Spectroscopic Probing of Mixed-Mode Adsorption of Ru(bpy)32+ to Silica

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Abstract
Evanescent wave excitation of fluorescence was used to study the adsorption of Ru(bpy)32+ from aqueous solution to three types of surfaces: bare silica, a dimethylethylsiloxane (C2) monolayer on silica, and a dimethyloctadecylsiloxane (C18) monolayer on silica. The solution pH was varied to investigate the nonpolar and electrostatic contributions to the free energy of adsorption for each surface. The pH dependence of the adsorption showed that the pKa is the same for each of the three surfaces, consistent with earlier conclusions that the acidity of derivatized silica surfaces is due to areas of exposed silica. The 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 C2 and C18 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 C2 surface and 100% for the C18 surface. Using Gouy−Chapman theory, the surface charge densities on each of the three surfaces, calculated from the electrostatic interaction energy of Ru(bpy)32+, were found to be within the range of literature values: 8.8(±0.1) × 10-7 mol/m2 for bare silica and 1.7(±0.1) × 10-7 mol/m2 for both the C18 and C2 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 phase.

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