title: Brønsted acids in ionic liquids: how acidity depends on the liquid structure.
abstract: 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.
authors.affiliation: QUILL, The Queen's University of Belfast, Belfast, BT9 5AG, UK. m.swadzba.kwasny@qub.ac.uk.
authors.forename: Jade A
authors.initials: JA
authors.lastname: McCune
authors.forename: Peizhao
authors.initials: P
authors.lastname: He
authors.forename: Marina
authors.initials: M
authors.lastname: Petkovic
authors.forename: Fergal
authors.initials: F
authors.lastname: Coleman
authors.forename: Julien
authors.initials: J
authors.lastname: Estager
authors.forename: John D
authors.initials: JD
authors.lastname: Holbrey
authors.forename: Kenneth R
authors.initials: KR
authors.lastname: Seddon
authors.forename: Małgorzata
authors.initials: M
authors.lastname: Swadźba-Kwaśny
mesh_terms: D000143:Acids / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D052578:Ionic Liquids / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure
chemical_list: D000143:Acids; D052578:Ionic Liquids