Polarity-Influenced Ion Transfer Kinetics at the Liquid-Liquid Interface

Because of the unique water/organic (w/o) interface structure, the interface between two immiscible electrolyte solutions (ITIES) is regarded as an effective biomimetic model for cell membranes and has been extensively used in drug delivery research. Nonpolar organic solvents such as dichloroethane (DCE) have been widely adopted as the organic phase in ITIES, yet the nonpolar molecular structure of the solvent greatly differs from the components of cell membranes, which may influence the ion transfer process across the w/o interface. Thus, in this work, tributyrin (Tri), an organic solvent structurally analogous to glycerophospholipids, was employed as a moderately polar oil phase in ITIES. Although the water/tributyrin (w/Tri) interface exhibited a narrower potential window and significantly slower ion transfer kinetics for tetrabutylammonium (TBA) compared to the conventional water/DCE (w/DCE) interface, a considerably faster transfer kinetics was observed at w/Tri than at w/DCE for a polar ion tolazoline (TOZ). This suggested that ion transfer kinetics depended on the polarity of both the ion and the organic solvent and was further confirmed by systematic measurements of ions with varying molecular polarities. On the basis of this, the direct ionophore-free transfers of the metal ions Cs+ and K+ were successfully achieved at the w/Tri interface for the first time. This finding not only expands the understanding of ITIES systems but also opens up new approaches to developing biomimetic sensing interfaces for polar bioactive molecules.