Designing the structure and relevant properties of semiclathrate hydrates by partly asymmetric alkylammonium salts

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 n-propyl, tri-n-butylammonium bromide (N₃₄₄₄Br) and tri-n-butyl, n-pentylammonium bromide (N₄₄₄₅Br). Their cation side chains are decremented or incremented from tetra-n-butylammonium (N₄₄₄₄ or TBA), which is one of the best fits for semiclathrate hydrate structures. The use of the widely used tetra-n-butylammonium bromide (N₄₄₄₄Br or TBAB) as an ionic guest, an increment of the carbon chain, i.e., N₄₄₄₅Br, 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 N₄₄₄₄Br (TBAB) hydrates. The N₃₄₄₄Br hydrate showed both significantly decreased melting temperature and fusion heat compared to the N₄₄₄₄Br (TBAB) hydrates. The phase behaviour of the N₃₄₄₄Br hydrate was found to be analogous to that of the N₄₄₄₄Br (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.