The way for improving the efficiency of ammonia and carbon dioxide absorption of choline chloride: Urea mixtures by modifying a choline cation

In the current work, two novel systems formed by chlorides of choline family (ChCl), namely ammonium salts with 2-hydroxyethyl and 2-(2-hydroxyethoxy)ethyl substituents, and urea (U) in a molar ratio 1 : 1 were obtained. Their densities (ρ), viscosities (η), refractive indices (nD) and properties related to the carbon dioxide and ammonia absorption were carefully investigated in a wide temperature range. The solubilities of NH3 and CO2 in the prepared mixtures at T = (303.2–333.2) K and p = (100–639) kPa were measured and linear correlation were used for fitting the experimental data. Based on this, the Henry's law constants both in a molality (Hm) and in a molar fraction scales (Hx), and the changes of enthalpy (ΔsolH∘), Gibbs free energy (ΔsolG∘) and entropy (ΔsolS∘) during the absorption process were calculated. The absorption capacity at 313.2 K and 101 kPa of the mixtures comprising new OH-grafted hydrogen bond acceptors (HBA) were found to be higher by 60% for NH3 and 17% for CO2 when one methyl in choline cation is exchanged by a 2-hydroxyethyl group, by 103% for NH3 and 27% for CO2 when two methyls are exchanged by two 2-hydroxyethyl groups, by 9% for NH3 and 51% for CO2 when one methyl is exchanged by a 2-(2-hydroxyethoxy)ethyl group, and by 30% for NH3 and 73% for CO2 when two methyls are exchanged by one 2-hydroxyethyl and one 2-(2-hydroxyethoxy)ethyl groups compared to the referent deep eutectic solvent (ChCl : 1.5 U). This assumes occurring the ammonia capture via most likely H-bonding interactions between the NH3 molecule(s) and the hydroxyalkyl fragments, while the ether's groups in HBA structure depress the positive effect of latter and hence decrease the general ammonia's solubility in the considered mixtures. In case of carbon dioxide, the absorption capacity of the studied mixtures correlates well with the free volume of the fluid.