(31a) Thermodynamics and Dynamics of Confined Nano-Phases

The availability of a wide range of novel synthetic nano-porous materials, including ordered mesoporous and microporous carbons, metal organic framework materials, and meoposorous organo-silicas, offers the prospect of important applications of these materials as fuel cell electrodes, supercapacitors, and sensors, as well as in more traditional areas such as separations and catalysis. The behavior of phases confined within such nano-scale cavities can be strikingly different from that of the bulk material, due to finite size and confinement effects. Vapor pressures, diffusion rates, reaction yields and rates can all change by orders of magnitude in small pores, and familiar laws and concepts for bulk phase systems may not apply.

The role of molecular modeling and simulation in studying effects of confinement and surface nanostructure on phase transitions[1,2] pressure enhancement in nano-pores and diffusion[3] will be discussed, with emphasis on simple pore geometries. Where possible, both experimental and molecular simulation results will be presented. These examples illustrate the effects of strong confinement, and also the breakdown of some macroscopic laws, such as Fick's Law of diffusion, and concepts such as Gibbs' surface thermodynamics, for nanophases confined within small pores. Recent molecular dynamics simulation studies of the transition between single file and Fickian diffusion will be described, including multiple diffusion mechanisms in a given material.

[1] 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). [2] C. Alba-Simionesco, B. Coasne, G. Dosseh, G. Dudziak, K.E. Gubbins, R. Radhakrishnan and M. ?liwinska-Bartkowiak, ?Effects of Confinement on Freezing and Melting?, Journal of Physics: Condensed Matter, 18, R15-R68 (2006). [3] J.D. Moore, J.C. Palmer, Y-C. Liu, T.J. Roussel, J.K. Brennan, and K.E. Gubbins, ?Adsorption and Diffusion of Argon Confined in Ordered and Disordered Microporous Carbons?, Applied Surface Science, DOI: 10.1016/j.apsusc.2009.12.071 (2010).