Effects and mechanisms of anion and cation on the gelation of nanosilica sol by all-atom molecular dynamics simulation: promotion or inhibition?

Abstract

Nanosilica sol (NSS) is prone to gelation due to the condensation of silicon hydroxyl at normal temperature and pressure, which is further exacerbated by the addition of electrolytes during production. Therefore, the effects of ions and the mechanism of gelation of NSS are crucial for its stability. Herein, all-atom molecular dynamics (AAMD) was carried out to explore the effects and mechanisms of cations (K⁺, Na⁺, Ca²⁺) and anions (Cl⁻, NO₃⁻, SO₄²⁻, PO₄³⁻) on the sol–gel transition. Results indicated that highly electrophilic cations (e.g., Ca²⁺) and anions with slightly stronger nucleophilicity than Si(OH)₃O⁻ (e.g., NO₃⁻) could inhibit gelation by preventing Si(OH)₄ and Si(OH)₃O⁻ from approaching the silica surface. Such inhibition is more pronounced in NSS with larger particle sizes. Our findings offer some critical insights into the effects of ions on the gel stability of NSS, which also contributes significantly to screening suitable electrolytes for the production of NSS.