Molecular Exchange Through Capsule Membranes Observed by Pulsed Field Gradient NMR

Molecular exchange through membranes of dispersed vesicles is studied using nuclear magnetic resonance spectroscopy combined with pulsed field gradients. Encapsulated molecules are differentiated from those in the continuous phase by their mean square displacement depending on a variable time interval Δ. Generally, two field gradient pulses are combined with a stimulated pulse echo sequence. In case of small capsules and vesicles with diameters below 1 µm, the Brownian motion of the capsules dominates the lateral motion for the encapsulated fraction and the echo decay curve can be analyzed with a simple analytical approach. In case of larger vesicles and capsules with diameters above 1 µm, 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 approximation. For very slow exchange processes with average residence times above 10 s, permeation is directly observed in a time resolved measurement on the non-equilibrium state. In all cases, a careful analysis yields data on the release of a given encapsulated ingredient as well as, under variation of the tracer molecule, on the specific permeability of the capsule walls for molecules of various size, polarity or flexibility.