Molecular simulation of nano-dispersed fluid phases

TL;DR

This study uses molecular dynamics to explore fluid phase equilibria involving nano-sized dispersed phases, revealing size-dependent effects on density and surface tension in nanoscopic gas bubbles within liquids.

Contribution

It provides new insights into the thermodynamic stability and curvature effects on nano-dispersed fluid phases through detailed molecular simulations.

Findings

Curvature causes liquid subsaturation and reduced density.

Small bubble diameter increases gas density at the center.

Surface tension depends on curvature, with a negative Tolman length.

Abstract

Fluid phase equilibria involving nano-dispersed phases, where at least one of the coexisting phases is confined to a small volume, are investigated by molecular dynamics simulation. Complementing previous studies on nanoscopic droplets, simulation volumes containing a nanoscopic gas bubble surrounded by a subsaturated liquid phase under tension, i.e. at negative pressure, are conducted in the canonical ensemble. The boundary conditions are chosen such that the phase equilibrium at the curved interface is thermodynamically stable. Two distinct size-dependent effects are found: Curvature induces a subsaturation of the system, leading to a smaller liquid density. For the gas in the centre of the bubble, the small diameter has an additional obverse effect, increasing its density. The curvature dependence of the surface tension is discussed by evaluating average radial density profiles to obtain the excess equimolar radius, which is found to be positive, corresponding to a negative Tolman length.