Coexistence curve for polystyrene–cyclohexane near the critical point

The coexistence curves for the system polystyrene–cyclohexane have been determined with a specially designed differential refractometer, which determines concentrations of the two coexisting phases with a precision of ± 0.01%. The plots of the coexistence curve data represented by log(X+−Xc) and log(Xc−X−) versus the reduced temperature ε = (Tc−T)/Tc yield largely curved lines. The shape of the coexistence curve very near the critical point, however, is asymptotically symmetric. The coexistence curve obtained with an over-all concentration at the critical concentration is represented by the relation X+−X− = Bεβ with β=0.348 and B=0.97 in the temperature region of Tc −T<2.0 °C. The critical point exponent β and the coefficient B vary only slightly depending on the over-all concentration. Although the data give a curved diameter, a precision of the present experiment is not sufficient to determine the asymptotic behavior of the diameter.