Phosphine oxides functionalized pillar[5]arenes for uranyl extraction: Solvent effect and thermodynamics

As pre-organized multidentate extractants, phosphine oxides functionalized pillar[5]arenes (POP5As) have showed superior efficiency for uranyl extraction. In this work, the effect of type of organic diluents including dichloromethane (DCM), 1,2-dichloroethane (DCE), 1-nitro-3-(trifluoromethyl)benzene (NTFB) and nitropropane (NP) with different polarity and the fundamental thermodynamic behaviour were investigated in the solvent extraction of uranyl by POP5As from nitric acid (1 M) solution. The extraction ability pertaining to spacer length (-CnH2n-, n = 2, 4, 6), diluent polarity and temperature has been evaluated by comparing the distribution coefficients. The extractant with longer spacer exhibited a better performance in all diluents examined. Particularly, influencing factors including structural variation of ligands and property of organic diluents for the extraction efficiency were discussed from the thermodynamic point of view (Delta H, Delta S and Delta G). Generally, the extraction processes of uranyl by POP5As were exothermic and spontaneous in nature. In the case of DCE, NTFB and NP, Delta H < 0 and Delta S < 0, the extraction process of uranyl is enthalpy driven whereas the entropy factor counteracts it. In DCM, Delta H < 0 and Delta S > 0, the extraction is synergistically driven by enthalpy and entropy.