Diffusion studies in silica gels

Abstract

Silica gel structure has interested scientists for nearly a hundred years. Most of the interest has centered around the control, applications, and characterization of the enormous surface-to-volume ratios that gels possess. A typical ratio is 500 m2/gm for dry silica gel. Since surface effects are responsible for silica gel's desirable properties, numerous determinations of surface areas, pore sizes, and pore densities have been made. On the other hand, quantitative data on hydrogel structure are not plentful in the literature. Two reasons for this de-emphasis on hydrogels are (1) the hydrogel is not as useful, and (2) there are no standard techniques that can be applied to the problem. Gas adsorption, X-ray, and density methods that can be applied to the problem of dry gel structure are not suitable for hydrogel study. With regard to (1), interest in silica hydrogel has recently experienced a slight upsurge because of its potential as an ideal medium for low temperature crystal growth. Some striking examples of crystal growth in gels have been reported, and lately optically perfect 1 era. hexagons of lead iodide less than 20 microns thick have been grown in silica gels in our lab. Exactly how the gel promotes sparse nucleation and regular, impurity-free growth is not clearly understood. I t will be seen that a reduction in diffusion rates, which might account for better growth of crystals, is not in evidence. Obviously, knowledge of silica hydrogel structure would aid in analysis of the gel effect, but (2) is now a stumbling block. Consequently, it is proposed that the method for study be a diffusion one, which, incidentally, involves a minimum of equipment. I t should be understood here that emphasis is more on a tentative approach than on the presentation of absolutely reliable results. In a previous paper (1), a method was proposed whereby diffusion coefficients of electrolytes can be obtained. The possible results of such an investigation have now been extended to include an order of magnitude estimate of the pore size in a gel. Diffusion coefficients are measured by analysis of constantly stirred electrolyte placed on the surface of a gel. If the pore sizes are so large that the solid portion of the gel does not interfere with the normal diffusion process (the blocking effect is negligible), there is a simple relationship between the aqueous diffusion coefficient and the gel coefficient, on the assumption that only volume effects are present: