The relationship between micro- and mesoporous substructures upon removal of colloidal silica from porous glasses subjected to alkaline treatment

Abstract

The effect of alkaline treatment of two basal liquation sodium borosilicate porous glasses on their micro- and mesoporous substructures has been studied. The morphology of pores has been investigated and the structural characteristics of micro- and mesoporosity have been determined by the equilibrium and kinetic adsorption-desorption methods at low, moderate, and high relative pressures. It has been established that the alkaline treatment leads to a substantial increase in the volume of mesopores and a noticeable decrease in the volume of micropores, which correlates with a reduction in the specific surface area of the mesopores. After the alkaline treatment, the trimodal distribution and the average diameter (0.5–0.6 nm) of micropores remain unchanged and ultramicropores do not arise in the range of 0.27–0.37 nm. It has been concluded that micropores represent only the regions of interglobular contacts of secondary silica in liquation channels with sizes of 1–2 diameters of an adsorbate molecule. The alkaline treatment is accompanied by the structuring and dissolution of silica globules in liquation channels; as a result, wide-porous glass samples with monomodal interglobular pores are obtained, while new micropores are not formed via the etching of channel walls in the glass matrix. Correlation dependences between the coordination number, porosity, and diameters of pores and globules of colloidal silica have been proposed for a large set of standard globular packings. It has been shown that, as the boron content in a porous glass rises, secondary silica globules in liquation channels grow, while their packing becomes looser.