Density Functional Theory of Inhomogeneous Liquids IV. Squared-gradient approximation and Classical Nucleation Theory

TL;DR

This paper develops a squared-gradient approximation within a density functional framework to model inhomogeneous liquids, providing explicit formulas, solutions for interfaces and clusters, and connecting to classical nucleation theory with improved insights into nucleation pathways.

Contribution

It introduces a new explicit expression for the SGA coefficient, solves the model for interfaces and clusters, and links the approach to classical nucleation theory with novel insights.

Findings

Quantitative accuracy of the model compared to simulations.

Piecewise-linear density profiles provide reasonable approximations.

Extension of classical nucleation theory to include non-classical effects.

Abstract

The Squared-Gradient approximation to the Modified-Core Van der Waals density functional theory model is developed. A simple, explicit expression for the SGA coefficient involving only the bulk equation of state and the interaction potential is given. The model is solved for planar interfaces and spherical clusters and is shown to be quantitatively accurate in comparisons to computer simulations. An approximate technique for solving the SGA based on piecewise-linear density profiles is introduced and is shown to give reasonable zeroth-order approximations to the numerical solution of the model. The piecewise-linear models of spherical clusters are shown to be a natural extension of Classical Nucleation Theory and serve to clarify some of the non-classical effects previously observed in liquid-vapor nucleation. Nucleation pathways are investigated using both constrained energy-minimization and steepest-descent techniques.