Effects of nanopore and fluid structure on anomalies and phase transitions of confined core-softened fluids

TL;DR

This study uses Molecular Dynamics simulations to explore how nanopore and wall structures influence the anomalies and phase transitions of a confined water-like fluid, revealing shifts in critical points and phase boundaries.

Contribution

It provides new insights into how confinement and wall structure affect the thermodynamic and dynamic anomalies of confined fluids, extending understanding beyond bulk behavior.

Findings

Density and diffusion extremum shift to lower temperatures under confinement.

Critical points vary non-monotonically with nanopore size for smooth walls.

Pressure and temperature of critical points change monotonically with pore size for structured walls.

Abstract

We use Molecular Dynamics simulations to study how the nanopore and the fluid structures affects the dynamic, thermodynamic and structural properties of a confined anomalous fluid. The fluid is modeled using an effective pair potential derived from the ST4 atomistic model for water. This system exhibits density, structural and dynamical anomalies and the vapor-liquid and liquid-liquid critical points similar to the quantities observed in bulk water. The confinement is modeled both by smooth and structured walls. The temperatures of extremum density and diffusion for the confined fluid show a shift to lower values while the pressures move to higher amounts for both smooth and structured confinement. In the case of smooth walls, the critical points and the limit between fluid and amorphous phases show a non-monotonic change in the temperatures and pressures when the the nanopore size is increase. In the case of structured walls the pressures and temperatures of the critical points varies monotonicaly with the porous size. Our results are explained on basis of the competition between the different length scales of the fluid and the wall-fluid interaction.