A dual‐boundary model for gas‐like/liquid‐like crossovers from subcritical to supercritical states

Supercritical fluids do not behave as a featureless single phase; instead, they retain gas‐like and liquid‐like characteristics. Existing descriptions usually emphasize a single Widom‐type line, whereas the finite width of the crossover remains insufficiently defined. Here we propose a dual‐boundary thermodynamic map that combines a critical‐tangent inner boundary with a hierarchy of reduced density‐pressure response contours. The inner boundary is defined by the tangent to the vapor–liquid coexistence curve at the critical point and represents the near‐critical central trend shared by several Widom‐line candidates. The outer response envelopes are described by the dimensionless response [ T r ( dρ r / dp r ) Tr ], which provides comparable response‐level contours for diverse pure fluids and selected refrigerant mixtures. Comparisons with experimental crossover observations for water, methane, and carbon dioxide, together with thermodynamic calculations for pure fluids and selected refrigerant mixtures, demonstrate the practical applicability and reproducibility of the proposed framework.