Kinetics of Silica Polymerization on Functionalized Surfaces: Implications for Reverse Osmosis Membranes

The general mechanisms of silica scaling through the polymerization of silicic acid at supersaturation have been predominantly studied in solutions. However, the pathway of silica polymerization occurring directly on surfaces, leading to silica precipitation, remains largely unexplored despite its wide-ranging implications for biomineralization processes, green material synthesis, and scaling in various engineered systems. In this study, we analyze the kinetics of silica polymerization from oversaturated solutions onto surfaces functionalized with various types of self-assembled monolayers (SAMs) or reverse osmosis (RO) membranes using a quartz crystal microbalance with dissipation. Upon contact with oversaturated silicic acid, the rate of silica polymerization on amine-terminated surfaces is nearly 6 times higher than that on carboxyl-, hydroxyl-, or methyl-SAMs. Silica polymerization on the surface of RO membranes over extended periods spontaneously transitions from a moderate to an accelerated regime, which corresponds to a structural transformation in silica scaling from the isotropic growth of aggregated particles to a gel-like glassy layer. Additionally, the presence of calcium ions in solutions significantly promotes silica scaling on membrane surfaces along with an increase in the viscoelastic properties of the formed scale layer. Our findings provide mechanistic insights into the molecular interactions between oversaturated silicic acid and functionalized surfaces, highlighting the critical roles of surface functional groups and coexisting ions in silica polymerization for scale formation on engineered surfaces.