Nucleation probability of Type-A tetra-n-butyl ammonium bromide semi-clathrate hydrate via the memory effect

Abstract

The memory effect in clathrate hydrates is known to produce recrystallization under milder conditions than those needed for the initial nucleation. Tetra-n-butyl ammonium bromide (TBAB) forms tetragonal (type A) as well as orthorhombic (type B) semi-clathrate hydrate in aqueous solution. Here we investigate the nucleation probability for recrystallization of type A TBAB hydrate via the memory effect. Using optical microscopy, we observed recrystallization in situ under various concentrations of aqueous solution. After their initial nucleation, the hydrate crystals were dissociated by raising the temperature and then cooled to slightly below the equilibrium temperature while measuring the nucleation probability and induction times for recrystallization. We found the nucleation probability to be highest at about 20 wt%, which is about half of the stoichiometric weight fraction of 40 wt%, and found the shortest induction time at about 30 wt%. These results suggest the importance of free water in solution to nucleate TBAB hydrate crystal with the memory effect. We also found that the nucleation occurred near the location of prior dissociation and the nucleation probability decreased when we increased the temperature of dissociation. Both of these findings support the concept that there exists a residual water structure from the dissociated hydrate that promotes nucleation.