Phase Transition of Tetra‑<i>n</i>‑butylammonium Bromide Hydrates Enclosing Krypton

Abstract

The phase equilibrium conditions for krypton (Kr)–tetra-<i>n</i>-butylammonium bromide (TBAB)–water systems were determined using an isochoric method. The pressure and temperature ranges were (0.06 to 1.0) MPa and (280 to 290) K, respectively, and TBAB solutions had TBAB molar fractions, <i>x</i>_TBAB, of 0.0062, 0.0138, 0.0234, and 0.0359. A second order transition of the TBAB hydrate was observed in all the Kr–TBAB–water systems. In the region at lower pressure than the phase transition point, the Kr–TBAB–water systems with low concentration (<i>x</i>_TBAB = 0.0062 and 0.0138) and high concentration (<i>x</i>_TBAB = 0.0234 and 0.0359) prefer to form TBAB·38H₂O and TBAB·26H₂O hydrates, respectively. However, a <i>new</i> TBAB hydrate was observed as a stable crystal structure in the higher pressure regions. Raman spectrum of the new TBAB hydrate shows band shapes remarkably similar to that of <i>pure</i> TBAB·38H₂O with the crystalline space group <i>Pmma</i> in the frequency ranges of the lattice for C–C stretching, C–H bending, the C–H stretching bands of the −CH₂ groups of TBA⁺ molecules, and the O–H stretching modes of water molecules, excluding the C–H stretching bands of the CH₃ groups of TBA⁺ molecules