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) = p(sigma)" or d(2)mu/dT(2) = mu(sig ma)" or both diverge when T --> Tc- on the phase boundary sigma. They queri ed the lattice-gas prediction that mu(sigma)" remains finite. Analysis of t wo-phase heat-capacity data provides, for the first time, evidence for such a Yang-Yang anomaly (mu(sigma)" --> +/- infinity) in propane and suggests an anomaly of opposite sign in CO2. A revision of standard scaling theory f ur fluid criticality is demanded: specifically, p - p(c) must appear in the ordering field. The coexistence diameter hence gains a \ T - T-c\(2 beta) term.