Brønsted acids in ionic liquids: How acidity depends on the liquid structure

Jade A. McCune, Peizhao He, Marina Petkovic, Fergal Coleman, Julien Estager, John D. Holbrey, Kenneth R. Seddon, Malgorzata Swadzba-Kwasny*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

39 Citations (Scopus)
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Gutmann Acceptor Number (AN) values have been determined for Brønsted acid–ionic liquid mixtures, over a wide compositional range. Four systems of general formula [C2mim][A]–HA (A− = bistriflamide, [NTf2]−; triflate, [OTf]−; mesylate, [OMs]−; or acetate, [OAc]−, [C2mim]+ = 1-ethyl-3-methylimidazolium cation) were studied. A library of Brønsted acidic systems of varying acidity was constructed and the AN parameter was found to be a convenient approach for quantifying their acidity. HOAc, HOMs and HOTf, when dissolved in ionic liquids, were found to associate with the respective anions to form hydrogen-bonded anionic clusters, [A(HA)x]−. In contrast, HNTf2 was solubilised as a discrete, undissociated molecule. AN values were sensitive to the presence of anionic clusters; acidity could be buffered to a particular AN by binding the solubilised acid in the anionic cluster form. Overall, a simple way to manipulate and quantify the Brønsted acidity of acid–ionic liquid mixtures was demonstrated, and measured AN values were related to liquid speciation.
Original languageEnglish
Pages (from-to)23233-23243
Number of pages11
JournalPhysical Chemistry Chemical Physics
Issue number42
Early online date11 Sept 2014
Publication statusPublished - 2014


  • ionic liquids, acidity


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