The anomalous behaviour of aliphatic fatty diamides: Chain length and hydrogen bonding interactions

Abstract

A series of five linear aliphatic diamides were synthesised via facile direct coupling of stearic acid (C18) with diamines of varying carbon number (18-n-18, n = 2, 4, 6, 8 and 10) in a catalyst-free neat reaction. The compounds were investigated as potential renewable phase change materials (PCMs) for latent heat thermal energy storage. The thermal properties of the diamides were predictably related to the diamine chain length. TGA measurements indicated that the diamides evaporated before decomposition, with evaporation onsets increasing from 325 °C for 18-2-18 to plateau at 337 °C for the longer-chain diamides. The diamides display negligible supercooling and melt and crystallize at 140 °C–150 °C within a narrow transition span (<5 °C). Notably, both their melting and crystallization temperatures versus n present a maximum at n = 4, explained by peak hydrogen bonding and chain length considerations. As the distance between the amide groups increases, the transition temperatures decrease exponentially owing to the subsequent decrease in intermolecular hydrogen bond strength. The latent heat of the diamides presented a sharp increase from 165 J/g for 18-2-18 to plateau at ~210 J/g for the other amides (n ≥ 4). XRD measurements support the thermal transition behaviour; it shows that all the diamides crystallize from the melt into the β-phase, except for 18-2-18 in which the β-phase co-existed with the less stable β′-phase. Overall, the study shows that the diamides are suitable alternatives to classical petrochemical derived PCMs, expanding the currently limited library of organic PCMs.