Introduction of Cyclic Structures into Phosphonium Salts As Potential Guest Substances for Ionic Clathrate Hydrates

An ionic clathrate hydrate composed of tri- n -butylbenzyl-phosphonium chloride has been investigated from the viewpoint of phase equilibrium (temperature-composition) relations in comparison with an ionic clathrate hydrate with tri- n -butyl(cyclohexylmethyl)phosphonium bromide. The highest dissociation temperature of the ionic clathrate hydrates composed of tri- n -butylbenzylphosphonium chloride and tri- n -butyl(cyclohexyl-methyl)phosphonium bromide were 274.4 ± 0.1 K and 276.0 ± 0.1 K, respectively at the atmospheric pressure. The dissociation enthalpy of tri- n -butylbenzyl-phosphonium chloride ionic clathrate hydrate was 192 ± 3 kJ·kg −1 . These results allow us to suggest that controlling steric structures in the guest compounds might be one of the options for tuning the equilibrium temperatures of ionic clathrate hydrates.