Silica gels are a diverse group of materials consisting of micro- or nanoporous silicon dioxide. It is produced in a variety of forms and finds use as a desiccant, purification medium, and catalyst support, among other uses.
Silica gels usually occur as a hard, brittle solid, though highly hydrated forms may be softer and more gel-like. Gels made for different applications are availble in various forms, such as irregular shapes or spherical beads, with varying pore sizes and states of hydration. Pure silica gel is colorless but many silica gel products contain pigments or dyes that change color upon hydration or significant changes in pH. Silica gel's high specific surface area (around 800 m2/g) allows it to adsorb water readily, making it useful as a desiccant (drying agent). Once saturated with water, the gel can be regenerated by heating it to 120 °C for 1–2 hours. Just like in the case of molecular sieves, heating the beads too hard may crack them. Certain types of silica gel will "pop" when exposed to enough water, causing them the spheres to break down.
A notable feature of many anhydrous silica gels is their tendency to release noticeable amounts of heat upon adsorbing water or other materials. This can be a somewhat reliable way of gauging the dryness of silica used for desiccation or water removal from solvents.
As with other forms of silicon dioxide, silica gel beads are relatively inert but will react with concentrated alkali, hot, concentrated phosphoric acid, hydrofluoric acid and some compounds of fluorine. However, they can absorb water and become soft.
Silica gel can be found in many products, ranging from clothes, shoes, all packaging, furniture, electronic and mechanical devices, inside small labeled bags.
While most caps of certain medical drug tubes contain silica gel, some also contain other drying agents that fizz on contact with water.
Silica gel litter used for adsorbing cat urine and other fluids can be cheaply purchased from various vet shops. Some may contain perfume, while most tend to have a few colored flakes (generally blue) mixed with the colorless ones, which, due to their large size can be easily removed by hand.
Lastly, silica gel beads can be bought in bulk from various suppliers.
Crystalline silica gel can be purchased from chemical suppliers or online.
Silica gels find many uses in a lab setting. Many commercial variants, such as the silica gel found in small packets in consumer products, are able to absorb atmospheric moisture and thus are used as desiccants. Indicating types such as those found in kitty litter provide a convenient test for water.
Though not often commercially available to the amateur, chromatography-grade silica is one of the most common stationary phases for column chromatography. In this application, the pores found in silica will adsorb various compounds from a solution of an organic solvent. The polar nature of most silica dictates that as a general rule, polar compounds will adhere more strongly and can only be removed with a more polar solvent. Highly nonpolar compounds, on the other hand, will freely wash away in a nonpolar solvent like hexanes. This trend is the basis for column chromatography and may allow for the separation of compounds in a mixture that are not easily separable by conventional means.
- Dry solvents
- Purification of gas streams
- Reactor catalyst
- Column chromatography
- Filtering extremely fine suspensions
Silica gel may irritate the skin when anhydrous. Repeated inhalation of crystalline silica dust can cause silicosis, but synthetic amorphous silica gel does not, as it is indurated. Very fine grades of silica gel should be dispensed outside or in a hood to prevent any airborne silica within a closed space.
Though largely unreactive, any silica gel intended for desiccant or adsorbent use should be stored in a tightly closed container to prevent moisture from entering. Fine, powdery silica gel should be stored in a robust, puncture-proof container or multiple layers of bagging to prevent leaks.
Silica gel on its own can be safely disposed of in ordinary household waste, though silica that is so fine it may become airborne should be disposed of in a bag or other secondary container. Silica containing heavy metal catalyst materials or other hazardous compounds should be disposed of as though it is that material.
Another option is to mix the silica gel with cement or gypsum and let it harden into a solid mass, which can be disposed like any other construction material waste.