The role of liquid water in crystalline hydrate dehydration. Copper sulphate pentahydrate

Simultaneous Differential Thermal Analysis/Thermogravimetric experiments ca rried out on one large single crystal, several small single crystals and po wdered crystals of pentahydrate copper sulphate have been used to demonstra te the role that retained liquid water plays in maintaining crystal morphol ogy during dehydration. Measured activation energies for stepwise dehydrati on in the system show the presence of solution-based transformations provid e lower energy paths for the dehydration steps and stress relieving mechani sms. Skeletal anhydrous crystals from large-sized pentahydrate copper sulph ate have the same morphology as the starting crystal on complete dehydratio n at controlled heating rates as long as a solution phase is maintained wit hin the crystal during decomposition. The athermal activation energies, in this work, are in agreement with those obtained by isothermal methods as lo ng as coincident reaction paths for the two techniques are maintained. The literature has been reviewed in the light of this work and a three-stage pr ocess is presented to rationalise the conflicting information obtained by w orkers using a variety of different experimental techniques.