FIRM BINDING AND LOOSE ATTACHMENT OF RU(II) COMPLEXES TO COLLOIDAL SILICA IN AQUEOUS-SOLUTION - EVIDENCE BASED ON BIPHOTONIC IONIZATION AND A KINETIC SALT EFFECT ON THE REACTION WITH HYDRATED ELECTRONS

The photoionization of tris-2,2'-bipyridyl Ru(II) complexes in dilute aqueous solution (pH = 9.8), yielding hydrated electrons, is enhanced by a factor of about 2.5 by colloidal silica. LiCl reduces the enhancement factor. The photoionization yield decreases with decreasing pH. Moreover, the presence of colloidal silica causes a drastic increase in the lifetime of the hydrated electrons. The results are interpreted in terms of firm binding and loose attachment of Ru(II) complexes to the negatively charged silica particles the extent of which is controlled by the silica concentration at fixed Ru(II) complex concentration. Firm binding causes a reduction in the ionization potential which results in higher photoionization yields. Loose attachment results in a kinetic salt effect on the rate constant of the reactions of hydrated electrons.