SPECTROSCOPIC PROBING OF MIXED-MODE ADSORPTION OF RU(BPY)(3)(2+) TO SILICA

Evanescent wave excitation of fluorescence was used to study the adsor ption of Ru(bpy)(3)(2+) from aqueous solution to three types of surfac es: bare silica, a dimethylethylsiloxane (C-2) monolayer on silica, an d a dimethyloctadecylsiloxane (C-18) monolayer on silica, The solution pH was varied to investigate the nonpolar and electrostatic contribut ions to the free energy of adsorption for each surface. The pH depende nce of the adsorption showed that the pK(a), is the same for each of t he three surfaces, consistent with earlier conclusions that the acidit y of derivatized silica surfaces is due to areas of exposed silica. Th e free energies of adsorption for the bare silica surface, -26,2(+/-0. 2) kJ/mol at pH 8, was attributed to electrostatic interactions alone, The free energy of adsorption for the C-2 and C(1)8 surfaces, -28.5(/-0.1) and -31.5(+/-0.1) kJ/mol, respectively were found to have both electrostatic and nonpolar contributions, with the latter being larger by 50% for the C-2 surface and 100% for the C-18 surface, Using Gouy- Chapman theory, the surface charge densities on each of the three surf aces, calculated from the electrostatic interaction energy of Ru(bpy)( 3)(2+), were found to be within the range of literature values: 8.8-(/-0.1) x 10(-7) mol/m(2) for bare silica and 1.7(+/-0.1) x 10(-7) mol/ m(2) for both the C-18 and C-2 surfaces. The results demonstrate that a cationic dye can be used to probe the silanol activity of chemically modified silica surfaces, The results support the picture that these chemically modified silica surfaces are acidic due to molecular-scale areas of contact between the bare silica substrate and the aqueous pha se.