Developing a fast estimation method for the pH in aqueous solutions made of ILs with protic constituents: The case of choline with amino acid and peptide anions

Abstract

This work aims to provide simple and fast estimations for the pH of protic Ionic Liquids (ILs) solutions, particularly for those constituted with choline cation ([Ch]⁺) and Amino Acid (AA) or Peptide (Pep) -based anions. pH calculation based on the pK_a of pILs are typically carried-out with a posteriori, after synthesis and measurement, or a priori procedures, through quantum chemical calculations. Here, we adapt the isoelectric point (pI) concept to perform a straightforward pH estimation for an aqueous solution of pILs using the pK_a from their parental ions. Moreover, a library of pH values for 1 M aqueous solution of 14 different in-house synthetized ILs based on AA and Pep is presented, including the first (pH, FTIR and TGA) characterization of some of those solvents. Following the pI approach and using the pK_a of the parental ions, the pH of a 1 M aqueous solution of these ILs has been predicted with suitable accuracy (RMSD ≈ 0.5 units of pH), fulfilling the main estimation purposes of this work. Despite the tabulated pK_a of each molecule being a more suitable input, software able to deploy fast pK_a predictions may be used as well, especially when no experimental values are available, but one must be aware that some systematic errors may appear. In summary, we present a fast and straightforward method able to simplify the pH prediction of pIL aqueous solutions, a matter that can be key in the design of IL-based technologies.