Accounting for Nanofilm Contributions in Interfacial Free Energy Calculations Using Classical Density Functional Theory

Abstract Fluid nanofilms play a fundamental role in nanoscale interfacial thermodynamics, yet their interfacial behavior and treatment within classical density functional theory (cDFT) warrant further investigation. We investigate nanofilm interfacial properties using a cDFT framework built upon the perturbed-chain statistical associating fluid theory (PC-SAFT) for both fluid–fluid and fluid–solid interfacial systems. Comparison with molecular simulations shows that the long-standing numerical discrepancies regarding free-standing nanofilms persist in the PC-SAFT functional predictions, supporting the view that they stem from thermal capillary-wave fluctuations rather than deficiencies of the density functional itself, as such fluctuations are inherently neglected in the mean-field approximation adopted by standard cDFT. Importantly, we establish a thermodynamically consistent framework for interfacial free energy (IFE) calculation, which explicitly incorporates nanofilm thermodynamic contributions and redefines the effective fluid volume by excluding the solid-phase region. The proposed method exhibits excellent consistency with another method based on the relationship between IFE and disjoining pressure for fluid systems, while both fluid–fluid and fluid–solid IFEs differ substantially from those predicted by the conventional method. We find that neglecting nanofilm contributions induces prominent size-dependent variations in the IFE and contact angle of hemicylindrical droplets, which is inconsistent with the extensive literature. Different methods also lead to opposite signs of the line tension for a hemispherical argon nanodroplet on a strongly lyophobic surface. The proposed framework provides a unified molecular-level basis for understanding interfacial processes involving nanofilms, including wetting, nucleation, adsorption, and other phenomena that rely on the accurate evaluation of IFEs.