Abstract
Gemini surfactants are made up of two surfactant-like moieties connected by a spacer group. This article reviews their phase behavior, considering successively the thermotropism of pure gemini surfactants, the lyotropism in gemini surfactant/water mixtures, and the ability of gemini surfactants to give rise to microemulsions in the presence of oil and water (and eventually of cosurfactant). From the qualitative viewpoint gemini surfactants have been found to behave similarly to the corresponding monomeric surfactants as far as phase behavior is concerned. However, the reported results indicate that the length of the spacer and its chemical nature represent very effective parameters for tuning the thermotropic and lyotropic behaviors. They are the key structural parameters that bring originality and usefulness to the gemini surfactant concept.