A quantum mechanics study of competitive interactions of a polymer within Ca2+ ion hydration Shell in an energy storage material

Polymers that dissolve and form gels in inorganic phase change materials (PCMs) are increasingly used in thermal energy storage applications. Here, we report a quantum mechanical Møller–Plesset perturbation theory (MP2) study of interactions of a 5-unit polyvinyl alcohol (PVA5) with partially hydrated Ca 2+ ion. The system represents the local interactions of polymer segments within an environment of liquid calcium chloride hexahydrate (CaCl 2 ·6H 2 O, CC6H) PCMs. To capture the deficiency of water in inorganic PCMs and explore role of additional water, we varied number of water molecules in Ca 2+ hydration shell (coordination number of 7) and explored ion-water-polymer interactions within [Ca(PVA5)(H 2 O) 6 ] 2+ and [Ca(H 2 O) 7 ] 2+ systems. In the [Ca(PVA5)(H 2 O) 6 ] 2+ system, one of the hydroxyl groups of PVA5 can potentially insert within the first hydration shell of Ca 2+ ion, while the remaining polymer units are available for hydrogen bonding with water molecules of the saturated ionic hydration shell. For both systems, the MP2 structural optimizations yielded O-Ca 2+ bond length 2.45 Å which agrees with the experimental values 2.38 to 2.52 Å for hydrated Ca 2+ ions. In addition to coordination bond with calcium ion, length of PVA5 allows the formation of three hydrogen bonds with the hydration water in the first shell of Ca 2+ . The formation of eight-member rings resulting from the collective hydrogen bonding and ion-dipole interactions is a unique feature of ion-polymer interactions in salogels. Further CCSD/Def2qzvp simulations showed that 5.68 kcal/mol of charge transfer contributes 60% to the interactions of Ca 2+ ions with water molecules in the primary hydration shell and hydrogen bonding of PVA5 to the ionic hydration shell. The results provide a molecular basis for understanding of solubility and gelation of PVA in the salt hydrates, enabling rational design of salogel-based thermal energy storage materials.