Temperature dependence of diffusion coefficient of carbon monoxide in water: A molecular dynamics study Ishwor Poudyal a , Narayan P. Adhikari a,b a Central Department of Physics, Tribhuvan University, Kirtipur, Kathmandu, Nepal b The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy abstract article info Article history: Received 1 September 2013 Received in revised form 6 January 2014 Accepted 9 January 2014 Available online 24 January 2014 Keywords: Water Carbon monoxide Diffusion coefficient Molecular dynamics Arrhenius behavior In the present work, molecular dynamics simulations of a mixture of carbon monoxide (CO) gas in SPC/E water (H 2 O) with CO as a solute and water as a solvent have been performed to understand the self-diffusion coefficient of the components, i.e. CO and water along with the mutual diffusion coefficients and structural properties of the system by studying the radial distribution function (RDF) of the components present in the system at different temperatures of 293 K, 303 K, 313 K, 323 K, and 333 K. The mole fraction of CO in the system is 0.018 and that of water is 0.982. The solvent–solvent, solute–solute and solute–solvent radial distribution functions have been estimated to study the structural properties of the system. The self-diffusion coefficient of CO is calculated using mean square displacement (MSD) and velocity autocorrelation function (VACF) and that of water is estimated using MSD method only. The self-diffusion coefficient of CO agrees within around 20% of the experimental results and that of water agrees within around 10% of the experimental results. The mutual diffusion coefficient of solute–solvent is calculated using Darken's relation. The temperature dependence of self-diffusion coefficients of CO and water and mutual diffusion coefficients of CO in water all follow the Arrhenius behavior from where we estimate the activation energy of the diffusion process. Thus estimated activation energies also agree to experimental values. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Molecular diffusion occurs as different species of a mixture move under the influence of concentration gradients [1]. It plays a key role in a variety of atmospheric and biospheric sciences. Diffusion is fundamental for transport of matter and for ionic conduction in disordered materials [2,3]. The kinetics of many microstructural changes that occur during preparation, processing and heat treatment of materials include diffusion; typical examples are nucleation of new phases, diffusive phase transfor- mation precipitation and dissolution of a second phase, homogenisation of alloys, recrystallization and thermal oxidation [1]. The application of diffusion is on doping during fabrication of microelectronic devices, the operation of solid electrolytes for batteries and fuel cells, surface harden- ing of steel through carburization or nitridation etc. [4]. The first attempt to measure self-diffusion (the most basic diffusion process) was that of physico-chemist Georg Karl Von Hevesy who studied self-diffusion in liquid [5] and in solid lead [6] by using a natural radioisotope 210 Pb and 212 Pb of lead. From the pioneering work of Alder and Wainwright [7,8] the simulation of diffusion coefficient has been an area of continuous research. The equations of Fick, the statistical interpretation of diffusion coefficient by Einstein and Smoluchowski and the Boltzmann–Matano method for concentration dependent diffusion coefficients opened the way for experimental techniques [4]. Now there is a magnificent increase in the application of computer modeling and simulation method in different areas of research [9–11] and also to study the diffusion process [2]. The study of the diffusion process has been done using a molecular dynamics simulation tech- nique [12–14]. Rahman and Stillincer considered 216 rigid water molecules to examine the structural and kinetic behavior of liquid water at 34.3 °C [15]. Malenkov et al. studied the physical, structural and dynamic properties of liquid water and ice [16,17]. Malenkov considered a huge system containing 3456 water molecules to study the correlation coefficient [17]. Due to the availability of the open source packages like GROMACS [18] and the different experimental techniques, Ringbom apparatus [19], diaphragm cell, infinite couple, Taylor dispersion, and nuclear magnetic resonance [20], the study of diffusion process is familiar in large scientific family. Despite the experimental study of the diffusion process of CO [21], to the best of our knowledge, there has been no molecular dynamics study on the diffusion of CO in water. Carbon monoxide (CO), as commonly recognized for its toxicological attributes, is a major component of smoke derived from fires and is also a major cause of deaths in fire. It is rapidly absorbed through the lungs and binds mostly to hemoglobin (Hb). The affinity of CO to Hb molecule is 200–240 times more than the affinity of oxygen; as a result, the carboxyhemoglobin level increases and this limits the oxygen carrying Journal of Molecular Liquids 194 (2014) 77–84 E-mail address: npadhikari@tucdp.edu.np (N.P. Adhikari). 0167-7322/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2014.01.004 Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq