Solubility of sodium chloride in phosphonium-based deep eutectic solvents

The solubility of sodium chloride (NaCl) in several phosphonium-based deep eutectic solvents (DESs) is studied in this work. Seven combinations of DESs were investigated. The solubility of NaCl increased with temperature in all DESs at all combinations. The type of salt and hydrogen bond donor (HBD) and the salt:HBD mole ratio of the DES affected the solubility of NaCl. The solubility of NaCl in DESs that use ethylene glycol or glycerol (neutral molecules) as HBDs is very low in comparison to DESs that use zinc (II) chloride (ZnCl₂) or ferric (III) chloride (FeCl₃) which are metal halides as complexing agents. DESs containing ZnCl₂produced the highest solubility. Ethyltriphenylphosphonium bromide:ZnCl₂DES possessed a NaCl solubility of 88.500 wt.% at 111 °C, and tetrabutylphosphonium bromide:ZnCl₂DES possessed NaCl solubilities of 70.120 wt.% and 75.000 wt.% at 100 °C and 160 °C, respectively. The thermodynamic activity coefficient non-random two-liquid (NRTL) model was used to correlate the solubility of NaCl in the studied DESs. The calculated values in some DESs showed satisfactory agreement with the experimental values.