Analogous (differenced) enthalpy-entropy-composition diagram for ethylene (1)-normal butane (2). The slopes of the rolling plane in this case are temperature and ethylene chemical potential. The composition of the equilibrium liquid state (red) is approximately 35% C2H4, the vapor state (green) 55%. The entropy-direction curvature of the red region is opposite to that of the blue and yellow regions, but that difference is difficult to see in this drawing, even in the full-size view.
Iowa State University