Hydration studies of a simple molecular solid

Abstract

This paper presents novel information on the thermal dehydration and rehydration of the molecular solid, Oxalic Acid Dihydrate. Although the procedure of the overall decomposition process is well-defined, the structural basis and mechanism of the dehydration process has been poorly studied. We show that the dehydration occurs in a planar manner, with a resultant semi-topotactic relationship between hydrated and dehydrated structures, reflected in the molecular packing. During rehydration, the reconstruction of the phase can be seen to occur at a 3-dimensional phase boundary reaction front, i.e. a recrystallisation of the dihydrate on the surface of the reactant product, with the topotactic relationship leading to texturing and possibly epitaxial relationships between partially dehydrated and rehydrated structures. The proposed mechanism is shown to be consistent with the measured kinetics of the process.