Published by Australian Academy of Science
Glossary | Putting on a good face – the chemistry of cosmetics

active ingredient. The ingredient that is responsible for producing the desired effect of a mixture of ingredients and for giving the product its main characteristic. The active ingredient is not necessarily the most common ingredient in a product.

amino acid. The basic building block of protein. All amino acids contain an amino (NH₂ ) end, a carboxyl end (COOH) and a side group (R). In proteins, amino acids are joined together when the NH₂ group of one forms a bond with the COOH group of the adjacent amino acid. The side group is what distinguishes each of the amino acids from the others.

There are 20 common amino acids: alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine.

carcinogen. A substance that is known to cause cancer.

humectant. A substance that can absorb water from moist surroundings.

hydrophilic. Describes a substance that absorbs, dissolves in or is attracted to water.

lead carbonate. A naturally occurring white amorphous powder with a chemical formula of PbCO₃. Used in exterior paints, ceramics, cements, processing of parchment and as a laboratory reagent.

lipophilic. Describes a substance that dissolves in or is attracted to fats, oils or other lipids. Lipophilic functional groups or molecules prefer to be in an environment where there is no water.

occlusion. The act of closing, shutting or stopping up. In cosmetics this usually refers to a shield or film that is spread onto the skin to slow or prevent moisture evaporation. This shield or film is usually made up of materials, such as oils and waxes that cannot be penetrated by water.

patch test. Test used to identify allergies due to chemicals coming in contact with skin. Individual chemicals are applied separately to the skin and then the skin is observed for reactions over a few days. For more information see Patch testing (dermatology.co.uk).

pH. The pH scale is used to measure the strength of acids and bases (or alkalis). The acid strength in the human stomach is about pH 2. Alkalis such as caustic soda and basic household cleaners have a pH of about 12 to 14. Neutral is pH 7, (ie, neither acidic or alkaline). The scale is logarithmic, so pH 4 is ten times as acidic as pH 5 and pH 2 is ten times as acidic as pH 3, and so on. For more information see About soil pH (National Aeronautics and Space Administration, USA).

polymer. Polymers are large molecules that are made up of many units (monomers) linked together in a chain. There are naturally occurring polymers (eg, starch and DNA) and synthetic polymers (eg, nylon and silicone). More information can be found at The basics – polymer definition and properties (Plastic Resource, USA), Introduction to polymers (Case Western Reserve University, USA) and History of polymers and plastics for teachers (Hands On Plastics, American Plastics Council).

polysaccharide. A carbohydrate made up of a long chain of simple sugar molecules joined together. Starch and cellulose are examples of polysaccharides.

skin allergy. An allergy is a hypersensitivity to substances in the environment which do not bother most people. Allergy to cosmetics usually manifests as a rash on the skin where the product has been applied. This condition is known as allergic contact dermatitis, and is often due to fragrances and preservatives in the cosmetic product. For more information see Allergic contact dermatitis (New Zealand Dermatological Society).

surface tension. A property of liquid surfaces that causes the surface layer to behave like a thin elastic 'skin'. Molecules in a liquid have attractive forces that hold them together. Molecules on the surface are attracted to molecules from all sides and below, but not from above. This results in a downward and sideways pull on molecules on the surface layer. For more information see Surface tension (Georgia State University, USA).