KINETICS AND MECHANISM OF THE DEHYDRATION OF ALPHA-NISO4-CENTER-DOT-6H2O

A study of the thermal dehydration of alpha-NiSO4.6H2O has been perfor med by power compensation differential scanning calorimetry in flowing nitrogen. No significant differences in behaviour were observed using either uncrushed crystalline powders or single crystal slabs cleaved parallel to {001}. In good agreement with previous findings, the kinet ic analysis of the thermal curves confirms the validity of a n = 2 Avr ami-Erofeev equation (AE2) in isothermal experiments at low (338-343 K ) temperatures or in the initial portions of variable temperature runs . The kinetic obedience is however of an 'order of reaction' type for the main portion of the variable temperature runs and, for isothermal experiments, in the upper part of the temperature range investigated. Values of activation energies and frequency factors are reported. Para llel studies by optical microscopy showed relevant changes of surface texture when partially (thermally or vacuum) dehydrated {001} cleaved surface were submitted to rehydration. This phenomenon (named orange p eel formation) indicates that a dehydrated layer forms on the crystal surfaces preceding the appearance of product crystals (germination or nucleation). Microscopy also revealed that reaction goes on inside the crystal and that product formation takes place in the bulk phase, fol lowing lattice collapse in experiments at high heating rates. Combined with previous results, these new experimental findings allow us to fo rmulate a mechanism for the present transformation, comprising three m ain rate processes: i) the reaction (detachment of water molecules fro m their lattice positions in the reactant); ii) the migration of the w ater molecules freed by the reaction through the initially formed, wat er-depleted layer enveloping the reactant crystal; iii) the crystalliz ation of such a layer to form the product.