Stains are typically removed by one or more of
the following strategies:
1. Dissolve the
stain in a solvent. This is basically how dry
cleaning works. A stain made of hydrocarbons (like
bicycle grease) can be removed by a hydrocarbon
solvent (like gasoline). Stains from fatty
substances like butter and chocolate can be
removed with organic solvents. The rule is "like
dissolves like": pick a solvent that is similar to
your stain, and you can wash the stain out.
2. Dissolve the stain using a surfactant.
Surfactants allow water to wet fabrics better, and
they can surround molecules in a stain and carry
them into solution.
Soap is a surfactant;
so are the sulfonates listed in the ingredients
for many spot removers and carpet cleaners. A
surfactant molecule contains long hydrocarbon tail
with a small polar head. The hydrocarbon tail of
the soap molecules surround (dissolve) grease,
while the polar ends dissolve in water; the net
result is that the grease/soap complex is water
soluble and gets washed away. This process is
called 'emulsification'. You can see it working if
you add soap to some oil-and-vinegar salad
dressing. The vinegar layer of the dressing gets
cloudy because the soap has surrounded little
droplets of oil and prevents them from rejoining
the oil layer.
Sulfonates are more often
used than soap these days. They work better in
'hard' water, which causes soap molecules to
precipitate from solution (forming bathtub rings
and 'soap scum').
3. Eat the stain.
Oxidizing agents like chlorine bleach, peroxides,
and borax attack the links that hold some
long-chain organic molecules in stains together.
The little fragments that are left are
water-soluble, and they wash away.
cleaning agents contain enzymes that speed up
reactions that digest proteins or fats in
4. Hide the stain Some detergents
contain 'whiteners' that whiten the fabric and do
nothing to clean! They contain a material that
absorbs ultraviolet light and re-emits it as
visible light. Get a black light shine it on your
clothes in a darkened room; if your laundry
detergent contains whiteners, your clothes will