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.

本工作借助单晶X射线结构分析手段，探究了四丁基溴化铵（tetrabutylammonium bromide, TBAB）离子包合物水合物的结构特征。本实验中合成的三种不同四方相TBAB离子包合物水合物，均以同一四方结构I型（TS-I）水晶格为基础，仅在溴阴离子的包合方式与四丁基铵阳离子的排布模式上存在差异。本研究证实了以下三点：（1）溴阴离子可嵌入水晶格，取代两个水分子；（2）阳离子的丁基基团不仅可嵌入TS-I水晶格的大型T、P空穴，还可嵌入小型D空穴；（3）基于TS-I的离子包合物水合物出现多型性的原因之一，是水骨架相邻层内的四隔间空穴存在多种交替排布模式。经化学分析确定了三种TBAB离子包合物水合物的组成，分别为TBAB·38.1H₂O、TBAB·32.5H₂O与TBAB·26.4H₂O，并通过差示扫描量热法（differential scanning calorimetry, DSC）测定了其熔融焓；基于上述实验结果，通过热力学计算得到了由TBAB与水生成TBAB水合物的生成焓。