On the applicability of the Stokes–Einstein relation for concentrated aqueous carbohydrate solutions

The viscosity and diffusion of carbohydrate solutions are critical for understanding biological processes, food science, and cryopreservation technology. However, the precise concentration thresholds at which the classical Stokes–Einstein (SE) relationship fails remain poorly defined. This study characterises the limits of the SE relation in aqueous glucose, trehalose, and raffinose solutions through all-atom molecular dynamics simulations. Using the OPLS-AA/1.14*CM1A-LBCC force field and the TIP4P/2005 water model at 298.15 K, we reproduce experimental trends in shear viscosity and self-diffusion coefficients across a wide concentration range. Our analysis reveals a systematic decoupling of viscosity and diffusion: as concentration increases, the effective hydrodynamic radius decreases, indicating deviations from SE behaviour. Within the present accuracy, the concentration dependence of the normalised SE product is similar for all three carbohydrates, suggesting no pronounced molecular-size dependence of the onset concentration. These findings further show that the Stokes–Einstein equation cannot be universally applied to concentrated carbohydrate systems without accounting for concentration effects.