Percolation line, response functions, and Voronoi polyhedra analysis in supercritical water

TL;DR

This study investigates the physical significance of percolation in supercritical water using simulations and theoretical models, revealing that response function extrema and structural changes are not always aligned with the percolation line.

Contribution

It introduces a comparative analysis of physical and configurational criteria for percolation in supercritical water and examines their relation to response functions and structural indicators.

Findings

Percolation lines are consistent across criteria.

Response function extrema do not generally coincide with percolation lines.

Maximum skewness of Voronoi neighbors correlates with compressibility extrema.

Abstract

The problem of a physical relevance (meaning) of percolation in supercritical fluids is addressed considering a primitive model of water. Two different criteria, physical and configurational, are used for the cluster definition in Monte Carlo simulations over a range of pressures to determine the percolation line and skewness, and a theoretical analytic equation of state is used to evaluate response functions. It is found that both criteria yield practically the same percolation line. However, unlike the findings for simple fluids, the loci of the response function extrema exhibit density/pressure dependence quite different from that of the percolation line. The only potential coincidence between the loci of the extrema of a thermodynamic property and a detectable structural change is found for the coefficient of isothermal compressibility and Voronoi neighbors distribution skewness maximum.