ABSTRACT Molecular simulation was used to investigate the coadsorption mechanism of Na+ and H2O on the hydroxylated silicon wafer. Water molecules can form a 3.5 Å-thickness bimolecular structure at the solid–liquid interface. Near the solid surface, the density of water can reach approximately 1300 kg/m3. Na+ does not bond with SiOH directly but adsorbs near SiOH in the form of hydrated ions. The adsorption of Na+ on the surface belongs to the ion hydration outer ring adsorption. The adsorption sites of Na+ are mainly located in the first water molecular layer. In the system, the peak value of the bond length distribution curve of water molecules is 0.98 Å, while the peak value of the bond angle distribution curve is 102°. Due to the polarisation of Na+, the peak value of the bond angle distribution curve of the water molecules decreases, especially at the interface water. Additionally, Na+ can destroy the hydrogen bonds in the system. AFM imaging confirmed that NaCl is adsorbed on the surface in the form of granular aggregates and the adsorption of Na+ by SiOH is weak in aqueous solution.