Translational diffusion in sucrose solutions in the vicinity of their glass transition temperature

The prediction of the stability of low-moisture products is complex, particularly close to the glass transition temperature, This study demonstrates that the relations used to evaluate the influence of temperature on the viscosity of carbohydrate media cannot be applied to the diffusivity. The translational diffusion coefficient of a fluorescent molecule (fluorescein) is measured in sucrose-water mixtures as a function of temperature, The main result is that the mobility of the fluorescein is not simply coupled to the viscosity of the diffusion medium at temperatures close to the glass transition temperature. Indeed the WLF equation, which gives a good prediction of the viscosity, does not allow the determination of the diffusivity in a temperature range close to T-g because the translational diffusion follows a weaker temperature dependence. Different possible explanations for this apparent decoupling between translational diffusion and viscosity are suggested: a small change in the hydrodynamic radius of the diffusing molecule due to low water content and/or a connection with the beta relaxation process.