Definition:
Evaporation is the free escape of vapour from the surface of a liquid bellow its boiling point. Factors affecting rate of evaporation:
1. Temperature: The rate of evaporation is directly proportional to the temperature of the liquid. The evaporation can be accelerated by increasing the temperature but it will cause decomposition of heat sensitive principles of many drugs. Many glycosides and alkaloids are decomposed at a temperature below 100oC. Hormones, vitamins, enzymes, antibiotics, malt extract need special treatment to avoid decomposition
2. Temperature and time of evaporation: It has been observed that exposure to a relatively high temperature for a short period of time (as in film evaporators) may be less harmful than exposure to a lower temperature for a longer period.
3. Temperature and moisture Some drug constituents decompose more readily in the presence of moisture if heated at a high temperature due to hydrolysis. To avoid this, the evaporation is done at a low temperature and then the final drying is done at a high temperature when only little moisture remains in it.
4. Types of product required: The selection of the method and equipment required for evaporation depends upon the type of product required (liquid, semisolid or solid).
5. Effect of concentration: During evaporation the upper layer tends to form a film and there is formation of precipitate in the product which results in lowering down the rate of evaporation. Therefore, efficient stirring is required which will prevent degradation of the product at the bottom due to excessive heat and also prevent deposition of solids.
6. Surface area: The rate of evaporation is directly proportional to the surface area of the evaporator.
7. Vapour pressure of the liquid to be evaporated: The rate of evaporation is directly proportional to the vapour pressure of the evaporating liquid. The rate of evaporation is maximum at its boiling point when the liquid has maximum vapour pressure.