The Influence of Water on Choline-Based Ionic Liquids

Choline and geranic acid (CAGE) based ionic liquids have been recently developed for applications in drug delivery. Understanding the microscopic structures of CAGE in the presence of water is critical for its continued use in biomedical applications since it will undoubtedly come into contact with water in physiological fluids. Water can drastically impact the physicochemical properties of the ionic liquids, including CAGE. Computational and experimental characterization, namely viscosity, conductivity, and self-diffusion coefficient, were employed here to understand the properties of equimolar CAGE (1:1 choline: geranic acid) in the presence of varying amounts of water. It was found that under stored conditions, 1:1 CAGE contained up to 0.20 mole fraction water. Experimental and computational studies indicate that microscopic intra-ionic interactions within CAGE are not substantially changed until the water content exceeds 0.65 mole fraction. At this point, we theorize that the geranate ions undergo reorganization to minimize contact between the hydrophobic tails and the water molecules. This is evidenced by the plateau in viscosity at this mole fraction, and the increased interactions between the tails of the anions. This suggests that CAGE could be used without pre-drying in most applications and can be diluted to induce the organization of the anions where desired.