Effect of gel modulus on the porosity of low-density silica

Abstract

The extent of shrinkage that occurs during the early stages of ambient pressure drying (APD) of silica gel is greatly affected by the stiffness of the silica network. This parameter was manipulated through modification of the second step of the two-step acid–base process. The modulus of the gel, which was measured with a new technique based on the compression of rod-shaped specimens, decreased to roughly the same extent by increasing the gel volume (through the addition of extra ethanol or water) or by decreasing the amount of base added. However, following APD and calcination, these modifications resulted in quite different pore volume distributions. In general, the pore volume corresponding to large and intermediate mesopores decreased with increasing gel volume; however, a uniform pore volume distribution (possibly due to localized rupture of the silica network during drying) was observed only with the addition of ethanol. Decreasing the amount of ammonium hydroxide produced the greatest loss of large and intermediate mesopores; this observation amplifies the importance of the highly branched silica clusters, created through the addition of base, to the stiffness of the gel.