Dynamics of solotes in supercooled aqueous carbohydrate solutions

H. CORTI; G. FRANK; P. LONGINOTTI; M. MARCONI

Lille. Francia

Simposio; 5th International Discussion Meeting on Relaxations in Complex Systems; 2005

We measured the diffusion coefficients of ferrocenemethanol, fluoresceine and fluoresceine-isocianate dextranes of different molecular weight in supercooled aqueous solutions of sucrose and trehalosa above the glass transition temperature using the fluorescence recovery after photobleaching (FRAP) technique. The decoupling of the diffusion coefficient of the solute and the solution viscosity was observed in these solutions above a critical T/Tg ratio which depends on the size of the solute molecule. The diffusion of solutes at temperatures bellow the decoupling limit is much higher than predicted using the Stokes-Einstein relationship, This behavior is similar to that found for ionic solutes and it would be related to structural heterogeneities due to the presence of water rich regions. Small solutes could diffuse more easily through these regions having local viscosities lower than the bulk viscosity. At temperatures close to the glass transition temperature the measured diffusion coefficient of solutes could be much larger than that calculated by extrapolating the high temperature value using the hydrodynamic model. Accordingly, the rate of diffusionally controlled deterioration reactions in aqueous systems close to the glass transition temperature could be several orders higher than predictions based on the classical continuum hydrodynamic theory.