Acid-base equilibria in ethaline. An approach providing a strategy for the pH modulation in deep eutectic solvents

The possibility of changing the pH in deep eutectic solvents when they are used as solvent media is fundamental in chemistry, since acid-base equilibria are of critical importance for almost all areas of chemistry, such as synthetic reactions, isolation of specific analytes, chromatographic separations, protein stability and enzyme catalysis. Such a possibility of modulating the pH of DESs is particularly interesting for enhancing the electrocatalytic properties of some electrode surfaces or promoting the stability and in some cases improving the activity of biorecognition systems such as antibodies, enzymes and aptamers that are particularly useful for biosensor applications. The aim of this investigation was the evaluation of the possibility of easily modulating the acid-base properties of the DES ethaline, adopted in our tests as the prototype of DESs because it displays a moderate viscosity at room temperature, by simply adding typical pH buffers such as weak acids or bases or salts consisting of weak acids and strong bases. This approach results to be particularly advantageous compared to that often used of modifying the proton activity of DESs by changing one or both of its precursors (HBA or HBD), since it allows their chemical-physical properties, such as density, viscosity, conductivity and surface tension, to be kept unchanged. With this purpose, acid-base equilibria in the DES ethaline were studied using cyclic voltammetry, pH measurements at glass electrodes, acid-base titrations and spectrophotometric measurements by exploiting suitable electrochemical probes, such as hydroquinone and alizarin: this last being an acid-base indicator. The results obtained with these measurements are all consistent with each other and provide an effective strategy to achieve pH modulation in DES, since the approach taken here in ethaline can be easily transferred to any other DES, provided it is capable of solubilizing the desired buffer species.