Now for the science bit

Dr Daniel Smart

Alcohol belongs to a large and diverse family of chemicals.

Alcohol is the term used for an organic chemical containing a ‘hydroxyl’ group - essentially, one oxygen atom covalently bonded to one hydrogen atom (R-OH).

The -OH functional group gives alcohols their unique physicochemical properties with respect to highly-similar chemicals, e.g. aldehydes and ketones.

Alcohols are immediately recognisable by name. The word suffix is OL.

The alcohol contained in wines, beers and spirits is known as ethanol, or ethyl alcohol, and can be made by various means such as fermenting sugars and starches or it can be artificially synthesized.

Ethanol is a simple alcohol described as aliphatic or 'grain' alcohol.

Aliphatic is the term used to describe the open-chain nature of carbon atoms.

Simple alcohols are named according to the number of carbon atoms (C) present in the molecule. In addition to the -OH group, each C atom is saturated with hydrogen atoms.
Other examples of simple alcohols include methanol (CH3OH), ethanol (C2H5OH), propanol (C3H7OH), butanol (C4H9OH), pentanol, hexanol, heptanol, octanol, nonanol, decanol, undecanol and dodecanol.

 1C 2C
3C
4C
5C
6C
7C
8C
9C
10C
11C
12C
 METH ETH
PROP
BUT
PENT
HEX
HEPT
OCT
NON
DEC
UNDEC
DODEC

Many cosmetic products contain denatured or specially denatured alcohol.

Denatured or specially denatured (SD) alcohol is simply ethanol with added adulterants, e.g. methanol and isopropanol, which renders it useless for consumption in beverages.

Simple alcohols are not ideal cosmetic constituents as they can dry and irritate the skin.  However, they are often used because of their antibacterial effects.

Ethanol and methanol are highly homologous and only differ in the C chain, i.e. in this instance by 1 C and 2 H’s.  The source of production, e.g. from grain or gas, plays no role in nomenclature.

Alcohol-free cosmetics are those that omit both methanol and ethanol, as well as propanol, butanol etc as they are all akin to ‘grain’ alcohol.

Less simple/more complex alcohols may not be aliphatic but aromatic. Their molecular structure is different to simple alcohols.

They contain an unsaturated ring structure in the carbon chain, e.g. a benzene ring.

The addition of such molecules make them dissimilar to aliphatic ‘grain’ alcohols.

An exception to this rule is benzyl alcohol. Benzyl alcohol belongs to the aromatic family but its molecular structure is closer to methanol.

Although these compounds are structurally quite different to methanol, ethanol etc,
benzyl alcohol contains an aromatic benzene ring and no long C chain (Cf. Fatty alcohols described below) and is ‘more’ similar to methanol, ethanol etc.

Examples of aromatic alcohols include phenoxyethanol - often used as a bactericide, insect repellent, antiseptic and solvent - and methylparabens used as an antimicrobial preservative.

Methylparabens contain a methyl group (R-CH3) like methanol (CH3OH) but, as methylparabens also contains an aromatic benzene ring and a carboxymethyl group, they are not aliphatic and so are not the same as ethanol.

Some people may have been put off using products containing methylparabens due to constant media speculation and uninformed views which suggest these may be harmful.  
Benzene and some benzene derivatives are known carcinogens. However, evidence is limited for other benzene-related compounds methylparabens.  Methylparabens are not carcinogenic at the levels used in cosmetics.

There is another group of alcohols known as fatty alcohols.

These are usually aliphatic. Simple alcohols ranging from octanol upwards can be classed as fatty alcohols. These include 16 C chain cetyl alcohol (CH3(CH2)15OH), 18 C chain stearyl alcohol (CH3(CH2)17OH) and cetearyl alcohol - a mixture of cetyl and stearyl alcohol. An exception is Eugenol which contains a branched C chain. Due to its molecular structure, Eugenol falls into the aromatic category.

Fatty alcohols differ from simple alcohols such as methanol and ethanol due to their chemical make up.

The major difference between these compounds and simple alcohols, e.g. methanol and ethanol etc, is the length of the aliphatic C chain.

Fatty alcohols contain LONG chains (> 8 C atoms), whereas, simple alcohols have SHORT chains (< 8 C atoms).  From this you may infer that simple alcohols ranging from octanol upwards can be classed as fatty alcohols.

Examples of these include 16 C chain cetyl alcohol (CH3(CH2)15OH), 18 C chain stearyl alcohol (CH3(CH2)17OH) and cetearyl alcohol - a mixture of cetyl and stearyl alcohol.

These are used in cosmetics for a variety of reasons. They are used as a lubricant to stabilise oil, water emulsifier (both cetyl), viscosifier/thickener, skin softener, non-ionic surfactant (all stearyl) and stable emulsifier (cetearyl).  These possess no drying or irritant properties.

About the Author

Dr Daniel Smart is a Research Scholar in the Department of Pathology, New York Medical College, USA.  Prior to this he completed a Ph.D in Biochemistry and an MSc in Toxicology at the University of Birmingham, UK, and a BSc in Pharmacology at the University of Liverpool, UK.