Molecular exchange through capsule membranes observed by pulsed field gradient NMR.
Mol. exchange through membranes of dispersed vesicles was studied using NMR spectroscopy combined with pulsed field gradients. Encapsulated mols. are differentiated from those in the continuous phase by their mean square displacement depending on a variable time interval D. Generally, two field gradient pulses are combined with a stimulated pulse echo sequence. In case of small capsules and vesicles with diams. 1 mm, the diffusion inside the encapsulated domain becomes the dominating phenomenon and the echo decay has to be fitted with a numerical approach based on a finite element approxn. For very slow exchange processes with av. residence times >10 s, permeation is directly obsd. in a time resolved measurement on the nonequil. state. In all cases, a careful anal. yields data on the release of a given encapsulated ingredient as well as, under variation of the tracer mol., on the specific permeability of the capsule walls for mols. of various size, polarity or flexibility.
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