Thermodynamic, interfacial, and structural properties of polyphasic microemulsion systems

Abstract

Three-dimensional phase diagram of the water 2% NaCl, dodecane, pentanol-sodium octylbenzenesulfonate system has been investigated. Topology of the three-phase Winsor III volume has been studied in detail. Brine can be considered as a pseudocomponent, therefore three-phase states are univariant at P and T constant. The representative points of compositions of aqueous, microemulsion, and organic phases are located respectively on three curves Γ 1, Γ 2, Γ 3, which have been characterized. Γ 1 and Γ 2 on the one hand and Γ 2 and Γ 3 on the other hand are converging; they form a single curve. This implies that the three-phase volume of the studied system presents two critical end points K 1,2 and K 2,3 where the microemulsion phase becomes respectively identical to the aqueous and the organic phases. γ mo, γ mw, and γ wo interfacial tensions vary all along the Γ 2 curve. Correlations between γ mo, γ mw, and γ wo values and the water content in the middle phase have been established. The lowest values are obtained in the vicinity of critical end points; in the intermediate part of the Γ 2 curve far from both critical end points, the tensions are still low. Application of scaling law theory shows that two mechanisms are operative in the achievement of low interfacial tensions in microemulsion systems. In three-phase states γ mo and the lowest γ mw interfacial tensions are associated with critical phenomena and thick interfaces; in two-phase regions, these tensions are probably due to a thin interface of alcohol and surfactant molecules. Light scattering experiments show that only microemulsion middle phases located either in the water-rich ( χ H 2O m > 70%) or the oil-rich ( χ H 2O m < 25%) parts of the Γ 2 curve give rise to light scattering characteristic of critical states.