engleski

Mesoscopic Dynamics and the Mechanism of Gelatin Sol-gel Transition

We present an extensive analysis of polypeptide dynamics in gelatin water solutions at the micron scale, revealing an emergent length scale and slow cooperative diffusion . Combining pulsed field gradient NMR diffusometry and low frequency conductivity spectroscopy1 we are able to closely track the motion of polypeptide chains during the transition from sol to gel, resolving two distinct diffusion regimes. At short timescales we observe an effective diffusion coefficient which is shown to correspond to single chain diffusion. This effective diffusion survives down to a characteristic, temperature-independent length, playing the role of a pore size. Simultaneous measurements of diffusion and conductivity also directly provide the number of non-bound chains in the solution, enabling us to follow chain aggregation and gel network formation . A concentration master curve is found for the aggregation process, with scaling exponents different from microscopic polarimetry measurements2 . This is explained by taking additional, emergent hydrophobic interactions between chains into account . At longer times (~1 s) both NMR and conductivity measurements detect an additional diffusion process, related to cooperative motion of chains and similar to dynamics observed in glasses and coloidal gels. Direct measurements of the four-point correlation function using PFG-NMR show a peak at the characteristic timescale of the slow process, confirming its cooperative nature . The slow process abruptly transforms from elastic to dissipative at the sol-gel transition . Our results prove that the gelation process of gelatin is significantly more complex than simple percolation2, 5 or phase separation6 models indicate, with emergent interactions playing an important role.

sol-gel transition; diffusion; NMR; nonlinear conductivity

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