Local intermolecular structure and dynamics in binary supercritical solutions. A molecular dynamics simulation study of methane in carbon dioxide Ioannis Skarmoutsos, Jannis Samios * University of Athens, Department of Chemistry, Laboratory of Physical Chemistry, Panepistimiopolis 157-71, Athens, Greece Available online 27 December 2005 Abstract The binary supercritical mixture methane – carbon dioxide (SC CH 4 –CO 2 ) with mole fraction of methane X = 0.2 was investigated at temperature T = 323.15 K and density q = 0.23814 g/cm 3 corresponding to a pressure 9.94 MPa, by using the molecular dynamics (MD) simulation technique in the canonical (NVT) ensemble. The local intermolecular structure of the fluid was investigated in terms of the appropriate pair radial distribution functions, which have been calculated and analyzed. The estimated average local coordination numbers and mole fractions around the species in the mixture have been used to explore the repulsive or attractive nature of the fluid. The results obtained have led to the conclusion that the SC CH 4 –CO 2 mixture at these conditions might be characterized as a repulsive one. This result supports the conclusions of a previously reported experimental work by Baglin et al. [F.G. Baglin, S.K. Murray, J.E. Daugherty, T.E. Palmer, W. Stanbery, Mol. Phys. 98 (2000) 409.]. The single reorientational dynamics of both the solute and solvent species in the mixture were also investigated by calculating the appropriate time correlation functions. The observed time dependence of these functions are discussed in relation with the local environment around the species, which in the case of the solute molecules seems to be strongly affected. D 2005 Elsevier B.V. All rights reserved. Keywords: Intermolecular structure and dynamics; Binary supercritical solution; Methane – carbon dioxide 1. Introduction In recent years, numerous experimental as well as theoret- ical investigations have been devoted to study the properties of supercritical (SC) solvents. This research remains a very active area of interest for many reasons. The efficiency, for instance, of some SC fluids (SCFs) for replacing toxic industrial solvents, and their ability to exhibit a catalytic role in the evolution of important chemical processes at mild conditions, such as fluid extraction, chromatography, chemical synthesis and analysis underlies this particular interest [1–3]. Further- more, one of the most important characteristics of SCFs is that their properties can be drastically changed from gas-like to liquid-like values with small changes of some thermodynamic variables, especially in the critical region. For example, small changes in the pressure of SCFs can cause significant differences in their density, because of their high compress- ibility value. Due to that fact, their dissolving capability could be tuned through such kind of small changes in the thermodynamic parameters, allowing the selective solvation of different types of solutes. These features make SCFs attractive alternatives to liquid solvents for use in the developments of new chemical processes [4]. One of the possible origins of this peculiar behavior of SCFs’ properties is the existence of local density inhomoge- neities. It has been pointed out that the compressibility of a fluid is directly related to the range over which microscopic fluctuations in density are correlated. As a result of this, the overall picture of a SCF in its compressible regime is that of an inhomogeneous medium with high and low-density regions (small cluster and molecular aggregates). In general, previ- ously reported studies in the literature have led to the conclusion that undoubtedly local density inhomogeneities are real and important phenomena in compressible SCFs [5– 29]. Furthermore, a thermodynamic state-point dependence of the extent of these fluctuations from mesoscopic to micro- scopic length-scale has been observed; this causes corre- sponding changes in solute solvation and reaction dynamics. However, a deeper and more quantitative understanding on the state-point dependence of the local density fluctuations and 0167-7322/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2005.11.023 * Corresponding author. E-mail address: isamios@cc.uoa.gr (J. Samios). Journal of Molecular Liquids 125 (2006) 181 – 186 www.elsevier.com/locate/molliq