Effects of enhanced clusterization of water at a surface of partially silylated nanosilica on adsorption of cations and anions from aqueous media

Abstract

Adsorption of metal species Ba(II), Sr(II), Zn(II), Ca(II), and Mg(II) and anions Cl−, Br−, and I− at a surface of unmodified and partially silylated nanosilica A-300 was studied in aqueous media upon changes in the degree of substitution of silanols to trimethylsilyl groups (ΘTMS) at ΘTMS = 0.272–0.483. The cation adsorption increases in series Sr(II) < Mg(II) < Zn(II) < Ca(II) < Ba(II) with increasing value of ΘTMS. Nanosilica at ΘTMS = 0.483 adsorbs 1.8 mmol/g of Ba(II) from 0.01 M BaCl2 solution that is three times higher than that for the unmodified silica. Among anions, the adsorption of Cl− is maximal (1.34 mmol/g of Cl− from 0.01 M CaCl2 solution) onto silylated nanosilica at ΘTMS = 0.272 that is eight times greater than that for the unmodified nanosilica. An increased adsorption ability of partially silylated nanosilica in comparison to unmodified silica can be explained by nonuniformity of a modified silica surface resulting in enhanced clusterization of adsorbed water that leads to reduction of its activity as a solvent. Therefore, the desolvation energy decreases for adsorbed cations and anions having smaller solvated shells near the modified silica surface.