Thermodynamic modeling of gas solubility in aqueous solutions of quaternary ammonium salts with the e-CPA equation of state

The study of gas solubility in aqueous electrolyte solutions is important, e.g. for hydrate applications, and it is also a challenging task, as metal halide salts show salting-out effects on gas in water, while some quaternary ammonium salts (QAS) show salting-in effects. This work presents a modeling study of gas solubility in aqueous solutions of several QAS (tetra-n-methyl-ammonium bromide, tetra-n-ethyl-ammonium bromide, tetra-n-propyl-ammonium bromide and tetra-n-butyl ammonium bromide) with the electrolyte Cubic-Plus-Association Equation of State (e-CPA). The ion size and ion-water interaction parameters are obtained by fitting the experimental data of mean ionic activity coefficients and osmotic coefficients of corresponding binary mixtures. The results show that e-CPA can reasonably correlate the mean ionic activity coefficients of QAS in aqueous solutions. The ion-gas interaction parameters are obtained by fitting the experimental data of gas solubility and the results show that e-CPA can correlate the gas solubilities (for nitrogen, carbon dioxide, methane and ethane) reasonably well from a quantitative point of view. For example, e-CPA gives deviations of 9.2% and 5.7% for the solubilities of carbon dioxide and methane in TBAB, respectively. The salting-in and salting-out effects and the influencing factors are also studied.