Surface-enhanced Raman Scattering of 4-mercaptobenzoic acid in silver colloid

The ionic strength and pH dependence of Surface Enhanced Raman Scattering (SERS) of p-Mercaptobenzoic acid (pMBA) in silver colloid has been investigated. Disassociative adsorption of pMBA at surfaces of the silver colloid was found to occur via both its sulfur and carboxylate groups. Titration curves at low (few mM) and high (100mM) ionic strengths indicated significantly different ionization coefficients of the carboxylate ion on colloidal silver particles. In low ionic strength solution, pMBA existed as p-sulphidobenzoate in basic solution at pH 8 and above. At pH 4, complete protonation of the carboxylate group had occurred and pMBA existed as p-sulphidobenzoic acid. In higher ionic strength (100mM), complete protonation of the carboxylate group occurred at pH 11 and the solution existed as p-sulphidobenzoic acid during further acidification down to pH 2. Aggregation effects of colloidal silver solutions could possibly explain this anomalous behavior.