Liquid-vapor critical behavior of the TIP4P/2005 water model: effects of NaCl solutes and hydrophobic confinement

TL;DR

This study investigates how ionic solutes and hydrophobic confinement affect the liquid-vapor critical behavior of water using molecular dynamics simulations with the TIP4P/2005 model, revealing shifts in critical temperature and pressure.

Contribution

It introduces a systematic analysis of the effects of NaCl solutes and hydrophobic confinement on water's critical point using advanced simulation techniques and a novel method for critical temperature determination.

Findings

Salt increases the critical temperature and pressure, aligning with experimental data.

Hydrophobic confinement depresses the critical point.

The critical temperature estimate is highly sensitive to van der Waals cutoff distance.

Abstract

The liquid-vapor critical behavior of water is strongly influenced by both ionic solutes and confinement. Molecular dynamics simulations of aqueous NaCl solutions using the TIP4P/2005 water model and the Madrid-2019 ion parameters reveal a systematic increase in the liquid-vapor critical temperature and pressure with salt concentration, consistent with experimental trends. In contrast, confinement between parallel hydrophobic plates leads to a depression of the critical point. The critical temperature was determined from the Binder cumulant crossing in the NVT ensemble, using a recently developed method originally applied to an active Brownian particle system. The reliability of this approach was verified through complementary NPT simulations. We further demonstrate the pronounced sensitivity of the estimated critical point to the van der Waals cutoff distance, underscoring the importance of properly accounting for long-range interactions. Our framework, which employs and tests an adjusted subbox method, can be readily extended to other critical phenomena, including the liquid-liquid critical point of water.