Spontaneous formation of unilamellar vesicles based on the surfactant 1-methylimidazolium bis-(2-ethylhexyl) phosphate, evaluated as a function of pH and in saline solution

Abstract

In this work, we report the formation of unilamellar vesicles in water based on the protic ionic liquid-surfactant 1-methylimidazolium bis-(2-ethylhexyl) phosphate (imim-DEHP) explored under different experimental conditions (surfactant concentration, pH and ionic strength). Surface tension measurements were made depending on the surfactant concentration at different pHs and in saline solution. The size of the vesicles was measured by dynamic light scattering (DLS) and images of the systems were obtained by atomic force microscopy (AFM). Our results showed that the surfactant self-assembled forming spontaneously vesicles at different pHs (from 2–8) and in saline solution. The vesicle formation, especially at pH ≥ 4, is independent of the imim-DEHP concentration and the ionic strength of the medium used, which demonstrates great stability in the systems formed. When the pH is lower than the pKa of the polar head group, the surfactant produces very large polydisperse aggregates, which is due to an agglomeration of smaller vesicles. Very interesting is that the dependence of the vesicle size with the pH value is reversible therefore, this system could be used as drug delivery under different conditions. In addition to this, imim-DEHP vesicles were coated with chitosan to improve biocompatibility and avoid possible effects of cytotoxicity of these systems. In summary, imim-DEHP is a promising surfactant to be used in biological applications, for example as a nanocarrier for the incorporation of bioactive compounds into living organisms.