Lower critical endpoint

In Figure 2.2-3 the curves Ig are the vapour pressure curves of the pure components which end in a critical point l=g. The curves l=g, h=g and h=g are vapour-liquid critical curves and the curves h=h are curves on which two liquid phases become critical. The points of intersection of a critical curve with a three-phase curve hhg is a critical endpoint. Distinction can be made between upper critical endpoints (UCEP) and lower critical endpoints (LCEP). The UCEP is highest temperature of a three-phase curve, the LCEP is the lowest temperature of a three-phase curve. The point of intersection of the hhg curve with a l/=g curve is a critical endpoint in which the li liquid phase and the vapour phase are critical in the presence of a non-critical l2 phase (h+(h=g)) and the point of intersection of the hhg curve with a h=h curve is a critical endpoint in which the two liquid phases h and // are critical in the presence of a non-critical vapour phase (h=h)+g)-... 

Three-phase, SLG equilibrium temperatures and pressures for binary mixtures of pentane and toluene with TPP are given in Tables II and III, respectively. A lower critical endpoint (LCEP) was observed for pentane-TPP mixtures, and is also denoted in Table II. 

Component Iv L Lower Critical Endpoint (LCEP) Experimental progressions frequently differ. 

Stacking the isothermal Gibbs triangles on top of each other results in a phase prism (see Fig. 1.3(a)), which represents the temperature-dependent phase behaviour of ternary water-oil-non-ionic surfactant systems. As discussed above, non-ionic surfactants mainly dissolve in the aqueous phase at low temperatures (2). Increasing the temperature one observes that this surfactant-rich water phase splits into two phases (a) and (c) at the temperature T of the lower critical endpoint cepp, i.e. the three-phase body appears. Subsequently, the lower water-rich phase (a) moves towards the water corner, while the surfactant-rich middle phase (c) moves towards the oil corner of the phase prism. At the temperature Tu of the upper critical endpoint cepa a surfactant-rich oil phase is formed by the combination of the two phases (c) and (b) and the three-phase body disappears. Each point in such a phase prism is unambiguously defined by the temperature T and two composition variables. It has proved useful [6] to choose the mass fraction of the oil in the... 

For systems containing oil, water, and a nonionic amphiphile, three-phase coexistence of the microemulsion with an oil-rich and a water-rich phase is possible only in a temperature interval Ti < T < T. At the lower critical endpoint T/, the microemulsion and the water-rich phase become identical, while at the upper critical endpoint the microemulsion merges with the oil-rich phase [83,84],... 

The question as to why the interfacial tensions are so low is an extremely interesting one, to which several answers have been given. It was once thought that the answer lay in the fact that the system is near a critical point [105]. That this is clearly not the case appears when one notes that the compositions of the oil-rich and water-rich phases that exhibit such low tensions are not at all similar in composition as they would be near a tricritical point. It has also been argued [84] that the interfacial tension is so low because the upper and lower critical endpoints are so close to one another in temperature. It is true that the tension of the oil/microemulsion interface, must vanish at one critical endpoint, and the tension of the water/microemulsion interface, cr, must vanish at the other. Furthermore, the oil/water interfacial tension, cto,, satisfies the inequality... 

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