  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 .
