The impact of oligomeric anions on the speciation of protic ionic liquids


The synthesis of protic ionic liquids based in carboxylate anions, too often admitted as being straightforward, is actually challenging and must be carefully addressed. This review discusses the importance of oligomeric anions, in particular those based on carboxylates, in the behavior and nature of protic ionic liquids. There is strong evidence in the literature that the synthesis, and subsequent purification, of protic ionic liquids involving carboxylic acids, leads to structures in the liquid phase with an acid:base proportion different from the expected 1:1 stoichiometry. The formation of the oligomeric anions, mostly dependent on the proton transfer from the Bronsted acid to the Bronsted base, may lead to a higher ionicity that suggests the formation of true ionic liquids. It is here stressed the relevance of deepening the understanding of the interactions between the species and the speciation of the liquid phase, combining experimental and theoretical approaches to establish foundations for insightful advances in the area. (C) 2020 Elsevier B.V. All rights reserved.



subject category

Thermodynamics; Chemistry, Physical; Engineering, Chemical


Martins, MAR; Carvalho, PJ; Santos, LMNBF; Pinho, SP; Coutinho, JAP

our authors


This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, 1MB/50011[2020 UIDP;50011;2020, CIMO-Mountain Research Center, UM/00690[2020, and CIQUP, University of Porto (Projects: PEstUID/QUI/00081/2013) financed by national funds through the Portuguese Foundation for Science and Technology (FCT)/MCTES. This work was also developed within the scope of the IndoPortuguese Program for Cooperation in Science & Technology OST[INT/Portugal/P-OU2017, financed by FCT and the Government of India. P.J.C. acknowledge FCT for his contract under the Investigator FCT 2015 contract number IF/00758/2015.

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