Resolving Discrepancies in Disjoining Pressure Predictions for Liquid Nanofilms from Molecular Simulations

Abstract

Literature values of disjoining pressure in liquid nanofilms from different molecular simulation methods show significant discrepancies. We demonstrate that these arise from neglecting long-range dispersion interactions and inconsistent definitions of film thickness in the original Peng method. A key insight is that long-range dispersion affects surface tension in a thickness-dependent manner, increasing it at large thickness but suppressing its enhancement at small thickness due to disjoining-pressure-induced normal compression and lateral expansion. This leads to crossover behavior in the surface tension of water nanofilms. Since disjoining pressure is obtained from the derivative of surface tension with respect to thickness, this nontrivial dependence strongly impacts its accuracy. With proper treatment of dispersion interactions and a consistent thickness definition, the revised Peng method agrees with the Bhatt method and yields more accurate Hamaker constants.

Figures (4)

• Figure 1: (a) Schematic representation of a thin water film confined in a pore. The film is in equilibrium with the bulk water through a Plateau-border meniscus and is surrounded by gas phases on both sides. The corresponding force balance acting on the film is also illustrated. (b) System setup for estimating the surface tension $\sigma_f$ and the corresponding force balance of the thin film in the absence of a meniscus. Large discrepancies observed in disjoining pressures calculated by Bhatt methodbhatt2003molecularbhatt2002molecular and Peng methodpeng2015methodologypeng2016modelling for (c) pure water and (d) pure argon systems.
• Figure 2: (a) Surface tension of the water nanofilm as a function of film thickness at 479 K. Solid lines represent the fitted curves. (b) Corresponding disjoining pressure as a function of film thickness. (c) Corresponding disjoining pressure as a function of the inverse cubic film thickness. Dashed lines represent linear fits. Our results are compared with those reported by Peng et al.peng2015methodology and Bhatt et al.bhatt2003molecular.
• Figure 3: Top panels: Surface tension of the argon nanofilm as a function of film thickness. Solid lines in top panels represent the fitted curves. Middle panels: Corresponding disjoining pressure as a function of film thickness. Bottom panels: Corresponding disjoining pressure as a function of the inverse cubic of the film thickness. Dashed lines represent linear fits. Left and right panels correspond to temperatures of 100.05 K and 105.93 K, respectively. Our results are compared with those reported by Peng et al.peng2016modelling and Bhatt et al.bhatt2002molecular.
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