The yang-yang anomaly in fluid criticality: experiment and scaling theory

Yang and Yang proved that the divergence of C(V)(T) at a gas-liquid critical point implies that either d(2)p/dT(2) identical withp(")(sigma) or d(2)&mgr;/dT(2) identical with&mgr;(")(sigma) or both diverge when T-->T(c)- on the phase boundary sigma. They queried the lattice-gas prediction that &mgr;(")(sigma) remains finite. Analysis of two-phase heat-capacity data provides, for the first time, evidence for such a Yang-Yang anomaly (&mgr;(")(sigma)-->+/-infinity) in propane and suggests an anomaly of opposite sign in CO (2). A revision of standard scaling theory for fluid criticality is demanded: specifically, p-p(c) must appear in the ordering field. The coexistence diameter hence gains a |T-T(c)|(2beta) term.