Adding salt (NaCl) to the solution of a basified emulsion of water and some other non-polar solvent tends to break down emulsions. This is true for some emulsions.
Some emulsions are caused by compounds that are soluble in both water and the non-polar solvent and because they don’t prefer either, they tend to cause both to mix, forming a third layer of emulsion. Sometimes the emulsion is so bad that there’s only 1 layer, no water layer or non-polar layer present.
Salt works by raising the polarity of the water, making less molecules soluble in it. This makes many molecules that are soluble in both the water and the non-polar solvent more soluble in the non-polar solvent. Part of this is caused by the salt taking up space in the water. The more salt you have in the water, the less space there is for other compounds. As the water becomes saturated with salt, many compounds becomes insoluble in the water and get pushed out by the salt. This technique doesn't work for all emulsions and actually makes some emulsions worse!
If you’ve ever done a salt based harmala extraction, it works by saturating the water with salt, causing many hydrochlorides to be insoluble in water. It just so happens that harmine and harmline HCl are insoluble is salt water, so they precipitate out of the water.
After adding the salt to clean an emulsion, it’s a good idea to heat the solution (no fire or sparks of course) to about 90 C or 10 C below the lowest boiling point of the non-polar solvent. This can help break the emulsion much faster. Be careful with this. With some solvents like naphtha, this is a fire hazard. But with solvents like DCM, there’s no danger of fire.
