STRUCTURE OF AMORPHOUS STARCH .2. MOLECULAR-INTERACTIONS WITH WATER

A detailed atomistic model for amorphous starch with water contents ty pical for this system at ambient atmospheric conditions was stimulated , extending the model for dry amorphous starch presented previously. T he interactions of the molecules in this system were found to be domin ated by the interactions of the hydroxyl groups of starch within thems elves as well as with the water molecules. The possibility of forming hydrogen bonds was dramatically increased through the introduction of the small water molecules. On the macroscopic level this was reflected in the increase of the cohesive energy density up to 17 x 10(8) N/m(3 ) in the structures with 23% water, which was parallel to the increase in the density of intermolecular hydrogen bonds. With increasing wate r content, the starch chains were moved further apart and the interact ion energy between them was reduced. Water was bound in the system wit h four hydrogen bonds, irrespective of whether the partner was a starc h or a water molecule. The binding energy was constant with increasing water content. The model calculations confirmed that an excess energy is needed to evaporate water form the system. The mobility of the wat er molecules was found to be distributed broadly over 3 orders of magn itude in the correlation times, and the movement of the molecules was anisotropic. The calculation of the chemical potential of the water co nfirmed the high affinity of dry starch toward water.