A continuous polymorphic transition of coordinating water molecules in CuSO 4·5H 2O

Abstract

Water molecules in freshly as-grown crystal of copper sulfate pentahydrate undergo continuous relocations between two effective lattice binding sites, before the stable polymorphic form is attained. The kinetics of such rearrangements was observed by high resolution temperature programmed desorption mass spectrometer (TPD-MS) fitted with a supersonic molecular-beam inlet which enables the probing of the sample at atmospheric pressure. By monitoring the water TPD curve, an interesting involved behavior is revealed, where H 2O molecules are seen, within the period of several days, to continuously rearrange between hydrate sites towards the final stable structural phase where three well-defined, fully resolved desorption peaks are observed. These structural variations were also detected by XRD measurements, yielding qualitative data supporting the conclusive TPD results as for the occurrence of a continuous morphological transition. The present findings combined with Raman data, strongly indicate the association of the observed structural transition with a stabilizing process of two bonds, occurring with different lengths, between two pairs of water molecules and the Cu 2+ cation. The growing of disordered hydrated crystals became irreproducible after a period of ca. two months probably due to contamination of ordered microcrystals in the laboratory. The phenomenon observed here questions theoretical predictions that rely on polymorphic transformations studies, and may thus encourage further experimental work in order to elucidate and test the validity of such theories.