Preparation of protic ionic liquids with minimal water content and 15N NMR study of proton transfer

Abstract

Low-molecular-weight Brønsted acids and amine bases were used to reproducibly prepare very dry, high-purity room-temperature protic ionic liquids (PILs). A series of eight amine bases and six Brønsted acids were combined to produce 48 mixtures, of which 18 were liquid at room temperature. The phase transitions and thermal decomposition temperatures were determined for each mixture; whereas viscosity, density and conductivity were determined for the room-temperature liquids. By utilising (15)N NMR it was possible to distinguish between neutral and ionised amine bases (ammonia vs. ammonium-type ion), which indicated that the protic ionic liquids were completely ionised when made as a stoichiometric mixture. However, a Walden plot comparison of fluidity and molar conductivity indicated the majority of PILs had much lower conductivity than predicted by viscosity unless the base contained excess proton-donating groups. This disparity is indicative of protic ionic molecules forming neutral aggregates or non-Newtonian fluid hydrogen-bonded networks with a secondary Grotthuss proton-hopping mechanism arising from polyprotic bases.

Physico-Chemical Properties of Non-Newtonian Shear Thickening Diisopropyl-ethylammonium-Based Protic Ionic Liquids and Their Mixtures with Water and Acetonitrile

“…Nevertheless, by increasing the added water in a PIL, presenting originally a non-Newtonian shear thinning behavior, Burrell et al showed that the flow behavior of the mixture changes from nonNewtonian shear thinning to Newtonian and then from Newtonian to non-Newtonian shear thickening. 49,50 Altin et al 56 have reported, in the case of the Newtonian IL 1-ethyl-3-methyl ethylsulfate, that depending on the concentration of Fe 2 O 3 particles in this IL the flow behavior of the mixture can change also from Newtonian to nonNewtonian shear thinning and shear thickening.…”

“…In fact, non-Newtonian behavior has been described for several AILs and PILs. [46][47][48][49][50] For example, Seddon et al 46 have shown that the alkylimidazolium tetrafluoroborate family (number of carbon atoms is typically 12) is thixotropic fluids whose viscosity decreases when increasing the shear rate. Kulkarni et al 48 have reported that the flow behavior may change from Newtonian to non-Newtonian by increasing the alkyl chain length on the cation like alkyl-methylimidazoliumbased AILs.…”

“…51 In this paper, the authors have presented information on the micellar structure changes based on rheology and freezefracture images. 51 On the basis of these papers, [49][50][51] we have also reported herein the rheological study of the selected PILs and mixtures to investigate briefly the rheological behaviors of these PILs, the impact of additions of water or acetonitrile on this property, and the formation of aggregates in these media.…”

“…Generally, ILs are described in the literature as Newtonian fluids. [42][43][44][45] Nevertheless, non-Newtonian behaviors have already been reported for some AILs [46][47][48] 49,50 Non-Newtonian shear thinning phenomena have recently been described in several PILs by Burrell et al 50 In this paper, Burrell et al have also investigated the effect of water on this property, and they showed that by increasing the water concentration the rheological behavior of the (PIL þ water) binary system changes from non-Newtonian shear thinning, to Newtonian, to non-Newtonian shear thickening behaviors. 50 More recently, a publication of Guo and Guo reports also that ionic liquid induced structure transition of nonionic wormlike micelles composed of Tween 80 and Brij 30.…”

Robust and versatile ionic liquid microarrays achieved by microcontact printing

“…Protic ILs, triethylammonium methylsulphonate (TAMS), EDAA and EDAF, were synthesized according to the procedure described before 36 . Briefly, the base solution (for example, triethylamine and ethylenediamine) and the acid solution (for example, methylsulphonic acid, acetic acid and formic acid) were purified, dried and handled under inert gas atmosphere.…”