Dilatation of Porous Glass

A porous body dilates when its surface energy changes, either by adsorption of a gas or by immersion in a liquid. A new equation is derived to describe the resultant strain and is shown to agree better with experiment than the equation proposed by Yates. The physical properties needed for the calculation include Young's modulus and surface area. Measurements are presented for a commercial porous glass that show that the modulus increases by a factor of ∼2, with negligible accompanying shrinkage, upon heating to 800°C. The increase is attributed to the presence of hydroxyls (nonbridging oxygens) in the solid phase that condense on heating. The surface area contains a large contribution from gel that precipitates in the pores during acid leaching. It is concluded that the gel is rigidly bound to the solid network, and therefore contributes to the dilation of the body Porous glasses show an anamolously large expansion when immersed in water, which is attributed to absorption of water into the solid phase. Diffusion of water into the solid phase is expected to be enhanced by the large concentration of nonbridging oxygens.