What is the role of surface interactions in the stability of microemulsions?

[Dario56]

Unlike emulsions, microemulsions are thermodynamically stable. Its stability is often explained by entropy changes brought about by dispersing liquid in another liquid, however this can't be the whole story behind its stability since dispersing liquids also happens in regular emulsions and they aren't stable thermodynamically.

I am not sure if I am correct, but I think that stability of microemulsions is due to the interactions between molecules on liquid interphase and emulsifier being better (stronger) than between molecules in the bulk of the liquid droplet in similar way when liquid spreads on the surface of the solid forming no contact angle with the surface. If interactions between emulsifier and surface molecules are more favourable than surface molecules have lower potential energy compared to the bulk and there is tendency to increase surface area between molecules and emulsifier or in other words there is tendency to create emulsion.

I wasn't able to find this explanation anywhere online though, but I think it has sense. What are your thoughts?

 

[Hillbillychemist]

Emulsions form when a non-polar liquid is encapsulated by an "emulsifier" which is typically a molecule such as soap that has a non-polar end (usually a hydrocarbon) and a polar end that can be a carboxylate or ammonium salt.

These encapsulations are called micelles and are dispersed in water which is the continuous phase. In ordinary emulsions, these micelles are large and can merge thus separating the non-polar fraction from the aqueous phase. In microemulsions the micelles are small and the surface charge is sufficiently high that they repel each other.

Typically, the micelles are small enough that they do not disperse light as ordinary emulsions and thus appear clear. Microemulsions can be distinguished from solutions by looking at print through the liquid. Microemulsions will display some distortion of the image whereas solutions will not.