Interaction of NaOH solutions with silica surfaces

Sodium adsorption on silica surfaces depends on the solution counter-ion. Here, we use NaOH solutions to investigate basic environments.

Sodium adsorption on hydroxylated silica surfaces from NaOH solutions were investigated through molecular dynamics with a dissociative force field, allowing for the development of secondary molecular species.

Across the NaOH concentrations (0.01 M - 1.0 M), ∼50% of the Na⁺ ions were concentrated in the surface region, developing silica surface charges between - 0.01 C/m² (0.01 M NaOH) and - 0.76 C/m² (1.0 M NaOH) due to surface site deprotonation. Five inner-sphere adsorption complexes were identified, including monodentate, bidentate, and tridentate configurations and two additional structures, with Na⁺ ions coordinated by bridging oxygen and hydroxyl groups or water molecules. Coordination of Na⁺ ions by bridging oxygen atoms indicates partial or complete incorporation of Na⁺ ions into the silica surface. Residence time analysis identified that Na⁺ ions coordinated by bridging oxygen atoms stayed adsorbed onto the surface four times longer than the mono/bi/tridentate species, indicating formation of relatively stable and persistent Na⁺ ion adsorption structures. Such inner-sphere complexes form only at NaOH concentrations of > 0.5 M. Na⁺ adsorption and lifetimes have implications for the stability of silica surfaces.