Tricritical Points in Multicomponent Fluid Mixtures: a Geometric Interpretation”

We consider special critical points in multicomponent fluid mixtures, where critical phenomena occur between three phases simultaneously. We show that although such points may be the intersection of three lines of critical points, they are not in general analogous to the tricritical points originally proposed by Gr iffiths. Rather the three lines are part of a single surface of critical points and the special point occurs where the boundary of the surface of critical points is a maximum with respect to a particular variable selected experimentally (usually temperature). We suggest experimental tests to determine whether the boundary is singular at the point, making it also a special point. Finally, we relate the entire discussion to the recently proposed concept of order for critical points and discuss the possibilities of special scaling. * Supported in part by the U. S. Atomic Energy Commission. t Lindemann Fellow $ Supported by NSF Grant No. GU-1590 and NASA Grant No. NGL 34-002-084. Permanent address: Riddich Laboratories, North Carolina State University, Raleigh, North Carolina 27607. ¶ Supported by NSF Grant No. GP-15428. (Submitted to Phys. Rev. A)