Spectroscopic Evidence for the Specific Na+ and K+ Interactions with the Hydrogen-bonded Water Molecules at the Electrolyte Aqueous Solution Surfaces

TL;DR

This study uses sum frequency generation vibrational spectroscopy to reveal how specific Na+ and K+ ions uniquely influence the hydrogen bonding structure of water molecules at electrolyte solution surfaces, challenging previous assumptions.

Contribution

It provides the first direct spectroscopic evidence of distinct Na+ and K+ interactions with water at the air/water interface, highlighting ion-specific effects.

Findings

Na+ and K+ ions cause different spectral features at the water surface

Ion concentration affects hydrogen bonding structure

First direct observation of alkali cation effects at water surface

Abstract

Sum frequency generation vibrational spectra of the water molecules at the NaF and KF aqueous solution surfaces showed significantly different spectral features and different concentration dependence. This result is the first direct observation of the cation effects of the simple alkali cations, which have been believed to be depleted from the aqueous surface, on the hydrogen bonding structure of the water molecules at the electrolyte solution surfaces. These observations may provide important clue to understand the fundamental phenomenon of ions at the air/water interface.