Detergents are less or non bio-degradable when compare with soaps. For detergents to be bio-degradable in nature, the non-polar part of the molecule should:
Soaps are formed by the reaction of alkaline, fats, and oils. These are the sodium salts of fats and oils. Fats and oils can easily be broken down into simple molecules by microbes and hence soaps are biodegradable.
Detergents are synthetic compounds generally ammonium or sulfate salts of long chain carboxylic acids. These synthetic compounds cannot be broken down into simple molecules by microbes and hence detergents are non-biodegradable.
If detergents have a straight chain, then it could be easily be broken by molecules.