The structure of the pH-dependent silica–water interface has been probed with two charged counter-ion chromophores Crystal Violet (CV) and Malachite Green (MG) by evanescent wave cavity ring-down electronic spectroscopy. The electronic spectra of these species in the charged interface have been measured directly over the wavelength range 570–680 nm. The effective CV interfacial counter-ion concentration is 185-fold larger than the bulk with a fully charged silica surface at pH 9, corresponding to a surface concentration of 0.26 molecules nm−2. The chromophore–silica surface association and dissociation kinetics indicate a binding energy at pH 9 >28 kJ mol−1 for each chromophore: a loosely bound molecular layer. The electronic spectrum of CV in the interface shows two maxima: one feature is blue-shifted 2 nm from the solution-phase maximum at 583 nm, and the second feature is red-shifted 24 nm. A Lorentzian deconstruction of the spectra indicates that they are consistent with two molecular environments: one closely associated with the surface sharing delocalised electrons and a second confined to the interface but randomly oriented.
