Effects of confinement between attractive and repulsive walls on the thermodynamics of an anomalous fluid

TL;DR

This study uses molecular dynamics simulations to explore how confinement between attractive and repulsive walls affects the thermodynamics and anomalies of a water-like anomalous fluid, revealing shifts in phase behavior and the potential for easier experimental detection of the liquid-liquid critical point.

Contribution

It demonstrates how confinement alters the phase diagram and anomalies of an anomalous fluid, including the displacement of the LLCP, and extends understanding to entire confined systems beyond homogeneous regions.

Findings

Confined fluid's phase diagram shifts to higher T, P, and density.

Structural, diffusion, and density anomalies persist under confinement.

The liquid-liquid critical point moves to higher temperature in confinement.

Abstract

We study by molecular dynamics simulations the thermodynamics of an anomalous fluid confined in a slit pore with one wall structured and attractive and another unstructured and repulsive. We find that the phase diagram of the homogeneous part of the confined fluid is shifted to higher temperatures, densities and pressures with respect to the bulk, but it can be rescaled on the bulk case. We calculate a moderate increase of mobility of the homogeneous confined fluid that we interpret as a consequence of the layering due to confinement and the collective modes due to long-range correlations. We show that, as in bulk, the confined fluid has structural, diffusion and density anomalies, that order in the water-like hierarchy, and a liquid-liquid critical point (LLCP). The overall anomalous region moves to higher temperatures, densities and pressure and the LLCP displaces to higher temperature compared to bulk. Motivated by experiments, we calculate also the phase diagram not just for the homogeneous part of the confined fluid but for the entire fluid in the pore and show that it is shifted towards higher pressures but preserves the thermodynamics, including the LLCP. Because our model has water-like properties, we argue that in experiments with supercooled water confined in slit pores with a width of > 3 nm if hydrophilic, and of > 1.5 nm if hydrophobic, the existence of the LLCP could be easier to test than in bulk, where it is not directly accessible.