COMPOSITION DEPENDENCE OF A FIELD VARIABLE ALONG THE BINARY-FLUID MIXTURE CRITICAL LOCUS

There are several classes of field variables used to extend the principle of critical-point universality to binary fluid mixtures. In particular, one can construct a field variable with sufficient flexibility to allow linearity in composition along the critical locus for the large class of binary systems with a critical line which is monotonic in temperature. This choice of hidden field variable has been used extensively in studies of vapor-liquid equilibrium, but a recent study of an alternative class of field variables suggests that linearity in composition along the critical locus cannot, in principle, be imposed exactly. It is demonstrated that several arguments presented by proponents of this proposition do not, in fact, apply to the field variable constructed in our studies.