Molecular exchange through membranes of poly(2-vinylpyridine-block-ethylene oxide) vesicles.

The molecular exchange of tracer molecules through the membranes of dispersed vesicles of the block copolymer poly(2-vinylpyridine-block-ethylene oxide) was studied by using NMR spectroscopy combined with pulsed field gradients. The hydrodynamic radius of the tracer molecules was varied systematically to obtain a permeability profile of the vesicle membrane. In addition, the effect of system parameters, such as temperature, pH value, vesicle size, and thickness of the vesicle membrane, was studied. In the case of rapid exchange with average residence times significantly smaller than 10 s, the permeation is observed under equilibrium conditions and the data are analyzed by using a simple analytical approach. For slow exchange processes with average residence times above 10 s, the permeation is monitored in a time-resolved measurement under nonequilibrium conditions. Generally, the transmembrane exchange rate of the tracer clearly depends on its hydrodynamic radius. The characteristics of this dependence indicate the presence of two different mechanisms of membrane penetration, one dominating for smaller and one for larger tracer molecules, respectively. The exchange rate also shows a significant dependence on the bilayer thickness and on the vesicle diameter. By contrast, no variation of the membrane permeability with the temperature or the pH value could be detected as long as the vesicles remain stable.