Water sorption by the calcium chloride/silica gel composite: The accelerating effect of the salt solution present in the pores

The kinetics of isothermal water sorption by the CaCl2/silica gel composite initiated by a small stepwise pressure rise over the sample has been investigated at a constant underlying plate temperature of 35°C. The initial portion of the kinetic curves is consistent with Fick’s diffusion model: the amount of sorbed water increases in proportion to the square root of the sorption time. This makes it possible to determine the effective diffusivity of water (D eff). At small amounts of sorbed water (w < 0.19 g/g), D eff changes slightly. The diffusivity of water in the composite pores (D) calculated for the same conditions is close to the Knudsen diffusivity of water vapor in mesopores. The D eff value grows with an increasing water content of the composite; that is, sorbed water accelerates water transport in the pores. This is likely due to the appearance of an extra diffusion channel, namely, diffusion through the aqueous solution of the salt, whose formation begins on the silica gel surface at w > 0.1 g/g. The contribution from this channel increases markedly when the amount of adsorbed water is above 0.25 g/g. This can be explained by the formation of the “connected” phase of the solution in the pores.