Analysis of trends in phase change enthalpy, entropy and temperature for alkanes, alcohols and fatty acids

Recent advances in phase change materials for thermal energy storage

“…41,42 Linear relationships between molar (denoted with subscript m) enthalpy changes (here referred to as Δ trs H m to indicate the inclusion of solid–solid transitions near the melting point, in addition to Δ fus H m ) and number of carbons in the alkyl chain, n , represented as:Δ trs H m = b 0 + b 1 n and between the corresponding molar entropy changes, Δ trs S m , and n as:Δ trs S m = c 0 + c 1 n were found for odd and even n for long-chain alkanes, alcohols and fatty acids, with nearly parallel lines for odd and even n within a given chemical family. 43 The slopes, b 1 and c 1 , were found to be nearly identical regardless of chemical family and even/odd n , indicating a nearly constant increase in Δ trs H m (and Δ trs S m ) per additional –CH 2 – group. 43 However, the enthalpy intercept, b 0 , reflects the structure, with least negative b 0 for fatty acids, then alcohols and alkanes, corresponding to the existence of strong intermolecular interactions in fatty acids in both the solid and liquid state, in comparison with alcohols and alkanes in which intermolecular interactions would be weaker in the liquid than the solid.…”

“…43 However, the enthalpy intercept, b 0 , reflects the structure, with least negative b 0 for fatty acids, then alcohols and alkanes, corresponding to the existence of strong intermolecular interactions in fatty acids in both the solid and liquid state, in comparison with alcohols and alkanes in which intermolecular interactions would be weaker in the liquid than the solid. 43 If such linear thermodynamic relationships are more general for organic phase change materials, they would allow estimates of enthalpy changes for potential PCMs for which experimental data are not available.…”