Interpretation of NMR diffusometry data regarding droplet size distributions in micro- and nanoemulsions
J.-H. Sommerlinga*, A. J. Simona, A. Haberb, M. Johnsb, G. Guthausenc, H. Nirschla aProcess Machines, Institute for Mechanical Process Engineering an Mechanics, Karlsruhe Institute of Technology bSchool of Mechanical and Chemical Engineering, University of Western Australia cPro2NMR, Institute for Mechanical Process Engineering and Mechanics and Institute of Biological Interfaces 4, Karlsruhe Institute of Technology *Corresponding author: [email protected]
Nuclear magnetic resonance (NMR) diffusometry allows to measure droplet size distributions in emulsions. This non-invasive, non-destructive method has several advantages compared to common mainly optical methods for droplet sizing. NMR requires almost no sample preparation, unlike optical methods or cryoTEM, where samples need to be transparent and preparation or dilution is necessary. The established method, combining the concept of Murday and Cotts for diffusion in a limited spherical geometry with Packer and Rees, assuming a log-normal distribution for the droplet sizes, has limitations regarding the distribution and size range of droplets. There are approaches to overcome the limitations raised by the statistical log-normal distribution. A very successful approach is a numerical method, the regularisation, which does not assume any distribution form. In this work, we discuss the interpretation of NMR PFG-STE data leading to droplet size distributions in accordance to the established methods and the regularisation. We also address the problem of the lower size limitations and propose a way to use the Stokes-Einstein equation to gain droplet size distributions (DSD) for emulsions with droplets smaller than 1 μm of OW emulsions.