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The dynamics of the water molecules in tetra-n-butyl-d36-ammonium bromide semiclathrate hydrate was investigated by quasi-elastic neutron scattering (QENS). The QENS results clearly revealed a fast reorientation motion of water molecules in the temperature range of 212–278 K. The mean jump distance of hydrogen atoms was within 1.5–2.0 Å. The relaxation time of water reorientation was estimated to be 100–410 ps with an activation energy of 10.2 ± 5.8 kJ·mol−1. The activation energy was in good agreement with the cleavage energy of hydrogen bonds. Such a short relaxation time of water reorientation is possibly due to strong interaction between a bromide anion and its surrounding water molecules (similar to so-called negative hydration), which suggests a unique strategy for designing efficient, safe, and inexpensive proton conductors having the framework of semiclathrate hydrates.

Reproduced from New J. Chem., 2023, 47, 231-237 with permission from the Royal Society of Chemistry., https://doi.org/10.1039/d2nj04361k, Ionic clathrate hydrates (ICHs) have been applied to thermal storage materials, which can be utilized as unusual thermal energy sources. In the present study, we developed a novel ICH including tri-n-butyl(cyclobutylmethyl)phosphonium bromide (P444(1c4)-Br) as an unprecedented guest species. The highest equilibrium temperature and the dissociation enthalpy of P444(1c4)-Br ICH were 279.5 ± 0.1 K and 202 ± 2 J g−1, respectively, at the mole fraction of 0.0255 ± 0.0008. The highest equilibrium temperature of P444(1c4)-Br ICH was slightly lower than that of the tetra-n-butylphosphonium bromide (P4444-Br) ICH. The dissociation enthalpy of P444(1c4)-Br ICH was larger than that of other ICHs with similar highest equilibrium temperatures. Cyclic hydrocarbon groups, together with normal, branched, saturated, and unsaturated hydrocarbon chains, can be one of the options for tuning the equilibrium temperature of ICHs.

Phase equilibrium (temperature–composition) relations of the tetra-n-butylphosphonium trifluoroacetate (TBP-TFA) + water binary system including solid (semiclathrate hydrate, SCH)-liquid equilibria and liquid-liquid equilibria were investigated. Phase diagram of TBP-TFA SCH exhibited peritectic behavior. The maximum decomposition temperature (peritectic temperature) of TBP-TFA SCH at atmospheric pressure were 275.9 ± 0.1 K. The stoichiometric composition of TBP-TFA SCH was located at the mole fraction of 0.027 ± 0.002. The decomposition enthalpy of TBP-TFA SCH was 199 ± 2 J/g. At temperatures above the lower critical solution temperature (LCST), the liquid-liquid phase separation appeared. Interestingly, the composition at the LCST was 0.026 ± 0.002, which is very similar to the stoichiometric composition of TBP-TFA SCH. It implies that both cluster-based solution structures are similar.

Shimada Jin, Tani Atsushi, Yamada Takeshi, et al. "Quasi-elastic neutron scattering studies on fast dynamics of water molecules in tetra-n-butylammonium bromide semiclathrate hydrate", Applied Physics Letters 123, 50 (2023) https://doi.org/10.1063/5.0157560., The dynamics of the water molecules in tetra-n-butyl-d36-ammonium bromide semiclathrate hydrate were investigated by quasi-elastic neutron scattering (QENS). The QENS results clearly revealed afast reorientation motion of water molecules in the temperature range of 212–278 K. The mean jumpdistance of hydrogen atoms was within 1.5‒2.0 Å. The relaxation time of water reorientation wasestimated to be 100‒410 ps with activation energy of 10.2±5.8 kJ·mol-1. The activation energy wasin good agreement with the cleavage energy of hydrogen bonds. Such a short relaxation time ofwater reorientation is possibly due to strong interaction between a bromide anion and its surroundingwater molecules (similar to so-called negative hydration), which suggests a unique strategy fordesigning efficient, safe, and inexpensive proton conductors having the framework of semiclathratehydrates.

Jin Shimada, Moe Yamada, Atsushi Tani et al. Thermodynamic Properties of Tetra-n-butylphosphonium Dicarboxylate Semiclathrate Hydrates. Journal of Chemical & Engineering Data, 67 (1), 67-73, January 13, © 2022 American Chemical Society. https://doi.org/10.1021/acs.jced.1c00741, 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.

This paper reports phase equilibrium (temperature–composition) relations of semiclathrate hydrates formed from tetra-n-butylphosphonium propionate (TBP-Pro) and butyrate (TBP-But) + water systems. Their maximum solid–liquid phase equilibrium temperatures at atmospheric pressure were located at (288.75 ± 0.06) K and the mole fraction x1 = 0.035 ± 0.001 and (287.01 ± 0.06) K and x1 = 0.028 ± 0.001, respectively. They showed equilibrium temperatures higher than those of tetra-n-butylphosphonium formate, acetate, and lactate semiclathrate hydrates. The dissociation enthalpies of TBP-Pro and TBP-But semiclathrate hydrates were (190 ± 5) J·g−1 and (204 ± 5) J·g−1, respectively. The temperature difference between formation and dissociation, that is, the maximum allowable degree of supercooling, was (17.7 ± 1.5) K for TBP-Pro semiclathrate hydrate and (15.4 ± 1.4) K for TBP-But one.

