Hygroscopic compounds are all those substances that attract water in vapor or liquid from its environment, thus its main application is as desiccants. Many react chemically with water such as metal hydrides or alkali metals. Others take as water of hydration in its crystal structure such as sodium sulfate. Water may also be physically adsorbed. In the latter two cases, retention is in a reversible mode and water can be desorbed. In the first case, it can not be recovery in a simple way.
The deliquescent materials are substances (in its most salts) that have a strong chemical affinity for moisture and absorb relatively high amounts of water if exposed to the atmosphere, forming a liquid. We have examples of deliquescent such as calcium chloride, ferric chloride, magnesium chloride, zinc chloride, potassium carbonate, potassium hydroxide and sodium hydroxide. The presence of these compounds in dilution with water modifies the properties of the same in relation to its pure state. These modifications are referred to as properties of a solution, constituent (viscosity, density, electrical conductivity, etc.) and colligative (decrease of the solvent vapor pressure, boiling point elevation, freezing point lowering and pressure osmotic) of special interest in this technology.
One of the mean applications of the hygroscopic compounds is absorption cycles used for refrigeration. These machines began to commercialize in the early 50´s, though his principle has been known for over a hundred years. The absorption cycles are physically based on the ability of some substances, such as water and certain salts, to absorb, in liquid phase, vapors of other substances such as ammonia and water, respectively. Similarity in this cycle water is the refrigerant and the hygroscopic compound the absorbent.
Examples of several known hygroscopic compounds are:
- Sodium chloride (halite) (NaCl).
- Calcium chloride (CaCl2).
- Sodium hydroxide (NaOH).
- Sulfuric acid (H2SO4).
- Copper sulphate (CuSO4).
- Phosphorus pentoxide (P2O5 or more correctly P4O10).
- Silica gel.
- Hydrated salts such as Na2SO4∙10H2O.
- LiBr (the most widely used at present, especially in absorption machines for cold generation).