Calorimetric and Structural Studies of Tetrabutylammonium Bromide Ionic Clathrate Hydrates

In the present work, characteristic
properties of tetrabutylammonium
bromide (TBAB) ionic clathrate hydrates structures were studied by
single-crystal X-ray structure analysis. The structures of three different
tetragonal TBAB ionic clathrate hydrates that were formed in our experiments
were based on the same water lattice of tetragonal structure I (TS-I)
differing in the ways of including bromide anions and arranging tetrabutylammonium
cations. We demonstrated that (1) Br– can be included
into the water lattice, replacing two water molecules, (2) the butyl
group of the cation can be inserted not only in large T and P cavities
but also in small D cavities of the water lattice TS-I, and (3) one
of the reasons for polytypism of ionic clathrate hydrates on the basis
of TS-I is the occurrence of alternative modes of arrangements of
four-compartment cavities in adjacent layers of the water framework.
The compositions of three TBAB ionic clathrate hydrates TBAB·38.1H2O, TBAB·32.5H2O, and TBAB·26.4H2O were determined by chemical analysis, and their enthalpies of fusion
were measured by differential scanning calorimetry (DSC). From the
obtained results, the enthalpies of the TBAB hydrate formation from
TBAB and water were calculated thermodynamically.