pH Measurements in Ionic Liquids

Deep eutectic solvents (DESs) and ionic liquids (ILs) have been significantly used in a variety of applications and one of the challenges is to understand the behaviour of protons in these non-aqueous media. The aim of this study is to investigate pH scales in DESs and ILs and to use the values to understand processes which occur in these media. The first stage of the study involved the development of a spectroscopic analytical method for measuring effective proton activity in IL media. Its pKIn data for bromophenol blue as an indicator were obtained in DESs and ionic liquids and it was found that the pKIn was influenced by the amount of the salt in the system. The value of pKIn was lower in DESs and ILs than in water. pKIn values were used to obtain pKa values for different organic acid in variety of DESs and in two different imidazolium-based ILs containing different anions. The order of acidity is controlled by the element of the anion within it and it was found that F- > HSO4-> Cl-> Ac- suggesting some correlation with hydrogen bond strength. In the second stage of this study a glass membrane electrode was used for the first time in DESs and ILs. The investigation successfully measured the pH scales of different DESs. It was shown that the response of a glass electrode was almost Nernstian in a DES with minimal water incorporated. The Nernstian slope increased as more water was added to the DES. A new, non-aqueous glass electrode was also made using Ag/AgCl in Ethaline reference electrodes. It was shown that this also had a Nernstian response showing that the glass membrane could function with only the traces of water naturally incorporated in the DESs. In the final part of this study the pH values of DESs were used to investigate the solubility of metal oxides in different DESs and the solubility was found to correlate well with the pH of the liquid. The formation of polyaniline was invetgated in solutions of differing pH both with and without water. The data showed the effect of both pH and mass transport on the rate of film growth. It was found that pH was not directly related to polymer growth rate but it showed that polymer only grew when an emulsion of monomer formed in solution. This significant discovery was only enabled by quantification of solution pH.