Ionic diffusion in silica sol–gel glasses

Abstract

Non-Fickian ionic diffusion in porous silica xerogel glasses was examined experimentally and modeled with a concentration-dependent diffusion coefficient. Smaller pore sizes resulted in a decreased diffusion rate, increased hysteresis, and greater dependence on the nature of other ionic species present. The pH-dependent non-Fickian diffusional dynamics within the porous matrix was attributed to pH-dependent chemistry of the silica surface species. Protonation of the silica surface was found to dramatically increase the rate of proton diffusion within the pores. The observed hysteresis was attributed to a competition between entry into pore openings versus transport within the porous network. Factors that affect ionic diffusion kinetics in nanoscale porous materials are relevant to chemical sensor and dopant delivery applications of porous silica materials. • Non-Fickian diffusion kinetics were adequately modeled with a homogeneous diffusion model. • Ionic diffusion was dependent on both pore size and other cations in solution. • The hysteresis in proton diffusion was attributed to charge accretion at pore openings.