Abstract:

This study presents a directly aggregated pseudo-ternary system. The three apexes of the investigated system represent a surfactant phase [Tween 80:propylene glycol (9:1 wt ratio)], an oil phase [isopropyl myristate:benzyl alc. (7.5:2.5 wt ratio)], and water. Within the pseudo-ternary system, water diln. line, termed N91 (90 wt% surfactant phase and 10 wt% oil phase), was found to represent transparent and thermodynamically stable compns. from conc. to high water diln. (>95 wt% water). Despite that the system was found to be directly aggregated, diln. line N91 exhibited classical L2→L1 phase inversion characteristics (at 39 wt% water). To explain this phenomenon, a novel structural interpretation regarding the obsd. inversion as an obstruction of the bidiscontinuous phase, consisting of oil and water-segregated domains, is proposed. The evaluation of diln. line N91 was based on elec. cond., SAXS, SD-NMR, rheometry, DSC and cryo-TEM. The structural transitions along water diln. line N91 were found to be as follows: 'pseudo L1' (pseudo direct surfactant-oil aggregates) → bi-discontinuous structure (of which partial is a hexagonal H1 mesophase) → L1. We concluded that the high concn. of low CPP (crit. packing parameter) surfactant plays a major role in detg. the system's geometry throughout the water diln. line. As a result, the proposed interpretation of the structural inversion obsd. along diln. line N91 differs from the classical U-type inversion interpretation (L2 →bicontinuous →L1). [on SciFinder(R)]