SMALL-ANGLE NEUTRON-SCATTERING FROM MICROEMULSIONS NEAR THE DISORDER LINE IN WATER FORMAMIDE OCTANE-CIEJ SYSTEMS

Small-angle neutron scattering measurements were performed on symmetric microemulsions containing equal volume fractions of water/formamide, n-octane, and a sufficient amount of n-alkyl polyglycol ether (C(i)E(j)). By changing the surfactant chain length in the order C8E3, C6E2, and C4E1 in pure water, as well as for given C8E3 and C6E2 by increasing the relative amount of formamide in the water/formamide mixture, the amphiphilic strength (the amphiphilicity) is reduced. We observe that the characteristic scattering peak becomes weaker and its position moves into q = 0 showing that the microstructure becomes disordered. However, a q-2 and a q-4 behavior for large q in film and bulk contrast, respectively, is still observed indicating the persistence of internal interfaces. As the microstructure is further weakened, 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 determine that this line has been passed from fits to the bulk scattering intensity. Simultaneously, the film scattering intensities show a transition from correlated to uncorrelated films as the disorder line is passed. Closely related to the weakening of the microstructure is the occurence of a nonwetting --> wetting transition in the vicinity of a Lifshitz line. Roughly speaking, microemulsions, that do show a scattering peak do not wet the water-oil interface. As predicted by Landau-Ginzburg theories, the wetting transition occurs on the microemulsion side of the disorder line. As the amphiphilicity is even further reduced, a tricritical point is reached with the interesting observation that substantial local structure still persists.