The influence of electrolyte concentration on the depth-dependent structural anisotropy of water at charged interfaces

Revealing the structural properties of water at charged interfaces is key for a better understanding of interfacial processes in various fields such as atmospheric chemistry, biology, and electrochemistry. One important aspect of the interfacial water structure in the presence of surface charges is its evolution with depth and how it varies with electrolyte concentration, which is still largely unknown despite various experimental efforts. In this work we investigate the anisotropic water structure in contact with insoluble charged surfactants using our recently developed depth-resolved vibrational spectroscopy which is based on a combination of phase resolved sum- and difference-frequency generation spectroscopy. By probing the line shape of the O-H stretch vibration of water in the first solvation layers of the surface charges and in interfacial regions further away from the phase boundary we obtain detailed insight into the field-induced orientational anisotropy and hydrogen-bonding properties of water molecules inside the electrical double layer. We find that the properties of the hydrogen-bond network in terms of hydrogen-bond strength and connectivity are nearly unaffected by the anisotropic molecular orientation. This is shown to hold throughout the double layer and for all measured electrolyte concentrations. The data, however, reveals significant changes of more than 40% in the amount of orientational anisotropy close to the interface as function of ionic strength, directly opposing a common and crucial assumption made in such investigations.