engleski

Low-frequency impedance spectroscopy: role of microscopic phase separation in gelation of aqueous gelatin

We present results of impedance spectroscopy on gelatin solutions using a specially constructed four-electrode impedance spectroscopy cell. Our method enables impedance measurements of conductive liquid samples down to the sub- Hertz frequencies, avoiding electrode polarization effects that usually cripple standard impedance analyzers. This enables conductivity studies of self-diffusion and collective phenomena in biological samples (macromolecules, cells, ...). Our results in aqueous solutions of gelatin reveal two relaxation processes in the dilute and semi-dilute solutions. Using PFG NMR diffusiometry we identify the faster process as a self-diffusion of gelatin molecules. Scaling relations for the self- diffusion indicate that we have a microscopic phase separation distinct from spinodal decomposition. After the formation of triple-helix junction sites and the onset of gelation, these hydrophobic triple-helices aggregate due to the attractive helix-helix interactions. This leads to the formation of microphase separation between gelatin molecules which are part of the gel macrostructure (gel phase) and those which are free (sol phase). The slower process indicates a collective phenomena with anomalous diffusion. This suggests that between the single helical segments joining three chains and the macroscopic gel there exists additional levels of organization. The scales inferred from these experiments lie in the micron range and on the same scale that TEM micrographs reveal polypeptide bundles forming a network.

sol-gel transition; gelatin; low-frequency; impedance spectroscopy; diffusion

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