertainty of that for the air-water interface; similarly, the water-graphene interface was studied by McCaffrey et al., showing again that the respective Gibbs free-energies agreed within error. MD simulations of this system, however, revealed that the enthalpic and entropic contributions in the mechanism of adsorption differed qualitatively from the air-water system. 10 Received: November 19, 2021 Accepted: December 27, 2021 Table 1. Anion Affinities for Air-Water and Hydrophobe- Water Interfaces Determined by DUV-SHG Spectroscopy ion/interface ΔG ads (kJ/mol) ref Br - /air-water ∼-1.4 25 Br - /dodecanol-water ∼-2 25 SCN - /air-water -7.5 ± 0.1 37 SCN - /dodecanol-water -6.7 ± 1.1 26 SCN - /graphene-water -8.5 ± 1.1 10 SCN - /toluene-water -7.3 ± 0.7 Letter pubs.acs.org/JPCL © XXXX American Chemical Society 222 https://doi.org/10.1021/acs.jpclett.1c03816 J. Phys. Chem. Lett. 2022, 13, 222-228 Downloaded via Shane Devlin on December 30, 2021 at 15:41:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.