Diffusion of sotutes in supercooled sugar aqueous solutions

LONGINOTTI, MARÍA PAULA; FRANK, GUILLERMO; MARCONI, MARIO; CORTI, HORACIO R

Mar del Plata. Buenos Aires. Argentina.

Simposio; ISOPOW 9. IX International Symposium on the properties of water.; 2004

The addition of certain solutes, such as polyols and saccharides, increase the glass transition temperature, Tg, of the aqueous solutions from 136 K, the Tg value of pure water, to values near ambient temperature. For this reason these solutions are commonly used for the stabilization of biomolecules, preventing their degradation by retarding the molecular mobility and consequently, deterioration reactions. Since deterioration reactions could be diffusion-controlled, it is very important for the modeling of cryopreservation to measure the diffusion of solutes in supercooled aqueous solutions as a function of temperature and composition. In this study we measured the diffusion coefficients of fluoresceine and ferrocenemethanol in aqueous solutions of sucrose and trehalose (cryoprotectant disaccharides) using the fluorescence recovery after photobleaching (FRAP) and voltammetry over microelectrodes techniques. Both methods, FRAP and microelectrode voltammetry, are very suitable for studies in supercooled fluids were the solute mobility is extremely slow due to the high viscosity and the conventional diffusion techniques can not be applied. The FRAP technique uses a high power Argon laser beam to generate in the sample a concentration gradient by irreversible photochemical bleaching of the fluorescent probe. The power of the beam is then reduced to prevent further photobleaching, while the beam intensity is still sufficient to excite the fluoresceine molecules that diffuse from the bulk into the bleache region. By measuring the evolution of the fluorescence intensity it is possible to determine the diffusion coefficient of this molecule in the sample. On the other hand, the use of microelectrodes allowed us to obtain the diffusion coefficients of electroactive solutes, such as ferrocenmethanol, by measuring the limiting current which establish even in highly resistive media such as supercooled aqueous solutions.  The results, which extend over a wide range of sugar concentration and temperature in the supercooled region, were analyzed on the basis of the classical continuum hydrodynamic theory (Stokes-Einstein relationship). It is shown that below a characteristic temperature the classical friction model fails to explain the diffusion of solutes in supercooled aqueous solutions. The decoupling between diffusion and viscosity is mainly due to an abrupt decrease of the viscosity close to the glass transition temperature. For trehalosa aqueous solutions the decoupling seams to appear at Tg/T » 0.8, a value slightly lower than that reported for sucrose solutions. Tha impact of the non-classical diffusion on the prediction of reaction kinetics in these systems is also discussed.