Kinetic and thermodynamic aspects of adsorption on silica nanoparticles. A pulse radiolysis study

The rate of the reaction of hydrated electron with methyl viologen, MV2+, and tris(2,2'-bipyridyl)ruthenium(II), Ru(bpy)32+, in the presence of colloidal silica nanoparticles was measured using the pulse radiolysis technique. The decrease in the rate of the reaction upon addition of the particles was used to determine the concentration of free adsorbate in the suspension. The adsorption equilibrium constant obtained for MV2+, K = (4.7 ± 0.3) × 105 M-1, is identical within the experimental error for all the silica particles studied, independent of particle size (4 nm ≤ r ≤ 10 nm). The maximum density of bound MV2+ at saturation was found to be (1.0 ± 0.05) × 10-6 mol m-2, also independent of particle size. For Ru(bpy)32+, a significantly smaller equilibrium constant was obtained, K = (1.3 ± 0.1) × 105 M-1, and the highest density of Ru(bpy)32+ adsorbed on the same particles is smaller by 30%, 0.70 × 10-6 mol m-2. The differences in adsorption parameters between the two adsorbates are attributed to the difference in molecular sizes and thus charge-density distribution within these two molecules. The adsorption equilibrium constants are independent of loading of adsorbate in the range of 50% to 100% coverage. It is emphasized across this study that the "density of binding sites" determined by using different probes might depend on the probe molecule used.