(193b) Wetting at the Nano-Scale

At the macro-scale the extent to which a liquid wets a solid substrate is usually described in terms of the contact angle, θ_c, and the surface tensions involved.  Depending on the liquid and substrate, the system is described as amphiphilic (‘wetting’, θ_c<90^o) or amphiphobic (‘non-wetting’, θ_c>90^o).  Such a description has a number of limitations; in particular, it breaks down for sufficiently small nano-scale systems, and is limited to describing liquid, as opposed to gaseous or solid, adsorbed films. At a more fundamental level, wetting is determined by the competition between the adsorbate-substrate intermolecular forces and the adsorbate-adsorbate forces.  Through a corresponding states analysis of the statistical mechanical description of such wetting systems it is possible to define a microscopic wetting parameter, α_w,, that is a measure of wetting that applies at all scales and for any kind of adsorbed film (gas, liquid or solid).^1,2

We illustrate the usefulness of this wetting parameter by considering the properties of a nano-phase confined within a porous material. In this case the dimensionless pore width, pore shape and wetting characteristics of the confined phase are of particular importance.  Examples drawn from both experiment and molecular simulation studies will be presented for phase separations,^2,3 selective adsorption in the case of mixtures,² and pressure enhancement,^4,5 with emphasis on simple pore geometries. These examples illustrate the central role played by wetting, and also the breakdown of some concepts and macroscopic laws, such as Gibbs’ surface thermodynamics for nano-phases confined within small pores.  In particular, we show the breakdown of the Kelvin equation for vapor-liquid condensation, and the Gibbs-Thomson equation for freezing in nano-pores.

1. R .Radhakrishnan, K.E. Gubbins and M. Śliwinska-Bartkowiak, “Global Phase Diagrams for Freezing in Porous Media”, Journal of Chemical Physics, 116, 1147-1155 (2002).
2. Keith E. Gubbins, Yun Long and Malgorzata Sliwinska-Bartkowiak, “Thermodynamics of Confined Nano-Phases”, Journal of Chemical Thermodynamics, 74, 169-183 (2014).
3. L.D. Gelb, K.E. Gubbins, R. Radhakrishnan and M. Sliwinska-Bartkowiak, “Phase Separation in Confined Systems”, Reports on Progress in Physics, 62, 1573-1659 (1999).
4. Yun Long, Jeremy C. Palmer, Benoit Coasne,  Małgorzata Śliwinska-Bartkowiak and Keith E. Gubbins, “Pressure enhancement in carbon nanopores: A major confinement effect”, Physical Chemistry Chemical Physics, 13, 17163-17170 (2011)
5. Yun Long, Jeremy C. Palmer, Benoit Coasne, Małgorzata Śliwinska-Bartkowiak, George Jackson, Erich A. Müller and Keith E. Gubbins, “On the Molecular Origin of High Pressure Effects in Nanoconfinement: Effects of Surface Chemistry and Roughness”, Journal of Chemical Physics, 139, 144701 (2013)