Phase behavior of the quinary mixture: H₂O-NaCl-dodecane-pentanol-SDS. Origin of the winsor III equilibria

Abstract

The phase behavior of the quinary mixture H₂O-NaCl-dodecane(D)-pentanol(P)-SDS is represented at fixed P and T in a serie of pseudoquaternary diagrams, each corresponding to a constant brine salinity. In practice, each diagram is itself constructed by examining a series of cuts X in which the brine over surfactant ratio is kept constant. This procedure allows one to describe completely the extent of the three-phase region Winsor II (W III) and to determine its mode of appearance. Our results show that this region, which starts to be observed at 4 g/l NaCl in water is generated by an indifferent state. In such a state, the compositions of the three coexisting phases are located along a straight line in the four-dimensional space of representation. As salinity goes above 5.5 g/l NaCl, the volume W III is bounded in the oil-rich region by a line of critical end points, LC₁. A second line of such points, LC₂, occurs in the water-rich region at salinities higher than 8.1 g/l NaCl. Then, the three-phase region W III appears by two critical end points only when salinity is higher than 8.1 g/l NaCl. Our experiments show that the lines LC₁ and LC₂, respectively, originate in the quaternary mixtures H₂O-D-P-SDS and H₂O-NaCl-P-SDS.