Thermodynamic stabilities of tetra-n-butylphosphonium acetate (TBP-Ace) semiclathrate hydrates in the presence of methane (CH 4 ), carbon dioxide (CO 2 ), nitrogen (N 2 ), or ethane (C 2 H 6 ) were measured in a pressure range up to approximately 5 MPa. The dissociation temperature of TBP-Ace + CH 4 , TBP-Ace + CO 2 , and TBP-Ace + N 2 semiclathrate hydrates increased drastically with an increase in pressure, which means that CH 4 , CO 2 , and N 2 molecules occupy the vacant cages of the TBP-Ace semiclathrate hydrate. On the other hand, the C 2 H 6 molecules hardly occupied the cages, resulting in small pressure dependence of the dissociation temperature. Raman spectra and powder X-ray diffraction patterns of TBP-Ace + CO 2 semiclathrate hydrate reveal that the phase transition occurs at 1.04 ± 0.04 MPa and 285.88 ± 0.05 K. One of the possible reasons why the phase transition occurs is that the carbonate and/or hydrogen carbonate anions derived from the CO 2 molecules are replaced with some of acetate anions in the TBP-Ace + CO 2 semiclathrate hydrate.

Minamikawa K., Shimada J., Sugahara T., et al. Thermodynamic Properties of Tetra-n-butylammonium and Tetra-n-butylphosphonium Hydroxycarboxylate Semiclathrate Hydrates. Journal of Chemical and Engineering Data 68, 3492 (2023); https://doi.org/10.1021/acs.jced.3c00575., 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.

Minamikawa K., Shimada J., Sugahara T., et al. Thermodynamic Properties of Tetra-n-butylammonium and Tetra-n-butylphosphonium Hydroxycarboxylate Semiclathrate Hydrates. Journal of Chemical and Engineering Data 68, 3492 (2023); https://doi.org/10.1021/acs.jced.3c00575., 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.

Shimada Jin, Tani Atsushi, Yamada Takeshi, et al. "Quasi-elastic neutron scattering studies on fast dynamics of water molecules in tetra-n-butylammonium bromide semiclathrate hydrate", Applied Physics Letters 123, 50 (2023) https://doi.org/10.1063/5.0157560., The dynamics of the water molecules in tetra-n-butyl-d36-ammonium bromide semiclathrate hydrate were investigated by quasi-elastic neutron scattering (QENS). The QENS results clearly revealed afast reorientation motion of water molecules in the temperature range of 212–278 K. The mean jumpdistance of hydrogen atoms was within 1.5‒2.0 Å. The relaxation time of water reorientation wasestimated to be 100‒410 ps with activation energy of 10.2±5.8 kJ·mol-1. The activation energy wasin good agreement with the cleavage energy of hydrogen bonds. Such a short relaxation time ofwater reorientation is possibly due to strong interaction between a bromide anion and its surroundingwater molecules (similar to so-called negative hydration), which suggests a unique strategy fordesigning efficient, safe, and inexpensive proton conductors having the framework of semiclathratehydrates.

Shimada Jin, Tani Atsushi, Yamada Takeshi, et al. "Quasi-elastic neutron scattering studies on fast dynamics of water molecules in tetra-n-butylammonium bromide semiclathrate hydrate", Applied Physics Letters 123, 50 (2023) https://doi.org/10.1063/5.0157560.

Shimada Jin, Tani Atsushi, Yamada Takeshi, et al. "Quasi-elastic neutron scattering studies on fast dynamics of water molecules in tetra-n-butylammonium bromide semiclathrate hydrate", Applied Physics Letters 123, 50 (2023) https://doi.org/10.1063/5.0157560., The dynamics of the water molecules in tetra-n-butyl-d36-ammonium bromide semiclathrate hydrate were investigated by quasi-elastic neutron scattering (QENS). The QENS results clearly revealed afast reorientation motion of water molecules in the temperature range of 212–278 K. The mean jumpdistance of hydrogen atoms was within 1.5‒2.0 Å. The relaxation time of water reorientation wasestimated to be 100‒410 ps with activation energy of 10.2±5.8 kJ·mol-1. The activation energy wasin good agreement with the cleavage energy of hydrogen bonds. Such a short relaxation time ofwater reorientation is possibly due to strong interaction between a bromide anion and its surroundingwater molecules (similar to so-called negative hydration), which suggests a unique strategy fordesigning efficient, safe, and inexpensive proton conductors having the framework of semiclathratehydrates.

Shimada Jin, Tani Atsushi, Yamada Takeshi, et al. "Quasi-elastic neutron scattering studies on fast dynamics of water molecules in tetra-n-butylammonium bromide semiclathrate hydrate", Applied Physics Letters 123, 50 (2023) https://doi.org/10.1063/5.0157560.

This paper reports phase equilibrium (temperature–composition) relations of semiclathrate hydrates formed from tetra-n-butylphosphonium propionate (TBP-Pro) and butyrate (TBP-But) + water systems. Their maximum solid–liquid phase equilibrium temperatures at atmospheric pressure were located at (288.75 ± 0.06) K and the mole fraction x1 = 0.035 ± 0.001 and (287.01 ± 0.06) K and x1 = 0.028 ± 0.001, respectively. They showed equilibrium temperatures higher than those of tetra-n-butylphosphonium formate, acetate, and lactate semiclathrate hydrates. The dissociation enthalpies of TBP-Pro and TBP-But semiclathrate hydrates were (190 ± 5) J·g−1 and (204 ± 5) J·g−1, respectively. The temperature difference between formation and dissociation, that is, the maximum allowable degree of supercooling, was (17.7 ± 1.5) K for TBP-Pro semiclathrate hydrate and (15.4 ± 1.4) K for TBP-But one.