[chris (OP)]

What makes some stains more stubborn / tricky to remove (if at all) than others?

Some things wash off easily, but others, once they are on a surface or a material, cannot be removed again. Tomato sauce and white fabrics and red-wines and cream carpets spring to mind...

What is the chemical basis for this?

[wolfekeeper]

Stains are typically pigments.

Tomato colour is mostly lycopene. Lycopene is very, very red, and is non polar and therefore insoluble in the 'universal solvent' aka water, and so tends to soak in and stay in fabrics, but is soluble in solvents:

https://en.wikipedia.org/wiki/Lycopene#Staining_and_removal

It also has a nasty habit of diffusing into plastics where it can't be removed.

Red wine, coffee etc are different, it's the tannins that are widely present in plants that are the biggest problem. I'm not entirely sure what the chemical basis is, but googling around seems to imply that tannins actively bond to fabrics as they dry. Best to remove them while they're still wet, but the stain can often still be removed later by using acids or alkalines to break them down.

[chris (OP)]

so tends to soak in and stay in fabrics

So is it binding to non-polar moieties in the chemicals that make up the fabric? Like the cotton fibres, for instance?

[alancalverd]

I'm not entirely sure what the chemical basis is, but googling around seems to imply that tannins actively bond to fabrics as they dry.

Yup! Tannic acid and vegetable tannins are used as mordants - substances that bind pigments to fabrics. Vital in the dyeing industry, and a pain to the well-dressed sloppy eater.

[wolfekeeper]

so tends to soak in and stay in fabrics

So is it binding to non-polar moieties in the chemicals that make up the fabric? Like the cotton fibres, for instance?

I don't think so, or not significantly, it's just more difficult to mobilise. You can mobilise it with solvents or even oils. Lycopene is actually a solid, and in tomato sauce it's presumably dissolved in the small amounts of fats and oils naturally in the tomato, and they would soak into the fabric, so detergents help as well. Lycopene is also broken down by UV, so leaving it out in the sun helps.

[chiralSPO]

(from the top of my head)

There are multiple types of stains, each of which have fairly different chemical explanations:

1) A strongly colored compound (like a pigment) gets on the article (for now lets just say it's cloth, though anything can get stained...), and stays there because of non-covalent bonds.

These stains can be removed by washing the pigment away using solvents with or without surfactants, or they can be degraded into less-colorful compounds, which is what bleach does, and either left in place or washed away.

2) A strongly colored compound becomes covalently attached.

This will not allow simple rinsing away of the stain. Chemical modification of the pigment, or cleavage of the attaching bonds would be required.

3) The original pigment (or actual material itself) undergoes a chemical reaction that produces a change in color. (ever notice how blue fabric often turns bright orange if you get some bleach on it?)

In this case there is no residue to be washed away. The article itself is fundamentally changed. Either find a way to reverse the reaction (this is not very common, but some color-changing chemical reactions are quite reversible, for instance pH-sensitive pigments aka indicators), or re-dye the article.

4) Colorless compounds that do not react with the item chemically, but still change how it interacts with light because of refractive index, which will change the way light scatters, like oil stains. Or there can even be "solvatochromic" effects on the dyes if you spill something more exotic on your clothes. (many dyes have a very significant change in dipole moment when they absorb light, so being in environments with different dielectric constants can have noticeable effects on the energy of the light absorbed.)

Here you just have to wash the offending liquid away, or wait for it to evaporate...

[chris (OP)]

@chiralSPO - thanks - brilliant answer.

[chiralSPO]

Any time! 

[MirrorBall7]

It all depends on the composition of the wine or, for example, greasy spots. The removal of pigments from the fabric is different for everyone, so nature has done it. Therefore now, they have made the most effective stain removers that are simultaneously non-toxic. For example, I always use <stain remover> for my clothes and my children's clothes. Initially, it is intended as a stain remover for kids' clothes. Still, I am happy to use this product for myself and my family because its chemical composition is the most gentle for fabric and human skin. If it accidentally falls on the children or me, nothing will happen.