Heat Capacity Anomaly in the Vicinity of the Tricritical and Upper Critical Points

摘要

The anomalous behavior of the isochoric heat capacity of a mixture of methane, pentane and heptane is studied experimentally in the vicinity of the liquid-vapor critical point in the cases when (a) the critical temperature T_c approaches the tricritical point T_(TCP) and (b) the critical temperature approaches the upper critical end point T_U. It is shown that in all cases, the singular part of the heat capacity of the mixture has the form C_(sing) = A|τ|~(-α), where τ = (T-T_c)/T_c and α ≈ 0.11. When T_c → T_U, amplitude A of the heat capacity anomaly is found to be approximately constant. At the same time, the amplitude of the anomaly tends to zero in the vicinity of the tricritical point: A ∝|τ_c|~ε, where τ_c = (T_c - T_(TCP))/T_(TCP) and ε = 1.6 - 1.7. The inevitable vanishing of this mode of the heat capacity anomaly leads to a negative value of the critical index α-bar characterizing the heat capacity anomaly at the tricritical point, while the tricritical point theory and the isomorphism hypothesis predict α-tilde = 0.5.