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

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 5.8 and for concentrations of LiCl, NaCl, NaBr, NaI, KCl, RbCl, and CsCl ranging from tens of micromolar to several hundred millimolar. For ionic strengths around 0.1-1 mM, SHG intensities increase reversibly by up to 15% compared to the condition of zero added salt because of optical phase matching and the 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 that 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 χ(2), are subject to ion specificity for the ions surveyed in this work in the order of χRbCl(2) = 1/2χNaCl(2) = 1/4χNaI(2).