Thermodynamic Properties of Tetra‑n‑butylammonium and Tetra‑n‑butylphosphonium Hydroxycarboxylate Semiclathrate Hydrates

Semiclathrate hydrate (SCH) has been expected to be one of the phase change materials suitable for cold energy storage. In the present study, the equilibrium temperature, dissociation enthalpy, and crystal structure of tetra-n-butylammonium­(TBA)-Tar, tetra-n-butylphosphonium­(TBP)-Tar, and TBA-Mala SCHs were investigated, where the hydroxycarboxylate anions were malate (-Mala) and tartarate (-Tar). The maximum equilibrium temperature of each SCH system at the stoichiometric composition was (279.54 ± 0.05) K for TBA-Tar SCH, (275.44 ± 0.05) K for TBP-Tar SCH, and (278.69 ± 0.05) K for TBP-Mala SCH at atmospheric pressure. Each dissociation enthalpy was (172 ± 3) kJ·kg–1 for TBA-Tar SCH, (161 ± 3) kJ·kg–1 for TBP-Tar SCH, and (172 ± 3) kJ·kg–1 for TBP-Mala SCH. The SCHs with hydroxy group(s) in the anion of onium salts are more environmentally friendly than the SCHs with a halide anion. The environmentally friendly SCHs measured in this study could be used as refrigerants in the logistics of vegetables and fruits because the equilibrium temperature is suitable to preserve their freshness.