Effect of Si content on diffusion coefficient of nickel nitrate in wet silica gel

The liquid-phase diffusion coefficient of nickel nitrate in wet silica gels with different Si content as well as in mesoporous silica gel was measured by the flow-cell method, where absorbance of a gel plate placed in a flow cell was monitored as a function of time. For diffusion in the mesoporous s ilica gel with pore diameter of 12 run, the tortuosity was as low as 1.2, s uggesting that the liquid-phase diffusion of nickel nitrate is not restrict ed in the large mesopores. In contrast, the diffusion coefficient in wet si lica gels monotonously decreased with increasing Si content in the gels, an d the tortuosity in the wet gel, 1.7-3.5, was larger than that in the mesop orous silica gel. The wet gel is considered to consist of crosslinked polym eric network of silica. The tortuosity in the wet gel would vary depending on the mesh size in the network and on the number of free side chain.