Biphasic region, interactions between

Di-(C7-Glu) also has a biphasic H/Li phase forming at 62 wt% surfactant existing to =60 °C. The H] phase melts over the range 62-80°C. Between the H and 1 phases is another biphasic region between 71 and 75 wt%. This, and the L( phase exist to >90 °C. A two phase loop occurs at low surfactant concentrations and temperatures (<8 wt% and <50 °C). The authors believe this to be the first observation of an upper critical solution temperature for a nonionic surfactant. The shape of the upper consolute loop is extremely unusual suggesting remarkable changes in micellar interactions. .. or the presence of impurities. One would be pleased to see these observations verified by other groups. Di-(Cg-Glu) shows only a L phase but with a wide miscibility gap to a dilute aqueous solution. This is not remarkable. 

The narrow biphasic gap in the diagram is essentially unaffected by interactions for negative values of x. On the other hand, if the interaction between solute segments is attractive then the biphasic region is abruptly broadened when x exceeds a small positive value. A critical point emerges at xi = 0.055. For xi values immediately above this critical value, the shallow concave curve delineates the loci of co-existing anisotropic phases, in addition to the isotropic and nematic phases at lower concentration within the narrow biphasic gap. At xi = 0.070 these phases co-exist at this triple point. 

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