Memory effect on semi-clathrate hydrate formation: A case study of tetragonal tetra-n-butyl ammonium bromide hydrate

Recrystallization in a tetragonal tetra-n-butyl ammonium bromide (hereafter, TBAB) semi-clathrate hydrate system has been investigated in TBAB aqueous solutions of two different concentrations (stoichiometric mole fraction for tetragonal TBAB hydrate formation and lower mole fraction than stoichiometric one) by use of optical microscopy. The recrystallization of TBAB hydrate has been observed under milder condition than that of the initial crystallization in both concentrations. In particular, solution of the lower concentration is easily recrystallized. This is the first observation of memory effect in a semi-clathrate hydrate system. In addition, the recrystallization occurs in the vicinity of the place where the last piece of initial crystal was dissociated. This implies that a small amount of residual structures remain in the dissociated water, but unfortunately they cannot be confirmed with Raman micro-spectroscopy.