Ab initio calculation of hydration and proton transfer on sulfonated nata de coco

Abstract

The repeating unit of sulfonated “nata de coco” is D-glucose sulfonate. This research aims to determine the most stable structures of sulfonated nata de coco polymer membrane, the energy and hydrogen bonds, in order to understand the characteristics, local hydration, and proton transfer on the membrane on the ab initio electronic structure calculation. The minimum energy structure for its monomer (two, three, four and five) are calculated by B3LYP/6-311G (d) method. The calculations show that there is no significant energy change on the structure interaction of two, three, four and five monomer of sulfonated nata de coco with one water molecule, which is about -18.82 kcal/mole. Those calculations that two monomers form of sulfonated nata de coco might be used to further calculation and research, because it can be considered as the representative for their polymer. The optimization and B3LYP/6-311G (d) calculation shows the amount of water molecule used for proton transfer is closely related to the formation of hydrogen bonding with sulfonic group. By the addition of one or two water molecule, the dissociated proton is stabilized by formation of hydronium ion. For further addition of water molecule (three to ten water molecules), the proton dissociation is also stabilized by the formation of Zundel ion and Eigen ion. The calculation of interaction energy with n water molecule (n = 1-10) shows that both energy change (∆E), and enthalpy change (∆H) are more negative. This implies that the interaction with water molecule is stronger. The bonding energy is about 14.0-16.5 kcal/mole per water molecule. On the addition of four and eight water molecules, proton dissociation forms two Zundel ion and two Eigen ions and causes lower bonding energy about 2 kcal/mole. Those optimization and energy calculations conclude that the formation of hydrogen bonding among water molecule and sulfonic group affects proton transfer on sulfonated nata de coco membrane.