Phase equilibrium relations of tetra-n-butylphosphonium propionate and butyrate semiclathrate hydrates

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.