Second harmonic generation (SHG) spectroscopy has been applied to probe thefused silica/water interface at pH 7 and the uncharged 1102 sapphire/water interface atpH 5.2 in contact with aqueous solutions of NaCl, NaBr, NaI, KCl, RbCl, and CsCl aslow as several 10 μM. For ionic strengths up to about 0.1 mM, the SHG responses wereobserved to increase, reversibly for all salts surveyed, when compared to the condition ofzero salt added. Further increases in the salt concentration led to monotonic decreases inthe SHG response. The SHG increases followed by decreases are found to be consistentwith recent reports of phase interference and phase matching in nonlinear optics. Byvarying the relative permittivity employed in common mean field theories used todescribe electrical double layers, and by comparing our results to available literature data,we find that models recapitulating the experimental observations are ones in which 1) therelative permittivity of the diffuse layer is that of bulk water, with other possible values aslow as 30, 2) the surface charge density varies with salt concentration, and 3) the chargein the Stern layer or its thickness vary with salt concentration. We also note that theexperimental data exhibit sensitivity depending on whether the salt concentration isincreased from low to high values or decreased from high to low values, which, however, is not borne out in the fits, at least within the current uncertainties associated with the model point estimates.

Relative permittivity in the electrical double layer from nonlinear optics

2019 Spring National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.