Structure of Amorphous Starch. 2. Molecular Interactions with Water

Abstract

A detailed atomistic model for amorphous starch with water contents typical for this system at ambient atmospheric conditions was stimulated, extending the model for dry amorphous starch presented previously. The interactions of the molecules in this system were found to be dominated by the interactions of the hydroxyl groups of starch within themselves 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 water content, the starch chains were moved further apart and the interaction energy between them was reduced. Water was bound in the system with four hydrogen bonds, irrespective of whether the partner was a starch 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 water molecules was found to be distributed broadly over 3 orders of magnitude in the correlation times, and the movement of the molecules was anisotropic. The calculation of the chemical potential of the water confirmed the high affinity of dry starch toward water.