On the Stability Range of Microemulsions: From the Tricritical Point to the Lamellar Phase in Water/Formamide‐Octane‐CiEj Systems

Symmetric microemulsions containing equal volume fractions of water/formamide, n‐octane and a sufficient amount of n‐alkyl polyglycol ether (CiEj) were studied for various surfactant chain lengths ranging from C4E1 to C14E5 in pure water, as well as for given C4E1 with increasing relative amounts of formamide in the water/formamide mixture (ψ in wt%). In both cases the amphiphilic strength (the amphiphilicity) is changed. Increasing the amphiphilicity one observes, as is well known, the occurrence of the lamellar phase. For the balanced state the lamellar phase occupies a large portion of the isothermal Gibbs triangle as is demonstrated with C10E5. We argue that with further increasing amphiphilicity the three phase body disappears at a four phase point. Decreasing the amphiphilicity the three phase body disappears passing a sequence of lines. First a nonwetting‐wetting transition can macroscopically be observed. Then, a disorder line is passed at which the behavior of the real space correlation function changes from a damped oscillatory behavior to a monotonically decreasing one. We determined previously that this line has been passed by small‐angle neutron scattering. Further, a Lifshitz line as predicted by Landau‐Ginzburg theories occurs until, as the amphiphilicity is even further reduced, a tricritical point is reached at which the three phase body disappears altogether. The results presented may serve as a basis for further experimental research and also stimulate the theoretical description of such systems.