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 welldefined,
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.