Estimating the disjoining pressure of liquid nanofilms from molecular dynamics simulations via implicit treatment of the bulk liquid phase

Abstract

In unsaturated soil mechanics of nanoporous geomaterials, the disjoining pressure ($Π$) arising from interparticle liquid nanofilms governs the effective stress and deformation behavior. However, experimentally determining $Π$ for nanofilms with thicknesses of only a few nanometers remains highly challenging, motivating the use of molecular simulations. Meanwhile, direct molecular dynamics simulations of systems with coexisting bulk liquid, bulk gas, and free-standing nanofilms are hindered by physical complexity and computational cost. Consequently, most existing simulation approaches estimate $Π$ using simplified systems that include only the bulk gas and nanofilm, thereby neglecting the actual state of the bulk liquid phase. Building upon a previous approach in which $Π$ is determined from the thickness derivative of the film surface tension, we propose a method that implicitly accounts for the bulk liquid phase and enables thermodynamically consistent estimation of both film thickness and disjoining pressure. The method self-consistently determines the properties of the nanofilm and the corresponding bulk liquid using pre-fitted pressure-density relationships obtained from separate bulk-phase simulations. Our results demonstrate that rigorous treatment of bulk liquid density is essential for accurate film thickness determination and reliable prediction of $Π$, particularly under high-$Π$ conditions. Compared with existing approaches, the proposed method exhibits improved agreement with literature data for water and argon nanofilms, highlighting its potential for quantitatively characterizing surface forces in nanofilm systems.