Corresponding States Theory of Adsorbed FILMS: Influence of Wetting and Wall Roughness

CORRESPONDING STATES THEORY OF ADSORBED FILMS: INFLUENCE OF WETTING AND WALL ROUGHNESS

Yun Long^a, An Rong^b, Małgorzata Śliwinska-Bartkowiak^c, Matthias Thommes^d and Keith E. Gubbins^b

^a Department of Chemical& Biomolecular Engineering, National University of Singapore, Singapore 117585

^b Department of Chemical& Biomolecular Engineering, North Carolina State University,

Raleigh, NC 27695, USA

^c Faculty of Physics, Adam Mickiewicz University, Poznan 61-614, Poland

^d Quantachrome Instruments, 1900 Corporate Drive, Boynton Beach, FL 33426, USA     A statistical mechanical analysis of adsorption shows that the equilibrium properties of the adsorbed film (isotherms, isosteric heats, phase change conditions, etc.) are determined by the dimensionless pore width, H, and pore geometry, and by a microscopic wetting parameter, α_w, that is defined in terms of intermolecular forces and solid structure; α_w is a measure of wetting that applies at all scales and for any kind of adsorbed film (gas, liquid or solid) [1,2].      Figure 1.  Experimental results for the effect of confinement in cylindrical pores on the melting point of various adsorbates.  Here CNT = carbon nanotube and α_w is the microscopic wetting parameter; high values of α_w indicate strong wetting.  For large pore widths the temperature shift is approximately linear, as required by the Gibbs-Thomson equation, but departures occur for smaller widths.

We illustrate the usefulness of this approach using examples drawn from both experimental and molecular simulation studies of gas-liquid and liquid-solid phase separations [3,4], and pressure enhancement in nanopores [5-7], 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 the equations of Kelvin, Laplace and Gibbs-Thomson, for nano-phases confined within small pores. They also suggest consistency tests for experimental data that should be applied in the event of capillary condensation or other phase changes in mesoporous materials.

The approach can be extended to account for geometric or energetic wall roughness [8], and to adsorbate molecules that interact with site-site potentials having both Lennard-Jones and point charge sites.

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.         C. Alba-Simionesco, B. Coasne, G. Dosseh, G. Dudziak, K.E. Gubbins, R. Radhakrishnan and M. Śliwinska-Bartkowiak, “Effects of Confinement on Freezing and Melting”, Journal of Physics: Condensed Matter, 18, R15-R68 (2006).

5.         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).

6.         H. Drozdowski, M. Kempinski, M. Śliwinska-Bartkowiak, M. Jazdzewska, Y. Long, J.C. Palmer and K.E. Gubbins, „Structural Analysis of the Behavior of Water Adsorbed in Activated Carbon Fibers”, Physical Chemistry Chemical Physics, 14, 7145-7153 (2012).

7.         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).

8.         M. Śliwinska-Bartkowiak, A. Sterczyńska, Y. Long and K.E. Gubbins, “Influence of Microroughness on the Wetting Properties of Nano-Porous Silica Matrices”, Molecular Physics, 112, 2365-2371 (2014).