CONTROL OF THE POROUS STRUCTURE OF SILICA-GEL BY THE PREPARATION PH AND DRYING

The decrease of pore dimensions with drying of silica gels as a functi on of the preparation pH is investigated. The gels of 8.2 wt% SiO2 in water are precipitated between pH 1 and 10 in a computerized static pH equipment. The shrinkage is monitored by thermoporometry, which is ap plied to the wet as well to the dried gels. The properties of the drie d gels are measured with four traditional techniques: transmission ele ctron microscopy, nitrogen sorption at 77 K, mercury penetration, and a Sears titration. The fixed pH influences the polymerization rate of silicate anions, and therefore the size and branching of the colloidal silica particles in the gel. At a low static pH of 1-2, very small bu t highly branched particles, approximately 3 nm diameter, are develope d in the gel with an extreme microporosity. With thermoporometry on hy drogels and N2 sorption on xerogels, it is established that each sol p article is surrounded by approximately eleven others. With thermoporom etry, it is determined that the particles are already clustered in the hydrogel. Shrinkage proceeds with drying, and no pores can be measure d by thermoporometry on xerogels. The pore radius, R(p), of 3.1 +/- 0. 5 nm decreases, and the specific pore volume decreases from 0.4 +/- 0. 1 ml g-1 to 0.13 +/- 0.05 ml g-1. At a high static pH above 4, less br anched and platey sol particles are formed with sizes less-than-or-equ al-to 26 nm. Before drying, the aggregates are highly porous and the p ores are too large to be determined by thermoporometry. At pH 4 and 5, shrinkage proceeds, and the pore radii, R(p), decrease to 2.8 +/- 0.6 and 6.8 +/- 3.0 nm, and the pore volumes to 0.8 +/- 0.1 and 0.9 +/- 0 .1 ml g-1, respectively. The mercury pore volume of the dried gels wit h a maximum of 2.8 +/- 0.2 ml g-1 at pH 5 is larger than both the ther moporometry and nitrogen pore volume, which is explained by macropores with radii of 1-2 mum. The pore radii and pore volumes of the macropo res cannot be measured with thermoporometry and nitrogen sorption. The gels measured with thermoporometry, mercury penetration and nitrogen adsorption exhibit a maximum in pore volume between pH 4 and 8. The co lloidal silica particles in the gel grow larger and repel each other a t high pH. Finally, at very high pH above 8, the gel weakens and suffe rs more from the capillary forces with drying, due to the decreasing a mount interparticle bonds.