Phase Behavior and Structural Characterization of Ionic Clathrate Hydrate Formed with Tetra-n-butylphosphonium Hydroxide: Discovery of Primitive Crystal Structure

This paper reports phase equilibrium measurements and crystal structure analysis on the ionic clathrate hydrate formed from tetra-n-butylphosphonium hydroxide (TBPOH). Phase equilibrium temperatures were measured in the mole fraction range of TBPOH in aqueous solution from 0.0072 to 0.0416. The highest ionic clathrate hydrate-solution equilibrium temperature was determined to be 290.2 K at a TBPOH mole fraction of 0.0340, which corresponds to the congruent composition. Single-crystal X-ray diffraction measurements were performed on the crystal formed at 288.7 K, and the chemical composition of the TBPOH hydrate crystal was determined to be TBPOH·29.6H₂O, which is consistent with the congruent composition obtained by the phase equilibrium measurement. The crystal structure of the TBPOH hydrate has a superstructure identical with Jeffrey's type I cubic structure, with an I43d space group with a lattice constant of 24.5191(13) Å. The TBPOH hydrate structure is compared with the same hydrate structure formed by the tetra-n-butylammonium fluoride. We provide a comprehensive overview of the dissociation temperature, the counteranion, and the hydrate structure regarding TBP and TBA salt hydrates. The dissociation temperatures decrease linearly with the increase in the partial molal volume of anions for TBA and TBP salt hydrates, changing the hydrate structures from the primitive cubic one that has the minimum hydration number.