Muromachi Sanehiro, Kida Masato, Morimoto Masato, Yamane Shogo, Takeya Satoshi
Energy Process Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, 305-8569, Japan.
Faculty of Engineering, Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, Hokkaido, Japan.
Phys Chem Chem Phys. 2022 Aug 3;24(30):18198-18204. doi: 10.1039/d2cp02625b.
Semiclathrate hydrates are host-guest materials that form from ionic guests and water. There are numerous options for ionic guests, such as quaternary ammonium salts, to tune the functional properties of these materials such as melting temperature, fusion heat, and gas capacity and selectivity. To design these materials, the stabilization mechanism of the side chains of quaternary ammonium salts must be understood based on both thermodynamic and crystallographic properties and relevant host-guest dynamics. In this paper, we studied semiclathrate hydrates formed from -propyl, tri--butylammonium bromide (NBr) and tri--butyl, -pentylammonium bromide (NBr). Their cation side chains are decremented or incremented from tetra--butylammonium (N or TBA), which is one of the best fits for semiclathrate hydrate structures. The use of the widely used tetra--butylammonium bromide (NBr or TBAB) as an ionic guest, an increment of the carbon chain, , NBr, caused disorders in its hydrate structure due to the oversizing of the cation. This suitably oversized cation selectively stabilized the orthorhombic structure, whose hydration number is relatively high. As a result, the fusion heat at the congruent composition of the hydrate phase was higher than that of the widely used NBr (TBAB) hydrates. The NBr hydrate showed both significantly decreased melting temperature and fusion heat compared to the NBr (TBAB) hydrates. The phase behaviour of the NBr hydrate was found to be analogous to that of the NBr (TBAB) hydrates. It was demonstrated that the semiclathrate hydrate structures and relevant properties can be modified by adjusting the alkyl side chain length of quaternary ammonium salts.
半笼形水合物是由离子客体和水形成的主客体材料。离子客体有多种选择，如季铵盐，可用于调节这些材料的功能特性，如熔点、熔化热、气体容量和选择性。为了设计这些材料，必须基于热力学和晶体学性质以及相关的主客体动力学来理解季铵盐侧链的稳定机制。在本文中，我们研究了由三丙基溴化铵（NPr₃Br）、三丁基溴化铵（NBu₃Br）和三丁基戊基溴化铵（NBu₃PenBr）形成的半笼形水合物。它们的阳离子侧链相对于四丁基铵（NBu₄或TBA）有所减少或增加，四丁基铵是最适合半笼形水合物结构的之一。使用广泛使用的四丁基溴化铵（NBu₄Br或TBAB）作为离子客体，增加碳链长度，即NBu₃PenBr，由于阳离子过大，导致其水合物结构紊乱。这种适当过大的阳离子选择性地稳定了正交结构，其水合数相对较高。结果，水合物相在全同组成时的熔化热高于广泛使用的NBu₄Br（TBAB）水合物。与NBu₄Br（TBAB）水合物相比，NPr₃Br水合物的熔点和熔化热均显著降低。发现NPr₃Br水合物的相行为与NBu₄Br（TBAB）水合物相似。结果表明，通过调节季铵盐的烷基侧链长度可以改变半笼形水合物的结构和相关性质。
