Humectants: Cosmetic excipients, UNIT I Classification cosmetic products
Humectants include those categories of substances which are used for moisturizing the skin. Their inclusion in the cosmetic products is important because the drug absorption is reported to be directly proportional to moisture content of the skin, which is usually 20% by weight. The more moisture content, the more will be the drug absorption. Besides the moisture content, the skin also remains soft due to the presence of sodium pyrrolidone carboxylate, which is present in the skin. Skin also contains some other natural humectants that are salts, urea and proteins.
Humectants are different from emollients, which are primarily hydrocarbon containing excipients. Humectants, are always present in an ionized form and contain polar groups example hydroxyl, amide etc which not only attract water but also hold it by a process known as absorption and thereby help in maintaining softness and pliability of the skin.
Humectants are also different from dessicants, since desiccants unlike humectants retain water by the process of adsorption and not absorption.
Because of the moisturizing effect imparted by them, humectants find wide application in cosmetic as well as personal care products. Besides the moisturizing effect they also find wide application in enhancing the aqueous solubility and penetration of the drug through the skin, which inturn leads to an increase in the activity time of the drug.
The mechanism by which the humectants enhance retention of moisture in the skin can be explained by the following equation,
aw = p/po = ERH/100
where aw is the water activity
p is the partial vapor pressure above the formulation product po is the partial vapor pressure for standard conditions
ERH is the equilibrium relative humidity
The humectants when incorporated in a cosmetic formulation result in the reduction of ERH, thereby enhancing the tendency of the formulation to retain moisture.
Choice of Humectants
Humectants in order to be suitable for inclusion into cosmetic formulations should be:
- Non irritant and non toxic to the skin after application
- Economical and easily available
- Ability to absorb atmospheric moisture
- Ability to retain the absorbed moisture under normal atmospheric humidity
- Organoleptically acceptable, preferably being colourless with no intense odour and good taste
- Compatibile with majority of ingredients used in preparation of cometics
- Non-corrosive to materials using for packing cosmetics.
- Ability to remain in liquid form under normal conditions of storage.
Urea, gel containing aloe vera, Glyceryl triacetate, 1,3-Propanediol, honey are a few examples of humectants included in the category of unusual humectants.
Special uses of Humectants
Besides their normal use in retention of moisture in the skin, humectants are also used for their special properties. The other special uses of humectants include:
- Used as excipients in injectable formulations
- Used as laxatives when taken in large quantity.
- Used for the synthesis of a variety of other nontoxic and biologically safe surfactants. Examples include the use of polyethylene glycol for the synthesis of Brij and Tween surfactants.
- Used as a additive in foods for keeping it moist and tasty.
- Used as a component of antistatic coatings for application on plastic materials.
Types of Humectants
Humectants can be classified as:
These humectants suffer from problems of corrosion and incompatibility with the other ingredients of the formulation and hence they find limited application in the formulation of cosmetics. Calcium chloride is an example of inorganic humectants.
Because of the desired qualities like compatibility with most of the ingredients used in the formulation of cosmetics, nontoxicity etc, they are widely used in the formulation of cosmetics. A few common examples included under this category include mannitol, polyethylene glycol, ethylene glycol etc.
Metal Organic humectants
This class of humectants also suffer from drawbacks like incompatibility with other formulation ingredients, pronounced taste and tendency to cause corrosion and thus have found restricted application in the formulation of cosmetics. Sodium lactate is an example of metal organic humectants which has been used as a component of skin creams.
Reference: e-PGPathshala NME-ICT
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