DFT simulation of the adsorption of sodium silicate species on kaolinite surfaces

Highlights: • Si(OH)₄ and SiO(OH)₃⁻ mainly adsorb on kaolinite Al surface (0 0 1). • The adsorption of SiO(OH)₃⁻ is more stable than that of Si(OH)₄. • The hybridization is the main interaction between Al surface and Si(OH)₄. • SiO(OH)₃⁻ adsorbs on Al surface by hybridization and electrostatic interactions. - Abstract: The adsorption of Si(OH)₄ molecules and the SiO(OH)₃⁻ anion on kaolinite surfaces was studied using density functional theory (DFT) calculations to investigate the dispersion mechanism of sodium silicate on kaolinite particles. The calculated results demonstrate that Si(OH)₄ and SiO(OH)₃⁻ primarily adsorb on kaolinite Al-terminated (0 0 1) surfaces. Both Si(OH)₄ and SiO(OH)₃⁻ bond with the Al-terminated surface by hybridization of the O_2p orbital and H_1s orbital. The unbonded O atom of SiO(OH)₃⁻ is notably active. The SiO(OH)₃⁻ anion can add more electrons and form a stronger electrostatic interaction with the Al surface. The adsorption of SiO(OH)₃⁻ is more stable than the adsorption of Si(OH)₄. After adsorption of sodium silicate, the surfaces of kaolinite can become more hydrophilic and carry more negative charge. Therefore, the adsorption of silicate species makes the fine particles of kaolinite disperse in aqueous solution more easily.