Magnetic field simulations in support of interdiffusion quantification with NMR

Modelling the mixing process of multicomponent liquids is still an open problem. In this work the possibility to use nuclear magnetic resonance (NMR) to measure concentration profiles during interdiffusion of a binary system is discussed. NMR is a non-invasive technique that can be used to measure diffusivities of liquids over the complete viscosity range even for optically non-transparent samples. This work discusses the difficulties that appear when measuring concentration profiles of a binary system. During the mixing process of two different liquids, the concentration gradient leads to a time-dependent distribution of magnetic susceptibility within the sample. NMR experiments as well as numerical simulations are used to show the magnetic field distortions introduced by the susceptibility of pure liquids as well as their mixture in the sample. If the field inhomogeneity is comparable to the chemical shift difference, the broadening of the NMR spectrum leads to important overlapping of the NMR lines. The lack of knowledge about the line shape, influenced by the field inhomogeneity, introduces errors in the concentration profile. Finally, a numerical method based on the simulation of the magnetic field distribution generated by a given concentration gradient is proposed to increase the accuracy of the concentration measurements.