The role of the intermolecular potential on the dynamics of ethylene confined in cylindrical nanopores{ Fernando J. A. L. Cruz,* a Erich A. Mu ¨ller b and Jose ´ P. B. Mota a Received 6th April 2011, Accepted 25th May 2011 DOI: 10.1039/c1ra00019e The influence of the molecular force field upon the transport properties of ethylene is studied by molecular dynamics simulations, addressing both the bulk fluid and the confined phases within pristine single-walled carbon nanotubes. Five different molecular models with different degrees of coarse-graining were selected, spanning from a simple isotropic Lennard–Jones sphere to a fully detailed all-atom description. The fluid was probed for its self-diffusion coefficient under isochoric (0.026 ¡ r (mol L 21 ) ¡ 15.751) and isothermal (220 ¡ T (K) ¡ 340) conditions, both in the sub- and supercritical regions (T bulk c = 282.4 K). Although the particular details of each potential model are seen to be nearly irrelevant to the bulk fluid dynamics, they are crucial to correctly describe the inhomogeneous system. The most important aspects affecting fluid transport are the existence/ absence of explicit electrostatic contributions and the molecular shape. The effect of temperature on the confined fluid self-diffusion is described by the Arrhenius law, D = Aexp(2E a /RT), and the nonlinear density dependencies of the activation energy (E a /R) and pre-exponential factor (A) have been fitted here to empirical equations. In spite of the quasi one-dimensional confining nature of SWCNTs, the isothermal results (T = 300 K) obtained for the bulk and confined systems collapse onto a unique master curve, D = D 0 r l , suggesting that the self-diffusion coefficient of a confined fluid can be estimated from its molecular density, an easily accessible property. 1. Introduction The confinement of fluids in nanoporous solids can be accompanied by striking effects on the molecular dynamics, 1–6 which do not necessarily possess an analogy in the bulk phase. Upon confinement, molecules interact with the solid walls to an extent that markedly depends on the chemical nature of the solid and its corresponding pore size. As the dimensions of the nanopore approach those of the molecule itself, the solid–fluid interactions tend to become dominant; 5,7 this is one of the main arguments used to explain why nanoconfined fluid dynamics exhibit sharp discrepancies when compared to the classical bulk phase behavior. In such situations, the most commonly accessed transport property is the self-diffusion coefficient, which apart from its fundamental and scientific importance, is of practical relevance for most industrial applications involving fluid processing such as separation and catalysis. Experimentally, however, the investigation of transport properties of nanocon- fined fluids is hindered by several aspects, including the poor characterization/chemical purity of the samples, inaccurate description of surface topology, and the very small pore size. An opportunity arises here to study diffusion of fluids in confined media using classical molecular dynamics simulations (MD), 8,9 thus obtaining an insight into both structural and dynamical features. As a structurally simple and well characterized model of cylin- drical nanopores, single-walled carbon nanotubes (SWCNTs) are excellent candidates for theoretical studies. Their structure can be rationalized as the cylindrical folding of a graphene sheet along a particular directional vector (n,m). 10,11 As the result of that folding, the carbon atoms on the solid walls may be arranged in three distinct topologies, namely armchair (n = m), zig-zag (m = 0) or chiral (n ? m) nanotubes. Each of those topologies exhibit different optical 12 and electronic 13 properties, and amongst other applications have been proposed as building blocks in composites, 14 chemical sensors, 15 field-effect transistors in nanoelectronics, 16 biomedical devices 17,18 and as separating media of organic vapors. 19–21 Their hydrophobicity places them as ideal candidates to explore molecular transport through biological membranes, and therefore the dynamics of confined water has been one of the main subjects of previous work in this area. 7,22,23 Recently, Liu et al. 23 calculated the potential energy surfaces for zig-zag and armchair SWCNTs, and showed that those two different topologies can interact differently when put into contact with water molecules. This observation was later related to the different diffusion mechanisms observed for water a Requimte/CQFB, Department of Chemistry, Faculdade de Cie ˆncias e Tecnologia, Universidade Nova de Lisboa, 2829-516, Caparica, Portugal. E-mail: f.cruz@dq.fct.unl.pt b Department of Chemical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK { Electronic supplementary information (ESI) available: Liu’s equation. See DOI: 10.1039/c1ra00019e RSC Advances Dynamic Article Links Cite this: RSC Advances, 2011, 1, 270–281 www.rsc.org/advances PAPER 270 | RSC Adv., 2011, 1, 270–281 This journal is ß The Royal Society of Chemistry 2011 Downloaded on 24 August 2011 Published on 02 August 2011 on http://pubs.rsc.org | doi:10.1039/C1RA00019E View Online