What does hydrophobic mean?
Hydrophobic is a term used to describe molecules that are not soluble in
water or other polar solvents. The word hydrophobic literally means "water hating". Other
ways to describe such molecules are Lipophilic (lipid loving) or non-polar.
The hydrocarbon tail of a soap molecule is hydrophobic and so is attracted to, and dissolves in
grease and dirt. The length of the hydrocarbon tail varies the solubility of the soap molecule in water. There is a balance
between the polar nature of the ionic head and the non-polar nature of the hydrocarbon tail and as the tail length increases, the
balance can tip too far in the lipid direction and the soap molecule becomes insoluble in water, and the soap will not work.
The balance is at its most efficient when the tail is 12 carbon atoms long - C12, and the solubility
of soap disappears at C22.
Author: Hannah Cope (document modification date: 28th April 2002)