Noura Dawass

6.1. I NTRODUCTION 6 115 cal potential which can be used to compute isotherms. Alternatively, solubilities of gases in solvents are directly computed using MC simulations in the grand– canonical and osmotic ensembles. MC simulations have been used to predict the solubility of gases in associating liquids [59, 148, 149] . MC simulations have been also widely-used to study the absorption of gases in solvents such as alco- hols [149, 191, 192] , ionic liquids [193, 194] , and deep eutectic solvents [143] . To the best of our knowledge, studies reporting MC simulations of the phase equi- libria of small gas molecules and MEG are lacking. A possible reason for the ab- sence of such studies is the fact that the simulation of dense liquids with strong intermolecular interactions, as in the case of MEG, is computationally demand- ing. MC simulations in open ensembles are often used to compute the solubility of solutes in liquids. In these ensembles, the solutes are added to or removed from the simulation box. For dense liquids and/or with the presence of strong interactions, such insertions can be challenging [58, 60] . To enhance the effi- ciency of molecular transfers in MC simulations, Shi and Maginn [59, 195] de- veloped the CFCMC (Continuous Fractional Component Monte Carlo) simula- tion method. In this method, the system is expanded using a so–called fractional molecule with a coupling parameter λ , which is used to vary the interactions between the fractional molecule and the surrounding molecules. In solubility calculations, a fractional molecule is used to gradually add/remove molecules to/from the solvent [196] . The presence of a fractional molecule does not af- fect the prediction of thermodynamic properties of the system [60, 197] . For a detailed discussion of the CFCMC method the reader is referred to the recent review by Rahbari et al. [62] . A prerequisite for successful MC simulations of pure and multi-component mixtures is the use of force fields that can adequately represent inter- and in- tramolecular interactions. Thus, another challenge of simulating gases in asso- ciating liquids, such as MEG, is the availability of force fields that provide accu- rate predictions of the desired properties. One of the most commonly-used force fields for a large number of gases and liquids is the Transferable Potentials for Phase Equilibria (TraPPE) force field [198, 199] . The TraPPE force field has been successfully used for the prediction of thermodynamic and transport properties of gases and liquids [149, 192, 193, 200– 202, 202– 205] . Cardona et al. [206] used TraPPE and other classical force fields to compute thermodynamic properties of pure MEG. The authors found that the united–atom version of TraPPE (TraPPE– UA) [199] is able to accurately predict thermodynamic properties of pure MEG, such as the density, isothermal compressibility, and heat of vaporisation. In this chapter, we aim to predict the solubilities of small gas molecules (CO 2 , CH 4 , N 2 , and H 2 S) in MEG using CFCMC simulations in the osmotic ensemble. To model MEG and the gases studied in this work (Ref. [171] ), the TraPPE–UA