Russell, Chishill asked:
Why do washing powders remove stains but not dyes?
So an answer to the first part; one of the main and important ingredients used is surfactants and the surfactant molecule is clever in the way that on one side it has a hydrophobic component, that’s a water-hating molecular chain. And on the other side, a hydrophilic water-loving component. The hydrophobic chain finds itself sticking to the stains on your clothes and the hydrophilic heads have a stronger attraction to water. They’re able to surround the dirts and roll it up into a small globular-type ball and the end result is that they’re able to lift the stain from your cloth, into the wash water.
Some of our detergents contain enzymes which are naturally derived molecules. Generally, we use different enzymes such proteases which break down proteins and amylase which breaks down starch and then finally, another major ingredient that we use, like most other detergent manufacturers is bleach. The bleach turns the stain into more soluble colourless particles that can be easily removed and carried away into the wash water. So, in actual fact, it can remove bleachable dye stains. So, to kind of answer the other part of the question, laundry detergents can remove certain dyes, as well as stains.
Most dyes are composed of molecules that these ingredients can’t target. Surfactants can't globuralize the dyes, nor can enzymes gobble them up, unless they're vegetable-based. But bleach can effect dyes and this is why, washing powders designed for colored clothes don’t contain any bleach.
I guess that the dye is more tightly bound to the fabric than the stain and hence is more difficult to remove. Therefore the powder isn't doing the discrimination between colour and stain, it's down to the chemistry. chris, Fri, 19th Jun 2009
Usually when dying fabric personally I add Vinigar to the final rince to set the color so it does not bleed or wash away with detergent..I always assumed the acid composure of the vinega made the color more permanant and therefore better for longer lasting color.
Biological washing powders use enzymes which could be responsible for their specificity: (breaking down stains but not dyes). RD, Mon, 22nd Jun 2009