Crossing the Widom Line: Evolution of Cohesive Strength and Cavity Formation in Supercritical Water

Abstract

Supercritical water (SCW) is a promising green solvent, yet optimizing its performance requires a fundamental understanding of the thermodynamic behavior and solvation mechanisms. Here, molecular dynamics simulations are used to investigate the thermodynamic responses, cohesive-strength evolution, and cavity-formation behavior of SCW along isobars, with the NIST benchmark data serving as a reference for the classical Widom line (WL) behavior. The origin of the high-pressure WL divergence is analyzed from the perspective of statistical fluctuations and intermolecular interactions. Furthermore, cohesive energy density and the state-dependent mean-radius cavity-formation free energy, together with their corresponding thermal response coefficients, are introduced to establish a thermodynamic connection with the classical WL. Building on this connection, the high-pressure “Widom region” is delineated, and the thermodynamically driven evolution sequence along the isobaric heating path is revealed. This work provides a thermodynamic framework for understanding the evolution of the Widom region and its relevance to the solvation behavior of SCW.