Comparing the Mechanical and Thermodynamic Definitions of Pressure in Ice Nucleation

TL;DR

This study compares mechanical and thermodynamic pressures in ice nucleation, revealing system-dependent differences and highlighting limitations of mechanical methods for estimating interfacial free energies.

Contribution

It provides the first direct comparison of mechanical and thermodynamic pressures in ice nucleation using molecular simulations, showing their agreement in some cases and discrepancies in others.

Findings

Mechanical and thermodynamic pressures agree for the ice nucleus in supercooled water.

Interfacial stress and free energy are comparable at the ice-water interface.

Basal interface shows higher interfacial stress than free energy, indicating system dependence.

Abstract

Crystal nucleation studies using hard-sphere and Lennard-Jones models have shown that the pressure within the nucleus is lower than that in the surrounding liquid. Here, we use the mechanical route to obtain it for an ice nucleus in supercooled water (TIP4P/Ice) at 1 bar and 247 K. From this (mechanical) pressure, we obtain the interfacial stress using a thermodynamic definition consistent with mechanical arguments. This pressure is compared with that of bulk ice at equal chemical potential (thermodynamic pressure), and the interfacial stress with the interfacial free energy. Furthermore, we investigate these properties on the basal plane. We find that, unlike in hard-sphere and Lennard-Jones systems, mechanical and thermodynamic pressures agree for the nucleus, and the interfacial stress and free energy are comparable. Yet the basal interface displays an interfacial stress nearly twice its interfacial free energy, suggesting that this agreement may be system dependent, underscoring the limitations of mechanical routes to solid-liquid interfacial free energies.