Interaction of NaOH solutions with silica surfaces

• Published in: Journal of colloid and interface science Vol. 516; no. C; pp. 128 - 137
• Main Authors: Rimsza, J.M., Jones, R.E., Criscenti, L.J.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 15.04.2018; Elsevier
• Subjects: Adsorption; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Molecular dynamics; Molecular simulations; NaOH; Silica; ReaxFF; BAD; vSFG; NaOH; Molecular dynamics; Silica; NMR; DFT; PDF; NPT; SHG; Adsorption; MD; NBO; QEQ; Molecular simulations
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2018.01.049

[Display omitted] 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.