What is the happening?
Dish detergent, shampoo, hand soap, and anything else you might use to wash things contain surfactants. A surfactant is usually a molecule with a hydrophilic "head" and a hydrophobic "tail". This means that it is able to bridge between water soluble and oil-soluble interfaces. An anionic surfactant is one with an anionic functional group at the "head" position. They are the most common surfactants used in industry, typical examples are lauryl sulfates, laureth sulfates, sulfonates and phosphate esters. If your soapy liquid of choice has one of these chemicals in its long list of ingredients, it should make a slime when you add just the right amount of salt to it.
When surfactants are in solution they can sometimes arrange themselves into these aggregate units called micelles, where all the surfactant molecules have their "tails" pointing inwards and "heads" pointing outwards, or vice versa. In the detergents that we used to make the slime, the micelles formed have anionic "head" groups sticking out into the solution, with their hydrophobic "tails" in the centre. This gives the micelles a certain charge density on its outer surface, which can be affected by the addition of salt. The ions from the dissolved salt, at the right concentration, can cause bigger micelles to form and pack closer together, resulting in a thicker mixture or gelling effect. But if salt concentrations are pushed beyond that optimal level, the micelle structures can breakdown and the mixture viscosity will decrease.
For more slime recipes and experiments, check out our comprehensive guide to SLIME!
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