A simple beam-bending technique permits measurement of the permeability and viscoelastic properties of a gel. Silica gel is found to be purely elastic in inert solvents such as ethyl formate, but viscoelastic in aqueous solutions. Measurements performed in buffer solutions show that the rate of relaxation increases with pH, a trend that is attributed to the increasing rate of attack on siloxane bonds. The permeability of the gels increases with pH, because coarsening increases the pore size; pore radii range from ca. 14 to 20 nm. The shear modulus decreases slightly as the pH increases, suggesting that the connectivity between clusters is reduced as the connectivity within clusters is increased by the dissolution/reprecipitation process.
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