Calculation of the Interfacial Tension of Nanostructured Interfaces by Molecular Dynamics Simulations Using Thermodynamic Integration
Final Report Abstract
This project is centred around the development of the so-called phantom-wall method for calculating solid-liquid surface tensions. This method is a thermodynamic-integration technique, which is used to calculate the difference in surface free energy (or, alternatively, surface tension) between a real surface and a flat, structureless model surface, the so-called phantom wall. The technique can be used in conjunction with molecular simulation methods; here, it has been implemented in the framework of atomistic molecular dynamics. The phantom wall method relies on a thermodynamically reversible pathway, by which the phantom wall is moved from its hiding place inside the solid surface to a final position well outside the solid. In the final position, only the phantom wall interacts with the liquid: the liquid has effectively been lifted off the solid by the movement of the phantom wall. The method has been developed and tested, initially with Lennard-Jones model systems, and it has been shown to faithfully reproduce known surface free energies for these systems, especially when the surface belongs to an anisotropic solid. As first applications, the phantom-wall method has been used to study the effect of nanoscale corrugation of surfaces on the wetting characteristics: surface free energies, surface tensions, and contact angles. For a Lennard-Jones model system with tunable surface interactions, it could be shown that corrugation amplifies existing surface characteristics. A wettable flat surface becomes more wettable when nanostructured. A non-wettable flat surface becomes even more solvent-repellent upon corrugation. Subsequently, the method was applied to a more realistic system, namely water at graphite surfaces. Graphite is inherently hydrophobic, and the hydrophobicity is enhanced when nanostructures are built on its surface. Concepts and approximations known from micrometer-scale structuring of surfaces can sometimes be applied also on the nanoscale, others cannot. It is found that the contact angle of a corrugated surface follows the Cassie-Baxter approximation if the holes or grooves in the surface are too small to be filled by water: the contact angle is additively composed of that of graphite and that of vacuum. For larger depressions in the surface, one would expect the Wenzel approximation to be valid: the contact angle would change with the increase of effective surface area produced by corrugation. This is, however, not found. Instead, the contact angle varies with the contour length of the surface nanostructure. This indicates that it is the edges of the nanostructure, which perturb most the water structure and thereby increase surface tension and contact angle. The phantom-wall method is currently applied to wetting phenomena on other technical surfaces, and it is being extended to dealing with non-planar surfaces, so it can be applied also to “solid substrates” such as carbon nanotubes (cylindrical geometry) and nanofiller particles (spherical geometry).
Publications
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“Interfacial excess free energies of solid-liquid interfaces by molecular dynamics simulation and thermodynamic integration”, Macromol. Rapid. Comm. 30, 864-870 (2009)
F. Leroy, D.J.V.A. dos Santos, and F. Müller-Plathe
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“Solid-liquid surface free energy of Lennard-Jones liquid on smooth and rough surfaces computed by molecular dynamics using the phantom-wall method”, J. Chem. Phys. 133, 044110 (2010)
F. Leroy and F. Müller-Plathe
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“Rationalization of the behavior of solid-liquid surface free energy of water in Cassie and Wenzel wetting states on rugged solid surfaces at the nanometer scale”, Langmuir 27, 637– 645 (2011)
F. Leroy and F. Müller-Plathe
