Conditioning Agents for Hair and Skin

Humectants in Personal Care Formulation 99
freezing point and to affect the solubility of certain compounds. Also, they
have been shown to increase product viscosity, retard the evaporation of water,
and act as a preservative and an emulsion stabilizer.
One of the aspects of humectants that is important, but not always appreciated,
is their ability to couple mutually incompatible materials together. The
humectant can act as a co-solubilizer for many materials in creating a clear
solution. The classic example of this is the coupling ability of glycerin with
sodium stearate in the formation of "clear" bar soaps. In fragrance solubilization,
materials such as propylene glycol, or dipropylene glycol are often
used as a "co-solvent" in conjunction with solubilizing materials such as
polysorbate-20.
Polyols are known to depress the freezing point of water solutions. The
CRC Handbook of Chemistry and Physics includes a number of tables delineating
the freezing-point depression of polyols in water solutions by concentration
(6). Humectants are added to water-based systems, especially surfactant
cleansers, to maintain clarity at low temperatures during shipping and to prevent
bottle cracking. The clarity of a surfactant cleansing system is maintained
chiefly through the polyol-humectant's ability to act as a co-solvent and antifreeze
for fatty materials, making it more difficult for these materials to come
out of solution. As the ambient temperature decreases, the longer-chain-length
fatty materials freeze out of solution first, followed by the shorter-chain-length
materials. At these reduced temperatures, their solubility in the surfactant
solution is significantly reduced. The addition of a polyol humectant helps to
enhance the solubility of these fats in the system.
Bottle cracking is a phenomenon that may occur when water-based systems
in sealed containers with insufficient expansion space are exposed to temperatures
low enough for the formulation of ice. As ice crystals form, an expansion
of the water contained in the product occurs that may be great enough to stress
the packaging to the point of breakage. The polyol-humectants act as antifreeze,
much the same as the antifreeze in a car's radiator, by reducing the
freezing point of water to a point where ice does not form at the temperatures
expected during shipping and storage.
VII. TYPES OF HUMECTANTS
As suggested, humectants are conditioning agents which have the ability to
attract and bind water. Because of this they are able to regulate the exchange
of moisture between a surface, such as hair or skin, and the air. By increasing
the level of moisture in the skin, its condition is improved. Hair also exhibits
improved condition when it is moisturized; however, the effectiveness of
humectants in hair is less than that of skin.