Excess Thermodynamic Properties of Mixtures Involving Xenon and Light Alkanes: A Study of Their Temperature Dependence by Computer Simulation

Abstract

As a natural extension of a previous work, excess molar enthalpies and excess molar volumes as a function of composition in a wide range of temperatures have been obtained for binary mixtures of xenon with ethane, propane, and n-butane by Monte Carlo computer simulation. Xenon was modeled by a simple spherical Lennard-Jones potential, and the TraPPE-UA force field was used to describe the n-alkanes. One of the main goals of this study is to investigate the temperature dependence of the excess properties for mixtures of xenon and n-alkanes and, if possible, to supplement the lack of experimental data. For all three systems, the simulation results predicted excess volumes in good agreement with the experimental data. As for the excess enthalpies, in the case of (xenon + ethane), the simulation results confirm the negative experimental result and the weak temperature dependence. In the case of (xenon + propane) and (xenon + n-butane), however, the simulation predicts negative excess enthalpies, but those estimated from experimental data are positive. Both excess volumes and enthalpies display a complex dependence on temperature that in some aspects resembles that found for mixtures of n-alkanes.The structure of the liquid mixtures was also investigated by calculating radial distribution functions [g αβ(r)] between each pair of interaction groups for all the binary systems at all temperatures. It is found that the mean distance between xenon and CH2 groups is systematically higher than the distance between xenon and CH3. In addition, the number of groups around xenon in the first coordination sphere was calculated and seems to be proportionally more populated by methyl groups than by methylene groups. The results seem to reflect a preferential and stronger interaction between xenon and CH3, in agreement with previous findings.

Solubility of xenon in liquid n-alkanes and cycloalkanes by computer simulation. Towards the perfect anaesthetic

“…Representing solvation enthalpy as a function of solvent's reduced temperature (also in Fig. 6 and table 5), it can be seen that the maxima are located in a relatively narrow temperature range, between 0.51 and 0.56 for all n-alkanes studied in this work and also for the dissolution of dT changes its sign is also very close to the reduced temperature where the mixing process for binary mixtures of n-alkanes and of light n-alkanes with xenon changes from endothermic to exothermic [17].…”

“…Interestingly, this is very close to the reduced temperature at which the excess enthalpy for equimolar mixtures of n-alkanes changes from positive to negative. This result was seen as an additional example of the similarity between (xenon + n-alkane) and (n-alkane + nalkane) interactions [17,24]. Monte Carlo simulations, using the TraPPE united atom model [25] and SAFT-VR calculations [26] produced Henry's constants in close agreement with experiment.…”

“…Noble gases and alkanes have long been used as model substances to study the effect of size, shape and flexibility on the thermodynamic properties of fluid mixtures [14][15][16][17][18]. The absence of molecular structure of the noble gases (in other words, their isotropic intermolecular potential) allows their use as reference particles, comparing the properties of their binary mixtures with components of different shapes and polarities.…”

Probing the Structure of Liquids with 129Xe NMR Spectroscopy: n-Alkanes, Cycloalkanes, and Branched Alkanes

“…Additionally, in order to find links between the solvent structure within xenon’s first coordination sphere and the experimentally obtained chemical shifts, the solutions were studied by molecular dynamics simulation, using a well established atomistic force field to model the alkane solvents, the optimized potentials for liquid simulations (OPLS-AA), and an optimized Lennard-Jones potential for liquid xenon . We have used these and other force fields in a number of previous studies − in which several thermodynamic properties for solutions of xenon in alkanes (Henry’s constants, excess volumes, excess enthalpies, etc.) were calculated and compared with theoretical results and our own experimental data.…”