Kinetics of monolayer particle deposition

Abstract

The kinetics of adsorption of a stable monodisperse spherocylindrical iron(III) hydroxide sol to a hydrophilic silica–titania surface have been measured at very low ionic strength over four orders of magnitude of bulk particle concentration, using a planar optical waveguide as the adsorbing substrate and recording the shift of the lightmode spectrum of the waveguide, from which the number of deposited particles can be accurately calculated as adsorption proceeds. Deposition is irreversible and saturates at submonolayer coverage. The kinetics are predominantly governed by electrostatic interactions and the geometry of exclusion of the adsorbed particles. The kinetics of deposition at all bulk concentrations could be fitted with just two common parameters, the particle surface potential ψ_p, and the hydration energy for adsorption U_h. ψ_p was ca. 8 mV, about an order of magnitude smaller than that deduced from measurements of the electrophoretic mobility. U_h was ca. 4 kT per particle, i.e. 0.3 mJ m^-2, much less than that required for complete dehydration of the particle/surface interfacial region.