SOLVENT EXTRACTION KINETICS THROUGH HOLLOW FIBRE AND FLAT MEMBRANES: INVESTIGATION WITH NITROXIDE RADICALS

摘要

Kinetics of extraction of stable nitroxide radicals ("spin probes") from organic solvents into water through hollow fibres and flat membranes was investigated. It was demonstrated that Mass Transfer Coefficients based on organic phase (MTCo) for exit through hollow fibres with mineral oil practically do not correlate with the hydrophobicity of low molecular weight radicals and are determined by diffusion through the organic solvent. In the experiments with a less viscous octane organic phase does not play a dominant role, but still the process cannot be explained simply by the influence of aqueous layer. Ascorbic acid reduced nitroxides in water and enhanced the rate of nitroxide exit from octane due to the decrease of transport resistance of unstirred aqueous layers. Addition of ascorbic acid up to 1 mol/dm~3 resulted only in the four-fold increase of MTCo for Tempo, which is less than the value predicted by the model of parallel diffusion and reaction in water. Low effect of ascorbic acid can be explained by addition of an interface resistance, which is usually neglected.