The interaction between alkali metal cations and Langmuir monolayers of anionic surfactants are examined in the context of the Law of Matching Water Affinities, which predicts preferential ion specific effects in aqueous solution based on the thermodynamic hydration properties of individual ions. Langmuir isotherms provide thermodynamic details while vibrational sum frequency spectroscopy (VSFS) yields molecular level details regarding changes in interfacial water structure and the hydration/orientation of the anionic headgroup. Combining thermodynamic and spectroscopic information while varying the anionic headgroup, alkali metal cation in solution, and charge density at the interface provides new insight into ion specific effects and reveals inconsistencies in the Law of Matching Water Affinities, particularly at high interfacial charge density.

Specific ion effects of alkali metal cations at a highly charged anionic interface

ACS Fall 2021

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