Pre-soak stain removers are used when clothing has a stain that we expect will be too difficult for normal laundry detergent to remove.
Normal laundry detergents often contain inexpensive ionic surfactants (such as sodium laureth sulfate) that work best in warm or hot water. But hot water can help set some stains. A spray-on pre-soak stain remover like Shout™ has non-ionic surfactants that start to work cold, as soon as they are sprayed on. Non-ionic surfactants work in hard water (unlike many ionic surfactants), work in acid or alkaline solutions, and have good cleaning, foaming, and emulsifying properties that can be delicately tuned by controlling how the molecule is made.
Tuning these surfactants is a matter of controlling how long the water loving end is, and how long the oil loving end is.
In the drawings shown above, three different polyethoxylated linear alcohols are shown. These are non-ionic surfactants that are designed to have different lengths of water loving ends (the ends on the left). Lauryl alcohol is the base. It is the chain of 12 carbon atoms on the right side of the molecule. It dissolves in fats and oils.
The left end of the molecule has (in these three cases) 3, 6, or 12 molecules of ethylene oxide added. The more ethylene oxide a molecule has, the more easily it will dissolve in water.
Molecules with less than 10 ethylene oxide units dissolve mainly in fats and oils. Those with more than 10 are water soluble. Those with 7 to 11 ethylene oxides are good for making water-in-oil emulsions (similar to butter or margarine). Those with 12 to 16 units are good for making oil-in-water emulsions (like mayonnaise). Those with 11 to 14 units are good for allowing water to wet fabrics easily. Those with 12 to 15 units make good detergents.
Shout™ uses several polyethoxylated linear alcohols for their different properties. Some smaller ones are used to lift oils from the fabric. Some larger ones are used to keep the oils in the water after they have been lifted off.
The alcohols can be linear (as in the lauryl alcohol shown) or branched, or have more complicated structure that includes cyclic molecules (loops). Linear alcohols are used where we want the detergent to break down easily in the environment. They pollute less, because bacteria can eat them easily.
Some formulas of Shout™ also include enzymes that help break down the proteins in blood and grass stains, and acrylic polymers to help them stay on the stain longer while in the washing machine.