Computational Analysis of Sugar Alcohols as Phase-Change Material: Insight into the Molecular Mechanism of Thermal Energy Storage

Sugar alcohols are one promising candidate for phase-change materials (PCMs) in energy industrial societies because of their large thermal storage capacity. In this paper, we investigate the melting point and enthalpy of fusion related to the thermal storage of six-carbon sugar alcohols (galactitol, mannitol, sorbitol, and iditol) by molecular dynamics simulations and elucidate physical principles required for new PCM design. The computational melting points and enthalpies of fusion reproduce the experimental trend qualitatively. On the basis of the energy decomposition analysis we find that their enthalpies of fusion originate mainly from the decrease in the number and strength of intermolecular hydrogen bonds upon melting. Furthermore, we examine the origin of the difference of enthalpy of fusion between the four isomers. The results show that the larger enthalpy of fusion of galactitol and mannitol comes from their stable solid phases associated with the absence of notable repulsive electrostatic interaction between oxygen atoms in the molecule. In accordance with these results and an additional statistical survey, we propose the 3-fold guideline for developing new sugar alcohol like PCMs with larger thermal storage: (1) linear elongation of a carbon backbone, (2) separated distribution of OH groups, and (3) even numbers of carbon atoms.