DYNAMICAL BEHAVIOR AND STRUCTURE OF CONCENTRATED WATER-IN-OIL MICROEMULSIONS IN THE SODIUM BIS(2-ETHYLHEXYL)PHOSPHATE SYSTEMS

Dynamical structures are elucidated by the dynamic light scattering (D LS) technique and theological measurements for sodium bis(2-ethylhexyl )phosphate (SDEHP)/n-hexane/water/sodium chloride systems in the conce ntrated region of the surfactant. For DLS measurements, the microstruc ture is deduced from the behavior of the initial relaxation and the lo ng-time relaxation which appears when the volume fraction Phi(BS) of t he molecular aggregates exceeds 15%-20%. Both of them indicate that th e network structure formed in the region Phi(BS) greater than or equal to 0.15-0.20 becomes confined into a smaller size scale as Phi(BS) go es up. The long-time relaxation may reflect the spontaneous structural relaxation of the transient network like a first-order reaction indep endent of scattered wave number and the overall diffusive relaxation, each rate of which also shows a steep reduction in the range 0.2 less than or equal to Phi(BS) less than or equal to 0.4 With increasing Phi (BS) Rheological measurements indicate that the relaxation time of ela stic stress is shorter for more concentrated SDEHP-microemulsions. Thi s is interpreted in terms of the formation of a more dense network str ucture for more concentrated systems, On the other hand, the character istic time for recovery of the network structure from a broken state b y a steady shear flow lengthens as Phi(BS) increases. The geometrical shape of the individual aggregates in the dilute region strongly influ ences the dynamical behavior of the microemulsion in the concentrated region.