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Available States Versus Available Space: Which Predicts the Self-Diffusivity of Dense Confined Fluids?

Gaurav Goel1, Jeetain Mittal2, Mark Pond1, Vincent K. Shen3, Jeffrey R. Errington4, and Thomas M. Truskett5. (1) Department of Chemical Engineering, The University of Texas at Austin, 1 University Station, C0400, Austin, TX 78705, (2) Laboratory of Chemical Physics, National Institutes of Health, 5 Memorial dr, Room 114, Bethesda, MD 20892, (3) Physical and Chemical Properties Division, NIST, 100 Bureau Dr. MS 8380, Gaithersburg, MD 20899-8380, (4) Chemical and Biological Engineering, University at Buffalo, 303 Furnas Hall, Buffalo, NY 14260, (5) Department of Chemical Engineering and Institute for Theoretical Chemistry, The University of Texas at Austin, 1 University Station, C0400, Austin, TX 78712

Confining a fluid to length scales on the order of few particle diameters changes both its static and dynamic properties. Classical density functional theories can often make reliable predictions concerning the former, but implications of confinement for dynamics remain challenging to forecast for even the most basic models. Recent computer simulations of simple, equilibrium fluids show that the relationship between excess entropy, a static property, and self-diffusivity, a dynamic quantity, is approximately independent of the degree of confinement. Do other static measures, such as those that characterize free or available volume, also strongly correlate with single-particle dynamics? Using computer simulations and theory, we investigate this question for a variety of fluids and fluid mixtures, namely the hard-sphere and the Gaussian core fluids confined to channels with strongly interacting boundaries. In particular, we critically analyze the validity of correlations between various static structural and dynamical properties for the above mentioned systems in dense as well as super-cooled regimes.