Question: How would you choose a surfactant based on the oil to water ratio of an emulsion?

Ruhina Miller answered on 15 Jun 2015:

In general, the ratio of oil:water doesn’t matter because of something called “Bancroft’s rule” – the phase in which the emulsifier (i.e. the compound that is stabilising the oil and water phases, in this case, the surfactant) is more stable in will be the main or continuous phase.

If I wanted an O/W emulsion for instance – drops of oil in bulk water, I would choose a surfactant that has a high hydrophilic-lipophilic balance (HLB) value. This is because high HLB value surfactants prefer the water or aqueous phase and would stabilise it with respect to the oil phase :).