Facilitated sulfate transfer across the nitrobenzene-water interface as mediated by hydrogen-bonding ionophores

Facilitated SO₄²- transfers by hydrogen bond-forming ionophores are investigated across the nitrobenzene (NB)-water interface by using polarography with a dropping electrolyte electrode. Bis-thiourea 1, α,α'-bis(N'-p-nitrophenylthioureylene)-m-xylene, is found to significantly facilitate the transfer of the highly hydrophilic SO ₄²- whereas its counterpart, N-(p-nitrophenyl)-N'- propylthiourea (ionophore 2), cannot. In contrast to the predominant formation of a 1:1 complex with SO₄^2- in the bulk NB phase, the SO₄²- transfer assisted by 1 is indeed based on the formation of a 1:2 complex between SO₄²- and ionophore, even under the condition of [SO₄²-]_aq ≫ [1]org. Such an exclusive formation of the 1:2 (SO₄²- to ionophore) complex at the NB-water interface is not observed with structurally similar bis-thiourea 3, α,α'-bis(N'-phenylthioureylene)-m-xylene, where p-nitrophenyl moietes of bis-thiourea 1 are simply replaced by phenyl groups. The facilitated transfer of SO₄^2- with bis-thiourea 1 is further compared to that of HPO₄²- and H₂PO₄- across the NB-water interface, which was previously shown to be assisted by 1 through the formation of the 1:1 and 2:1 (anion to ionophore) complexes, respectively. On the basis of these examinations, unique binding behaviors of hydrogen bond-forming ionophores at the NB-water interface are discussed, with a view towards development of ionophore-based anion-selective chemical sensors.