The influence of solution chemistry on the initial rate of perylene adsorption to a nonporous inorganic silica surface was investigated. Fluorescence was utilized to monitor the loss of perylene from aqueous solution. At constant ionic strength, the initial rate of adsorption decreased with increasing pH for all background electrolyte compositions. The observed adsorption rates were correlated with the aqueous activity coefficient (gamma(i)(w)) of perylene and the surface speciation of silica. At low pH, the rate of perylene adsorption appeared to depend solely on its gamma(i)(w). At neutral to high pH, binding of cations at the silica surface became increasingly important in determining adsorption rates. Binding of Na+ at the silica surface decreased the rate of perylene adsorption, whereas binding of Ca2+ at the surface increased the adsorption rate. The reasons for this difference are not presently known, but may relate to the structure of water at the solution-silica surface when different cations are present in the interfacial region.
