Specifics about Specific Ion Adsorption from Heterodyne-Detected Second Harmonic Generation

TL;DR

This study uses heterodyne-detected second harmonic generation spectroscopy to investigate ion-specific adsorption phenomena at charged silica/water interfaces, revealing how ion size and type influence interfacial properties and SHG responses.

Contribution

It demonstrates the application of heterodyne-detected SHG to distinguish diffuse and Stern layer effects and quantifies ion-specific interfacial behaviors at silica/water interfaces.

Findings

SHG intensity varies with ionic strength and ion type.

Charge density increases with decreasing cation size.

Stern layer properties show ion specificity, diffuse layer remains invariant.

Abstract

Ion specific outcomes at aqueous interfaces remain among the most enigmatic phenomena in interfacial chemistry. Here, charged fused silica/water interfaces have been probed by homodyne- and heterodyne-detected (HD) second harmonic generation (SHG) spectroscopy at pH 7 and pH 5.8 and for concentrations of LiCl, NaCl, NaBr, NaI, KCl, RbCl, and CsCl ranging from 10 mc microM to several 100 mM. For ionic strengths around 0.1 mM to 1 mM, SHG intensities increase reversibly by up to 15% compared to the condition of zero added salt because of optical phase matching and electrical double layer. For ionic strengths above 1 mM, use of any combination of cations and anions produces decreases in SHG response by as much as 50%, trending with ion softness when compared to the condition of zero added salt. Gouy- Chapman model fits to homodyned SHG intensities for the alkali halides studied here show charge densities increase significantly with decreasing cation size. HD-SHG measurements indicate diffuse layer properties probed by the SHG process are invariant with ion identity, while Stern layer properties, as reported by chi(2), are subject to ion specificity for the ions surveyed in this work in the order of chi(2)RbCl = 1/2 chi(2)NaCl = 1/4 chi(2)NaI .