Depth Resolved Nonlinear Vibrational Spectroscopy a Charged Liquid Interfaces

Molecules of solvents with a strong dipole moment have the ability to reorient as a response to an external DC field. At charged interfaces, the topmost layers involve molecules and ions that directly interact with the field source. In consequence, the molecular network at the vicinity of a charged phase boundary reorient within an unclear extent. Furthermore, the DC field towards the bulk is gradually screened by all charged particles in the interfacial region, and the field influence completely vanishes once the bulk is reached. Within this model, the anisotropy of charged interfaces has two components, one related with symmetry breaking by preferential molecular ordering, and the other, that arises from the presence of the field. Stablishing a clear connection between the surface macroscopic observables and the microstructure will open new ways to understand as well as to control relevant interfacial properties and processes. Nevertheless, accessing information that combines depth information and chemical sensitivity is experimentally non-trivial.

With our new depth resolved vibrational spectroscopy tool, in which we combine Sum- and Difference-Frequency Generation spectroscopy (SFG and DFG, respectively), we can extract precise depth information within regions with broken centro-symmetry  and correlate it to specific vibrational features[1]. To explore the effect of the ionic strength on the interfacial depth profiles, we performed measurements on systems with positive and negative charges at the topmost layer, and at relatively high and low concentrations of a monovalent inorganic salt. Based on these measurements we can draw conclusions about the spatial evolution of the electric fields as well as the extents and respective length scale of induced molecular ordering within the interfacial region..