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| - pH Dependence of the Crystal Violet Adsorption Isotherm at the Silica−Water Interface
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| - The pH-dependent adsorption isotherms for the charged chromophore crystal violet, CV+, have been measuredwith three different bases by a free-running cavity implementation of evanescent wave cavity ring-downspectroscopy. The ratio of the maximal absorbance measurements at pH 5.10 and 9.05 is consistent with aQ2:Q3 silanol site ratio of 72.8:27.2. The adsorption isotherms have been interpreted in terms a cooperativebinding adsorption allowing more than one ionic species to bind to each silanol group. The surface concentrationis consistent with a silanol charge density of 1.92 ± 0.55 nm-2 and a total neutralized interface layer structureextending 9 nm from the surface. Binding constants and stoichiometric coefficients are derived for CV+ toboth the Q2 and Q3 sites. A variation of the adsorption isotherm with base is observed so that the isothermat pH 9.05 adjusted with ammonium hydroxide sets up a competitive acid−base equilibrium with the SiOHgroups with only 49% of the surface silanol sites dissociated. The implications for functionalized surfaces inchromatography are discussed.
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