Photochemistry on Colloidal Silica Solutions

Abstract

Two probe molecules, Ruthenium tris-bipyridyl, Ru(II), and 4-(1-pyrenyl)butyltrimethylammonium bromide, PN+ have been used to investigate the nature of colloidal silica particles in water. The fluorescence spectra of the two probes show that the silica surface is very polar and similar to water. Quenching studies of the excited state of RuII and PN+ by anionic quenching molecules show that the particles are negatively charged but that the charge is not as effective as that on sodium lauryl sulfate micelles. Quenching studies with cationic quenchers show that the cations are bound strongly to the silica particles but do not move as readily around the surface as on anionic micelles. A small steric effect is observed with neutral quenchers. Several charge transfer reactions, including photo-ionization are strongly affected by the silica particles. The studies show many similarities to anionic micelles; they differ from micelles in two important aspects: (a) they do not solubilize neutral organic molecules and (b) cationic organic molecules such as PN+, hexadecyltrimethylammonium bromide, and hexadecylpyridinium chloride, tend to cluster on the silica surface rather than disperse uniformly around it as with ionic micelles.