The preferential affinity of selected anions to interfaces exposing organic hydrophobic moieties plays, in connection to the incompletely understood Hofmeister effects, a vital role in a number of complex biological processes. The prototypical model surface for studying ion adsorption has been the air/water interface, where potential effects of the nature of the hydrophobic surface have been neglected. Here, we investigate the interactions of the monovalent anions Cl, Br-, I-, and SCN- with a hydrophobic solid surface exposing methyl groups to the solution. Molecular information is obtained using vibrational sum frequency spectroscopy by targeting water molecules in the first monolayer as well as the C≡N stretch of the adsorbing SCN- anion. Although the relative affinity for the interface follows the Hofmeister ordering with SCN- &gt; I- &gt; Br- &gt;Cl-, chlorine, and bromide are found to be expelled from the interfacial region, with thiocyanate showing a net enrichment. Molecular dynamics simulations of NaSCN solutions at the interface with air and with an alkane monolayer provide additional quantitative insight and confirm the comparatively higher affinity of SCN- anions to methyl-terminated hydrophobic surfaces.

Do monovalent anions preferentially adsorb to extended hydrophobic surfaces exposing methyl groups?

2019 Fall National Meeting

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