Breaking the Symmetry of Ions at the Air–Water Interface

Abstract

In this Article, we investigate the impact of the fluid-air interface on the dissolved electrolyte potassium ferrocyanide K4[Fe(CN)6] by infrared-visible sum-frequency generation spectroscopy (SFG) and surface tension measurements. SFG is a surface specific nonlinear optical technique that records the vibrational spectrum of the adsorbed species with little to no contribution from the bulk phase. According to the selection rules, only modes which are simultaneously Raman and Infrared active can contribute to the SFG spectrum. Since [Fe(CN)6]4– belongs to the point group Oh, no vibrational mode is SFG active, unless a deformation takes place. Our spectra provide experimental evidence for a symmetry reduction of the ions at the interface to subgroups with no center of inversion. The distortion of the ions is the direct consequence of the asymmetric environment at the interface and is further discussed in this paper. The SFG spectra of the interfacial water were used to gain further insights in the distribution of the ions at the interface. Furthermore, the combined data of the surface tension equilibrium isotherm and the SFG spectra suggest a nonmonotonous interfacial concentration profile of the ion.