COLL 157 |
| We have studied the encapsulation and exchange dynamics of different probe molecules in colloidal dispersions of hollow polymeric capsules by pulsed field gradient (PFG)-NMR spectroscopy. The capsules are prepared by self-assembly of polyanions and polycations onto the colloidal silica particles, followed by dissolution of the silica core in hydrofluoric acid (HF). Permeability of the capsule wall towards different molecular species is a key issue for further development of these capsule as drug carriers. In this paper, we have studied the permeation of polyethylene oxides (PEO) of different molecular weight. In case of PEOs, in PFG-NMR, two echo decay components with diffusion coefficients corresponding to free PEO and capsules, respectively, are found. In addition, diffusion-relaxation correlation spectroscopy (DRCOSY) reveal two peaks with different diffusion coefficients, but almost identical relaxation times. This implies the presence of mobile PEO chains in the capsule interior. The exchange behaviour is analysed by a two-site exchange model of free and encapsulated PEO. Exchange times or permeation rates through the capsule wall are extracted for different molecular weight. Permeation rates are decreasing with increasing molecular weight of PEO. While for short chains (Mw < 10000 Da) the dependence on Mw is strongly pronounced, another, less pronounced dependence is found for larger Mw. This suggests a transition between two different mechanisms of permeation as the molecular weight is increased. We also investigate the diffusion behaviour of phenol in presence of capsule dispersion. It is observed that phenol is adsorbing on the capsule wall rather than staying in the capsule interior. By analyzing the diffusion and relaxation data, we successfully extract the exchange time of the phenol between the bound and free sites which was ~ 30 ms. We are also able to extract the fraction of phenol, adsorbed on the capsule wall. |
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Fundamental Research in Colloid and Surface Chemistry
6:00 PM-8:00 PM, Monday, August 20, 2007 BCEC -- East Registration, Poster
Division of Colloid & Surface Chemistry |