Coexistence Curve and Interfacial Width

The bilayer composed of two coexisting phases is characterized by two types of parameters. These are the compositions of coexisting phases ( )1 and ( )2 and the spatial extension of the interface separating these two phases. Experimental depth resolution p of an applied profiling technique is not a prime concern when measuring the profile ( )(z) plateaux yielding concentrations ( )1 and (j. However the interfacial width w may be evaluated properly only if the resolution p is assured to be smaller than the measured width wexp. A quadratic correction procedure is used as a standard to extract w from wexp  

It is of interest to trace the temperature dependence of these two types of observables. The locus of (jq and as a function of temperature yields the coexistence curve in the composition-temperature plane for the studied binary blend. A related data set on the interfacial width is plotted as a function of temperature. 

Both the equilibrium interfacial width and the coexistence compositions vary with the interaction parameter % (see Eqs. 8 and 10). The parameter % is, in turn, temperature dependent. The coexistence conditions, such as those of Figs. 7 and 8, may be described with different level of precision by various approaches yielding different forms of the % parameter [9]. 

For isotopic blends both approaches by Bates et al. [68,69] estimate properly the T 1 dependent (enthalpic) term of the more realistic interaction parameter form  

The higher precision in describing the experimentally determined coexistence conditions is obtained by assuming that the % parameter is -dependent. This has been known at least from the late 1970s (see [9] and references therein). Recently the interaction parameters, linear [125, 126] as well as parabolic with 

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