Mechanism and kinetics of dehydration of epsomite crystals formed in the presence of organic additives

The thermal dehydration of epsomite (MgSO4 center dot 7H(2)O) crystals grown in the presence and absence of organic additives (phosphonates, carboxylic acids, and polyacrylic acid derivatives) was studied by means of thermogravimetry (TG), differential scanning calorimetry (DSC), X-ray thermodiffraction (XRTD), and environmental scanning electron microscopy (ESEM). In situ XRTD analyses (in air, 30% relative humidity) show an epsomite -> hexahydrite (MgSO4 center dot 6H(2)O) transition at 25-38 degrees C, followed by formation of amorphous phase(s) at T > 43-48 degrees C, and MgSO4 crystallization at similar to 300 degrees C. Kinetic parameters (E-alpha and A) were determined for the main dehydration step (25-160 degrees C), which corresponds to a MgSO4 center dot 7H(2)O -> MgSO4 center dot H2O transition, by applying two isoconversional methods to nonisothermal TG data obtained at different heating rates (beta = 1, 3, and 5 K center dot min(-1)). In situ, hot-stage ESEM observations of the thermal dehydration of epsomite crystals are consistent with the nonisothermal kinetic study and, along with XRTD results, allow us to propose a dehydration mechanism which includes an early nucleation and growth event, followed by the advancement of the reaction interface (3D phase boundary reaction). Both E-alpha and A values increase in the presence of the most effective crystallization inhibitors tested. H-bonding between additives and epsomite crystal surfaces is consistent with Fourier transform infrared spectroscopy (FTIR) and may account for this effect. The increase of E-alpha values can be related to the excess energy required to break additive-water bonds in the reactant. These results are likely to further our understanding of the interaction mechanisms between salt hydrates and organic additives which act as growth inhibitors/modifiers.