This drawing shows the molar Gibbs energy plotted against pressure and temperature (GPT). The doubly transformed function intersects itself in the curve of equilibrium, liquid-vapor states, and the surface is often referred to as a “swallowtail catastrophe.” The transparent yellow extensions of the surface represent metastable states — liquid to the left, vapor to the right. Since the unstable part of the surface would fold back on itself and conceal both the intersection curve and the metastable extensions, it has been omitted from the drawing. The slope of the (white) isotherms in the pressure direction is the volume - vapor volumes to the left, liquid to the right.
Iowa State University