New phosphonium ionic liquid with neodecanoate anion as butyric acid extractant

Liquid–liquid equilibrium data on butyric acid (BA) extraction by new ionic liquid (IL) tetradecyl(trihexyl)phosphonium neodecanoate ([C14C6C6C6P][NDec]) are presented and correlated using a model developed earlier. Extraction performance of the new IL is comparable with that of previously tested phosphonium ILs with decanoate and phosphinate anions. High loading of IL with BA of more than 12 BA molecules per one IL ion pair was achieved at aqueous BA concentration of 1.2 mol dm−3. The overall extraction mechanism includes competitive extraction of BA and water, and coextraction of BA with water. Both these sub-mechanisms act simultaneously, the first one dominates below IL loading by acid of 2 and the second one above this value. Dynamic viscosity of water saturated [C14C6C6C6P][NDec] is slightly higher compared to a similar IL with decanoate anion [C14C6C6C6P][Dec] (104 and 96 mPa s). However, at 298 K dried [C14C6C6C6P][Dec] is solid, while [C14C6C6C6P][NDec] is liquid even below 253 K which is a great advantage. Viscosity of equilibrium organic phases shows a maximum at IL loading by BA of about 1.6 which corresponds to the minimal solute (BA + water) content and maximal molar fraction of IL due to water release from the solvent.