Studies of hydrogen bonding in protonic ionic liquids and their binary mixtures

Abstract

It is nearly thirty years since the first report of hydrogen-bonding in room-temperature ionic liquids, which was met by an immediate chorus of derision. Today, it is universally accepted, but still has not received the degree of attention that it deserves. The focus of this thesis was the study of hydrogen-bonding in protonic ionic liquids and their binary mixtures. To achieve it, several high purity alkylammonium nitrate ionic liquids were synthesised and characterised by elemental analysis, Karl Fisher titration, 1H and 13C NMR, DSC and TGA. The study was focussed on two aspects, firstly the comparison of the homologous [N0 0 0 n][NO3] series (n = 2, 3, or 4) and secondly the related series [N0 0 0 2][NO3], [N0 0 2 2][NO3], [N0 2 2 2][NO3], and [N2 2 2 2][NO3] (where there is no hydrogen-bonding in the last salt). This is the first systematic study of mixtures of protonic alkylammonium nitrate salts. Physicochemical properties, such as density, dynamic viscosity and ionic conductivity, were determined to investigate how hydrogen bonds influence the macroscopic features of these pure compounds and their binary mixtures, in a collaboration with a research group at ITQB (Lisbon). In addition, a mathematical model based on a neural network was performed to establish the relationship between physical properties and the intrinsic characteristics of these systems, such as temperature, composition and molecular weight, in a collaboration with a research group from Complutense University of Madrid. All these ionic liquids: ethylammonium nitrate, propylammonium nitrate, butylammonium nitrate, diethylammonium nitrate, triethylammonium nitrate, and tetraethylammonium nitrate, their binary mixtures and their deuteriated analogues were investigated by the vibrational spectroscopic methods - infrared, and Raman spectroscopies, and inelastic neutron scattering - and the results were supported by theoretical calculations performed at ISIS Rutherford Appleton Laboratories. The selective deuteriation of the pure salts, and their deuteriated binary mixtures, was an important tool to determine the manner in which the hydrogen bonding networks were organised. In a preliminary study, the temperature dependence of the proton dynamics of pure methylammonium nitrate and its deuteriated analogue were evaluated by quasi-elastic neutron scattering (QENS), collected at ISIS Rutherford Appleton Laboratories. The results reported here suggest that a jump diffusional model accurately represents its molecular motion. This thesis reports, for the first time, the actual hydrogen bond frequencies at low wavenumber for a series of protonic ionic liquids. This work, and its future extensions, thus may have a significant impact on our understanding of the relationship between the chemical structure of ionic liquids and the physicochemical properties.