Diffusion of 77000 g/mol dextran in submicron polyelectrolyte capsule dispersions measured using PFG-NMR

Pulsed field gradient NMR (PFG-NMR) spectroscopy is introduced to probe the distribution and dynamics of long dextran sugars (77 kDa) in heterogeneous solutions containing submicron hollow polymeric capsules. The capsules are made using the layer-by-layer (LBL) self-assembly method of strong polyelectrolytes onto silica microspheres, followed by a dissolution of the silica core. The resulting capsules are completely permeable for water and small ions, but large molecules, such as polydextran, are found to be in slow exchange between the capsule interior and the surrounding water. The spin-lattice relaxation time (T-1) for the aliphatic protons on the dextran is extracted from the variation of the apparent population of encapsulated dextran. The spin-spin relaxation time (T-2) of the encapsulated dextran is estimated by a combined diffusion-relaxation measurement. Both relaxation times are used to calculate the population of encapsulated dextran and to discuss the state of the encapsulated dextran. The NMR measurements here indicate that the dextran may preferentially select the capsule interior, resulting in an enriched dextran concentration inside the capsules. The encapsulated dextran appears to be in a liquidlike state rather than being immobilized on the capsule wall.