Semiclathrate hydrate (SCH) is one of the phase change materials suitable for cold energy storage. Thermodynamic properties of SCHs, such as an equilibrium temperature and the dissociation enthalpy, depend on the size and shape of the guest substances. In the present study, to reveal the effects of steric conformations of the guest anions on the thermodynamic properties of SCHs, tetra-n-butylphosphonium dicarboxylate (TBP-DC) SCHs, where the anion was oxalate (TBP-Oxa), malonate (TBP-Mal), succinate (TBP-Suc), glutarate (TBP-Glu), maleate (TBP-Male), or fumarate (TBP-Fum), were investigated. TBP-Oxa, -Mal, -Suc, and -Fum SCHs had similar equilibrium temperatures, whereas the equilibrium temperatures of TBP-Glu and -Male SCHs were higher. This suggests that the size and conformation of glutarate and maleate anions are appropriate for the cage structures of SCHs. Moreover, we compared the equilibrium temperatures of TBP-Suc, -Male, and -Fum SCHs because TBP-Suc, -Male, and -Fum have similar anion structures. The equilibrium temperature of TBP-Suc SCH was similar to that of TBP-Fum SCH, whereas TBP-Male SCH showed a higher equilibrium temperature. This result implies that the succinate anion is accommodated in the trans conformation, similar to the fumarate anion, in the hydrate cages.

中文翻译：

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四正丁基鏻二羧酸半包合物水合物的热力学性质

半笼形水合物（SCH）是一种适用于冷能储存的相变材料。SCH 的热力学性质，例如平衡温度和解离焓，取决于客体物质的大小和形状。在本研究中，为了揭示客体阴离子的空间构象对 SCHs 热力学性质的影响，tetra -n-二羧酸丁基鏻 (TBP-DC) SCH，其中阴离子为草酸根 (TBP-Oxa)、丙二酸根 (TBP-Mal)、琥珀酸根 (TBP-Suc)、戊二酸根 (TBP-Glu)、马来酸根 (TBP-Male)，或富马酸盐（TBP-Fum）进行了研究。TBP-Oxa、-Mal、-Suc 和-Fum SCHs 具有相似的平衡温度，而TBP-Glu 和-Male SCHs 的平衡温度更高。这表明戊二酸和马来酸阴离子的大小和构象适合 SCH 的笼结构。此外，我们比较了 TBP-Suc、-Male 和 -Fum SCH 的平衡温度，因为 TBP-Suc、-Male 和 -Fum 具有相似的阴离子结构。TBP-Suc SCH的平衡温度与TBP-Fum SCH相似，而TBP-Male SCH的平衡温度更高。该结果表明琥珀酸阴离子被容纳在反式构象，类似于富马酸阴离子，在水合物笼中。