Silica gel is a partially dehydrated form of polymeric colloidal silicic acid. It has an amorphous micro-porous structure with a distribution of pore opening sizes that range from 3-60 angstroms. It has a very high specific surface area and will adsorb water by both adsorption and capillary condensation. It is effective at temperatures below 77degF, but will lose some of its adsorption capacity as the temperature rises.
The adsorption characteristics of silica gels make them useful as desiccants, for example in the packaging of delicate items such as electronics and artwork to slow down degradation caused by moisture. It can also be used to help dry out items that are in the process of being restored by removing unwanted moisture before re-conditioning.
It is produced by mixing an aqueous solution of an alkali metal silicate with an aqueous emulsion of a saturated lower aliphatic carboxylic acid ester and acidifying the mixture with a mineral acid to form a gelatinous precipitate of silica. The resulting gel is then washed to remove soluble salts and dried.
A variety of different silica gels are available for use depending on the application. They include indicating and non-indicating grades with a variety of mesh sizes and hybrid gels. All of these have similar moisture capacities within the 30-60% RH range that is desirable for museum environments.
Adding carbon materials such as expanded graphite or multi-walled carbon nanotubes can enhance the performance of silica gels for some applications. These materials increase the thermal conductivity of the gel and can also improve the adsorption capacity.