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Density of silica gel is determined by the amount of alkali metal ions present in the gel. Gels prepared from soluble alkali metal silicates show the lowest collapse temperatures.
The solubility of silica gel was also studied as a function of pH of the dissolution medium. Aristov et al. concluded that grain size impacted the activation energy, and that the resulting voids were smaller than those produced by spheroidizing silica gel without fused quartz.
Water vapor permeation is calculated as the mass fraction of water absorbed in relation to the dry mass of the film. At 100% relative humidity (RH) and 23 degC, the mass fraction of absorbed water is 0.35 grams per gram of dry silica gel. This value is in the range of the literature.
Films prepared with low-density polyethylene monolayer (LD-PE-LD) and calcium oxide film had water vapor permeation coefficients that were notably higher than those for pure PE-LD films. However, N2 permeability was lower, and CO2 permeability was not.
Unlike polar gases, non-polar gases are not able to diffuse through the polar groups in silica gel. However, the presence of free OH-groups in silica gel may explain the water vapor permeation behavior.
The sorption and permeation of silica gel/water systems was investigated using a permeation-testing device, type AQUATRAN, manufactured by Mocon Inc. Data was collected in five different cycles. Each cycle consisted of a desorption phase followed by a regeneration phase. During each cycle, the temperature was varied from 70 degC to 0% RH.
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