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.