Interactions and phase transitions in micellar and microemulsion systems studied by small angle neutron scattering

Owing to their amphiphilic nature, surfacant molecules spontaneously self-assemble into various forms of aggregates in aqueous and hydrocarbon solvents. These aggregates are often so well defined and sufficiently uniform in size that the suspension can be treated effectively as one- or two-component supramolecular liquids. Ionic surfacants as sodium dodecyl sulfate (SDS) form normal micelles in water. These micelles together with their counterions can be regarded as a strongly coupled two-component fluid. On the other hand sodium di-2-ethylhexylsulfosuccinate (AOT) forms reverse micelles in hydrocarbons (oils). These reverse micelles can solubilize large amounts of water and become microemulsions. These microemulsion droplets bear no net charged and interact with each other via Van der Waals forces analogous to atoms in simple liquids. Thus, ATO microemulsion system shows a gas-liquid type phase transition. BY exploiting the existing liquid theories the SANS spectra can be satisfactorily analysed in terms of well-defined interparticle interactions. For ionic micelles one can obtain the surface and aggregation number at arbitrary concentrations and for microemulsions one obtains the range depth of the attractive interaction near the critical point. (orig.)