Equilibrium Phase Relations and Dissociation Enthalpies of Tri-n-butylalkenylphosphonium Bromide Semiclathrate Hydrates

Semiclathrate hydrates (SCHs) have received attention as thermal storage materials because of their equilibrium temperature and relatively large dissociation enthalpy. The alkyl length of quaternary onium cations significantly affects the equilibrium temperature and dissociation enthalpy of quaternary onium salt SCHs. To investigate the effects of the double bond introduced in onium cations, in the present study, the thermodynamic stability of tri-n-butylalkenylphosphonium bromide (P444(alkenyl)-Br: alkenyl = 3-butenyl (3.1) or 4-pentenyl (4.1)) SCH was investigated. The equilibrium temperatures and the dissociation enthalpies were determined to be (277.5 +/- 0.1) K and (199 +/- 2) J/g for P444(3.1)-Br SCH, and (274.8 +/- 0.1) K and (188 +/- 2) J/g for P444(4.1)-Br SCH. The equilibrium temperatures of P444(3.1)-Br SCH and P444(4. 1)-Br SCH were lower than those of the corresponding normal alkyl SCHs (P4444-Br and P4445-Br), although the equilibrium temperature of tri-n-butylallylphosphonium bromide SCH was higher than that of the P4443-Br SCH.

Automated summary

The present study investigates the thermodynamic stability of semiclathrate hydrates (SCHs) of tri-n-butylalkenylphosphonium bromide (P444(alkenyl)-Br: alkenyl = 3-butenyl (3.1) or 4-pentenyl (4.1)). The introduction of double bonds is found to lower the equilibrium temperature of the SCHs and affect their dissociation enthalpy.