Features of local ordering of biocompatible ionic liquids: The case of choline-based amino acid ionic liquids

Choline-based amino acid ionic liquids, having low toxicity, high biocompatibility and biodegradability, are among the most promising compounds as solvents and reaction media. Their microscopic ordering, determined by interionic interactions, can significantly affect their physicochemical behavior. We present here a detailed study of the interionic interactions and local structure in [Ch][Gly], [Ch][Ala], and [Ch][Pro] using the integral equation method in the framework of the 1D-RISM (Reference Interaction Site Model) approach. The features of local ordering in these ionic liquids were analyzed in terms of the potentials of mean force and radial distribution functions. It was found that the key structural units of the studied compounds are counterion pairs that mutually coordinate each other. This cation-anion association is realized in a cooperative manner, involving the carboxylate group of the anions and the hydroxyl and onium groups of the cation. Moreover, there are indications for the formation of weak anion-anion associates in [Ch][Gly] and [Ch][Ala] and weak triple cation-anion-cation aggregates in [Ch][Ala] and [Ch][Pro]. The present contribution provides the molecular level information on the structure of this novel class of biocompatible ionic liquids that is required for their use as bioactive compounds in various applications. (C) 2019 Elsevier B.V. All rights reserved.