Current-stimulated release of solutes solubilized in water-immiscible room temperature ionic liquids (RTILs)

Room temperature ionic liquids (RTILs), salts which are liquid at room temperature, may be water-soluble or water immiscible, depending on the combination of cation and anion. They are efficient solvents for a wide range of solutes including drugs. The water-immiscible RTILs studied in this paper (the 1-butyl, hexyl and octyl 3-methyl imidazolium (BMIM, HMIM, and OMIM) hexafluorophosphate (PF(6)(-)) salts) can act as drug reservoirs. Passage of an electric current through these immiscible liquids can enhance the release of some solutes into an aqueous medium. Current flow (over the range 1-5 mA) increased the release rate of a solubilized hydrophilic solute, (3)H-sucrose, and of a model hydrophobic drug, (3)H-dexametasone. A threefold increase in the release rate of both sucrose and dexamethasone into water was observed under some conditions although the effect of application of current was not always linear. OMIM[PF(6)] was the most responsive liquid. Some measurable physical properties of the ionic liquids change on the application of current. For example, the surface tension of the three RTILs studied decreased significantly on application of current for 15 min (from 47.8 mNm(-1) to 36.2 mNm(-1) for BMIM) but the effect on the surface tension of the OMIM salt was small. Only a small decrease in the viscosity of RTILs was observed. Although the mechanisms of the enhanced release are not yet elucidated, RTILs are potentially interesting depots for electrically controlled drug delivery.