Adsorption of potential-determining ions on porous glasses of different compositions
The adsorption characteristics are studied for nano- and ultraporous glasses (PGs) produced from sodium borosilicate glasses and a glass containing small amounts of fluoride ions and phosphorus oxide by acid (HCl) leaching and additional alkaline (KOH) and thermal treatment. The surface charges σ0 of PGs are determined by continuous potentiometric titration in 10−3−1 M NaCl, KCl, and (C2H5)4NCl solutions. Only negative surface charges of PGs are observed for all investigated systems. The |σ0| value is predetermined by the following factors: the composition of PG, the pore radius in the nanometer region (r ≤ 13 nm), the specificity of counterions, the content of secondary silica in the pore space, and the temperature of the additional thermal treatment of the membranes. The introduction of fluoride ions and phosphorus oxide into sodium borosilicate glass, an increase in the pore sizes and the amount of the secondary silica in PGs, and a rise in the specificity of counterions enhance the |σ0| values, which decrease with a rise in the temperature of the thermal treatment due to the surface dehydration and dehydroxylation. For ultraporous glasses (r > 13 nm), the surface charge is almost independent of the pore radius.