Diffusion coefficient estimations of radiometals in target solutions using microfluidic devices

Abstract

Radiometals re-emerge as a promising alternative in nuclear imaging. Cyclotron production of such radiometals using liquid targets solve critical impurity problems seen with current conventional production methods. Purification employing liquid-liquid extraction in microfluidic environments has recently been investigated. Fluid behaviour in microfluidic environments are deeply laminar, and one of the dominant factors in mass transfer in such environments is diffusion. For optimal design, numerical models are being developed to theoretically describe mass transfer in a microfluidic environment. To achieve this, diffusion coefficients of radiometals in their respective liquid target solutions need to be known. Exploiting the laminar flow properties in microfluidics, microfluidic devices have been employed in determining diffusion coefficients. This study presents a simplified 2D theoretical description of mass transfer in single phase flows in microfluidic channels. It has been attempted to verify the proposed model by determining the diffusion coefficient with methylene blue in an aqueous solution for direct comparison with literature. The method was tested for microfluidic devices of varying geometries, different flow rates and alternative setups, and yielded overestimations of a factor of two regarding literature. Consistently finding this overestimation, along with the discovery of several small mistakes in work presented in literature, does not render the method inaccurate. Further research in what range microfluidic devices remain an appropriate tool for determining diffusion coefficients is required. Diffusion coefficients for 68Ga in target solutions of varying concentrations of zinc nitrate, dissolved in aqueous solutions of varying concentrations of nitric acid were investigated. The influence on concentration of nitric acid was minimal, while the diffusion coefficient was found to be inversely proportional to the viscosity of the target solution, in correspondence with the general behaviour of several empirical correlation relations for finding the diffusion coefficient.