In grade school chemistry, one may recall that vaporization is a process during which a liquid turns into a gas. There are three types of vaporization: evaporation, boiling and sublimation. The first two types are explained here, for the benefit of novice formulators.
Evaporation is a phase transition from the liquid phase to gas phase that occurs at temperatures below the boiling temperature at a given pressure. Further, evaporation is the process by which molecules in a liquid state (e.g. water) spontaneously become gaseous (e.g. water vapor). Generally, evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed to a significant volume of gas. Evaporation tends to occur at the surface.
Boiling is also a type of phase transition; however, it is the rapid vaporization of a liquid. Boiling typically occurs when a liquid is heated to its boiling point, the temperature where vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. Therefore, a liquid may boil when the pressure of the surrounding atmosphere is sufficiently reduced, such as in the use of a vacuum pump or at high altitudes.
The evaporation process is important to the cosmetic chemist since moisture on the surface of the skin is always evaporating, resulting in dry skin. Dry skin is a common complaint of dermatologist patients. Proper treatment generally includes application of a topical product. The evaporation of water from skin also can be slowed by addition of oils to the skin, referred to as emollients, or water can be can be replenished in the skin by use of humectants.