Optimizing Noble Gas-Water Interactions via Monte Carlo Simulations.

Oliver Warr,Junju Mu,Andrew Masters,Chris J Ballentine

doi:10.1021/acs.jpcb.5b06389

Oliver Warr, Junju Mu + Show 2 more

Open Access

https://doi.org/10.1021/acs.jpcb.5b06389

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Abstract

In this work we present optimized noble gas-water Lennard-Jones 6-12 pair potentials for each noble gas. Given the significantly different atomic nature of water and the noble gases, the standard Lorentz-Berthelot mixing rules produce inaccurate unlike molecular interactions between these two species. Consequently, we find simulated Henry's coefficients deviate significantly from their experimental counterparts for the investigated thermodynamic range (293-353 K at 1 and 10 atm), due to a poor unlike potential well term (εij). Where εij is too high or low, so too is the strength of the resultant noble gas-water interaction. This observed inadequacy in using the Lorentz-Berthelot mixing rules is countered in this work by scaling εij for helium, neon, argon, and krypton by factors of 0.91, 0.8, 1.1, and 1.05, respectively, to reach a much improved agreement with experimental Henry's coefficients. Due to the highly sensitive nature of the xenon εij term, coupled with the reasonable agreement of the initial values, no scaling factor is applied for this noble gas. These resulting optimized pair potentials also accurately predict partitioning within a CO2-H2O binary phase system as well as diffusion coefficients in ambient water. This further supports the quality of these interaction potentials. Consequently, they can now form a well-grounded basis for the future molecular modeling of multiphase geological systems.

Highlights

When trace amounts of noble gases are present within a gas− water binary phase system, they exist as solute particles within both phases.[1−3] The exact ratio of noble gases in each phase, the partitioning, is dependent on environmental factors such as temperature and phase composition.[2,3]
Using the isothermal−isobaric (NPT) Gibbs ensemble Monte Carlo (GEMC) technique,[30] the Towhee simulation package[31] is adapted to simulate a 2-box binary phase system composed of a water-rich phase with an adjacent noble gas phase
From both previous studies and the GEMC simulation solubility data in this study it has been proven that the Lorentz−Berthelot combining rules are frequently inadequate for generating suitable unlike Lennard-Jones 6-12 potentials

Summary

Introduction

They provide information on the formation and evolution of natural geological systems,[4,5] constrain groundwater recharge rates,[6,7] and allow us to reconstruct palaeotemperatures[8,9] and model ocean circulation patterns.[10,11] The partitioning of noble gases can play an important economic role as it can quantify the origin and evolution of hydrocarbon reservoirs[12,13] and valuable ore deposits.[14,15] Their utility in this role is intrinsically linked to an intimate understanding of noble gas behavior within multiphase fluid systems. One major limitation of this approach is the uncertainty associated with extrapolation between and beyond defined experimental conditions for which noble gas data have been generated (e.g., refs 2, 17, and 18)

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Conclusion