Scaled charges at work: Salting out and interfacial tension of methane with electrolyte solutions from computer simulations

Abstract

The solubility of methane in water decreases when a small amount of salt is present. This is usually denoted as the salting out effect (i.e., the methane is expelled from the solution when it contains small amounts of salt). The effect is important, for instance the solubility is reduced by a factor of three in a 4 m (mol/kg) NaCl solution. Some years ago we showed that the salting out effect of methane in water can be described qualitatively by molecular models using computer simulations. However the salting out effect was overestimated. In fact, it was found that the solubility of methane was reduced by a factor of eight. This points to limitations in the force field used. In this work we have carried out direct coexistence simulations to describe the salting out effect of methane in water using a recently proposed force field (denoted as Madrid-2019) based on the use of scaled charges for the ions and the TIP4P/2005 force field for water. For NaCl the results of the Madrid-2019 force field significantly improve the description of salting out of methane. For other salts the results are quite reasonable. Thus the reduction of the charge of the ions also seems to be able to improve the description of salting out effect of methane in water. Besides this we shall show that the brine-methane interface exhibits an increased interfacial tension as compared to that of the water-methane system. It is well known that electrolytes tend to increase the surface tension of liquid water, and this seems also to be the case for the interface between water and methane.

“In Silico” Seawater

“…51,52 The diffusion coefficient of CO 2 in brines has already been investigated by molecular simulation (see for instance ref 53.). However, similarly to the case of the salting out effect of methane 54 we have preliminary results indicating that the solubility of CO2 in a NaCl solution may not be described correctly by most of the current force fields. In addition, it would be necessary to fit the interaction parameters of CO2 with the main ionic components of seawater beyond standard LB rules.…”

Interfacial behavior of the decane + brine + surfactant system in the presence of carbon dioxide, methane, and their mixture

“…Note that the IFT of the water + CH 4 /CO 2 and brine + CH 4 /CO 2 systems depends nonmonotonically on pressure. [72][73][74][75][79][80][81][82][83][84][85][86][87][88] Furthermore, the IFTs of the alkane + water + CH 4 /CO 2 and alkane + brine + CH 4 /CO 2 systems increased with decreasing x CH 4 /x CO 2 . Here, the influence of methane was found to be less pronounced than that of carbon dioxide.…”

“…38,40 Details of the interfacial behavior of the water + CH 4 /CO 2 and brine + CH 4 / CO 2 systems have been described by us and others. [72][73][74][75][80][81][82][83][84][86][87][88] Here the enrichment of the interfacial region with CH 4 and CO 2 depends nonmonotonically on pressure. Note that the behavior of the IFT can be further understood by means of the Gibbs adsorption equation:…”

Thermal conductivity of aqueous solutions of reline, ethaline, and glyceline deep eutectic solvents; a molecular dynamics simulation study

“…As discussed in detail in the studies of Perkins et al [49,51] and Liu et al [18], charge scaling in ILs and DESs is essential because electrostatic interactions are usually overpredicted. Recently, Blazquez et al [52] showed that the scaling of charges of even simple electrolytes such as NaCl can lead to an improved prediction of the salting out effect of methane in water. In the past few years, various studies have shown that simulations using the GAFF force field combined with scaled charges yield relatively accurate thermodynamic and transport properties of neat reline, ethaline, and glyceline [6,16,34,35,49,51,53].…”