Encyclopedia of Nuclear Magnetic Resonance, Advances in NMR (Volume 9)
A simple electrolyte solution consists of at least three different constituents, namely the solvent molecules, cations and anions. Many systems, e.g. those of biological, geological or industrial interest, consist of mixed solvents and/or mixed electrolytes and have correspondingly higher numbers of constituents. Hence, electrolyte solutions are typical multicomponent systems and therefore not easily accessible to many investigational methods, including a number of spectroscopic techniques, where signal contributions from different constituents can overlap. However, NMR spectroscopy represents an exception among the spectroscopic techniques since it offers important advantages with respect to liquid multicomponent solutions, and electrolyte solutions are particularly well suited to investigation by NMR techniques.
The main advantages of NMR are: (i) the generally narrow resonance lines in liquid electrolytes, enabling the detection of small chemical shift differences or small line broadening effects; and (ii) the selectivity of NMR allows an overlap-free observation of distinct constituents, even in a complex mixture. This selectivity, the most important advantage, comes from normally well resolved NMR lines due to the chemical shift differences, e.g. in 1H or 13C spectra of electrolyte solutions of simple ions, allowing the distinction of solvent molecules and ionic species by NMR of the same nuclide. Even more important for the selectivity of NMR is the huge number of different nuclides from the whole Periodic Table which can be utilized, in particular when ionic species or their nearest neighborhood can be observed. A third advantage of NMR, especially for equilibrium studies, arises from the fact that the NMR experiment at the extremely low energies in the rf region does not disturb the thermal equilibrium of the system under observation.
The aim of NMR studies of electrolyte solutions is to gain information about the structure and dynamics of liquid systems on a microscopic level. Thus, in contrast to the well-known applications of NMR to the study of molecular structures, we are dealing here with structural and dynamical effects, which are caused by intermolecular interactions. These intermolecular interactions are often relatively weak and compared with chemical bonds less sensitive to distance and orientation, and therefore the ‘structure’ of a liquid solution is less sharply defined.1 The best way to describe the structure of liquids and solutions is the use of pair correlation functions (or radial distribution functions) gij(r), which are defined by statistical mechanics and which represent a measure of the probability of finding a molecule (or atom) i at a distance r from molecule (or atom) j . Unfortunately, NMR techniques do not yield gij(r) directly, but certain nuclear magnetic relaxation rates 1/T1 are related to an integral over a pair correlation function. We will see that in NMR studies of electrolytes changes in the dynamics of the solvent molecules and of the ionic species are also often interpreted as a consequence of structural changes, and therefore sometimes a mixture of a structural and a dynamical description is used.2
The observation and detection of intermolecular interactions requires the measurement of quantities which are sensitive to intermolecular processes. This is the reason why in NMR studies on electrolyte solutions the measurement of nuclear magnetic relaxation times T1 and T2 (Relaxation: An Introduction) predominates by far over the measurement of chemical shifts and J couplings, the latter playing an almost negligible role.
The most characteristic property of electrolyte solutions is the electrical conductivity, that is the charge transport by migration of ionic species in an electric field. Therefore it is clear that an understanding of the translational motions in electrolytes is of outstanding importance. Here also NMR offers appropriate and unique measuring techniques for the study of translational displacements of selected species, namely the pulsed magnetic field gradient (PFG) spin echo techniques. These allow the selective measurement of translational diffusion coefficients (see Diffusion and Flow in Fluids and Diffusion Measurements by Magnetic Field Gradient Methods) of solvent molecules as well as of many ionic species, and also allow mobility measurements3 of charged particles (see Electrophoretic NMR). Taking all these special advantages of NMR
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