Volume changes in the critical solution region

Abstract

Recent theoretical and experimental studies suggest that (∂2V/∂x2)T, p and (∂2H/∂x2T, p vanish at the critical solution point. We report dilatometric measurements of volume changes in the system n-C6H14 + n-C6F14 at 21 compositions and eight temperatures in the region of its critical solution point at t0 = 22.71°C and xc = 0.35. By mixing two standard mixtures of mole fractions which bracket xc, the magnitude of the volume change is reduced by a factor of ten, permitting the use of a finer capillary. Qualitatively the curves of VEm against x become nearly flat as one approaches the critical point; associated with this is a large increase in the coefficient of thermal expansion. Curve fitting in the critical region is difficult, but analytical and graphical treatments give similar results. At the critical composition (∂2Vm/∂x2)T, p = − 16 cm3 mol−1 at 24.000°C and decreases in magnitude to about − 4 cm3 mol−1 at 22.715°C, about 0.01 K above the critical temperature. This latter value is closer to zero by a factor of about four than any value reported previously and lends further support to the view that the limiting value is in fact zero.