Vapor-like liquid coexistence densities affect the extension of the critical point's influence zone

TL;DR

This paper demonstrates that the wide critical influence zone in vapor-liquid coexistence is due to vapor-like low-density behavior of the liquid phase, affecting properties like density and viscosity.

Contribution

It reveals that the extensive critical influence zone is caused by vapor-like effects in low-density liquid phases, supported by pressure analysis in a simple ethane model.

Findings

Critical influence zone extends up to tens of degrees below critical temperature.

Low-density liquid phases behave like vapor phases within the influence zone.

Transition temperature can be predicted through pressure analysis of coexisting phases.

Abstract

The critical point affects the coexistence behavior of the vapor-liquid equilibrium densities. The length of the critical influence zone is under debate because for some properties, like shear viscosity, the extension is only a few degrees, while for others, such as the density order parameter, the critical influence zone range covers up to hundreds of degrees below the critical temperature. Here we show that for a simple molecular potential of ethane, the critical influence zone covers a wide zone of tens of degrees (below the critical temperature) down to a transition temperature, at which the apparent critical influence zone vanishes and the transition temperature can be predicted through a pressure analysis of the coexisting bulk liquid phase. The liquid phases within the apparent critical influence zone show low densities, making them behave internally like their corresponding vapor phases. Therefore, the experimentally observed wide extension of the critical influence zone is due to a vapor-like effect due to low bulk liquid phase densities.