Phase inversion characteristics observed upon water dilution of a bidiscontinuous phase

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 alcohol (7.5:2.5 wt ratio)], and water. Within the pseudo-ternary system, water dilution line, termed N91 (90 wt% surfactant phase and 10 wt% oil phase), was found to represent transparent and thermodynamically stable compositions from concentrate to high water dilution (>95 wt% water). Despite that the system was found to be directly aggregated, dilution line N91 exhibited classical L2→L1 phase inversion characteristics (at 39 wt% water). To explain this phenomenon, a novel structural interpretation regarding the observed inversion as an obstruction of the bidiscontinuous phase, consisting of oil and water-segregated domains, is proposed. The evaluation of dilution line N91 was based on electrical conductivity, SAXS, SD-NMR, rheometry, DSC and cryo-TEM.The structural transitions along water dilution 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 concentration of low CPP (critical packing parameter) surfactant plays a major role in determining the system's geometry throughout the water dilution line. As a result, the proposed interpretation of the structural inversion observed along dilution line N91 differs from the classical U-type inversion interpretation (L2 →bicontinuous →L1).