INVESTIGADORES
LONGINOTTI Maria Paula
congresos y reuniones científicas
Título:
Dynamic in super-cooled aqueous solution and the diffusion-viscosity decoupling
Autor/es:
H. R. CORTI; M. P. LONGINOTTI
Lugar:
Montevideo, Uruguay
Reunión:
Conferencia; 6th International Conference of Biological Physics; 2007
Institución organizadora:
Argentinian Biopysical Society
Resumen:
The diffusion of ionic (fluoresceine) and neutral (ferrocene methanol) solutes in super-cooled aqueous solutions containing trehalose and sucrose were measured by employing FRAP (fluorescence recovery after photobleaching) and chronoamperometry on cylindrical microelectrodes techniques. In both cases the decoupling of diffusion and viscosity was observed a Tg/T >0.75, being Tg the glass transition temperature. That is, close to the glass transition the diffusion coefficient turns to be much higher than that predicted by the classical hydrodynamic model of Stokes-Einstein. The enhanced diffusion in super-cooled aqueous solutions is of practical interest in cryopreservation and cryoestabilization practices and should be studied systematically for other solutes and aqueous glass forming systems. On the other hand, the dielectric relaxation properties of mono- and disaccharides aqueous solutions were analyzed as a function of temperature and water contect, from the pure saccharide to dilute solutions. It is observed the secondary b relaxation of aqueous solutions at water concentration above 15 wt% have a common behaviour when the dielectric relaxation time is scaled according to the glass transition temperature. This behaviour appears to be common to all carbohydrates (glucose, fructose, sucrose and maltose) over a wide range of concentrations and temperatures. At water contents lower than 15 wt%, the relaxation rates become faster with lower water contents. This change of behaviour with water content arises for a water concentration close to the percolation concentration of the water structure observed in dynamic simulation studies by Molinero et al. AT Tg/T > 0.75 a breakdown in the relaxation times is observed for all the carbohydrates analyzed in this work. At this temperature, a separation between the a and b relaxation times is observed. As the a relaxation is related with the viscosity of the sample and the b relaxation with the mobility of hydration water and mobile groups of carbohydrate, the separation of both processes can be related to the decoupling temperature found for some transport coefficients in many glass formers.