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Phase Behavior and Structural Characterization of Ionic Clathrate Hydrate Formed with Tetra‑<i>n</i>‑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-<i>n</i>-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·29.6H<sub>2</sub>O, 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 <i>I</i>4Ì…3<i>d</i> space group with a lattice constant of 24.5191(13) Ã….
The TBPOH hydrate structure is compared with the same hydrate structure
formed by the tetra-<i>n</i>-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