Structure and dynamics of maltooligomer-water solutions and glasses

Molecular dynamics simulations of mixtures of carbohydrates and water are performed for a homologous series of maltooligomers from glucose to maltopentose at water contents ranging from 0 to 100 weight % and for temperatures between 270 and 370 K. The specific volume of maltooligomer water mixtures follows volume additivity up to 70 weight % maltooligomer content and the specific volume as well as the free volume are independent of the maltoligomer molecular weight. In the glassy state, however, the free volume fraction increases with increasing maltooligomer molecular weight, in qualitative agreement with recent experimental results. From the radial distribution functions (RDFs), it is inferred that at very high maltooligomer contents water forms small clusters of about six water molecules. The RDFs also demonstrate that the distance between a carbohydrate hydroxyl group and water is the same as between hydroxyl groups on neighboring carbohydrate molecules. Regarding the diffusion of water in the maltooligomer-water systems, an Arrhenius- type temperature dependence for all simulations where the diffusive limit could be reached within the simulation time was observed, independent of the state of the matrix. The mobility of the maltooligomer molecules, conversely, becomes vanishingly slow in the approach to the glassy state, leading to a decoupling of the mobilities of water and maltooligomers. In contrast to other simulation studies we do not observe a specific length scale associated with the diffusion of water in concentrated states although hopping movements can be observed in individual trajectories. We speculate that the diffusion of water in carbohydrate matrices is analogous to the beta relaxation modes of glass-forming materials. This analogy leads to a dynamic interpretation for the plasticizing effect of water in carbohydrate glasses.