Dehydration of α-oxalic acid dihydrate: Structural, spectroscopic and thermal study with implications on the disruption of water molecular bridges in soil organic matter

Water molecule bridges (WaMB), representing linkages between functional groups in organic matter, are hypothesized to stabilize the supramolecular structure of the soil organic matter (SOM). As the underlying mechanisms are largely unknown, partly due to the high heterogeneity, complexity and amorphicity in SOM, a more general understanding on the processes potentially underlying WaMB formation and disruption can be accomplished by exploring a simpler model system. We examined in a first step and for the first time the mechanism of water liberation in a well-defined model system, alpha-oxalic acid dihydrate, using XRPD, FT-IR, DSC and solid state H-1 NMR. In a second step, we evaluated the potential of the transfer of this knowledge to the processes occurring in SOM. According to XRPD, dehydration occurs at about 70 degrees C and it is accompanied by a change in the crystal system from monoclinic to orthorhombic. Temperature-dependent FT-IR spectra, apart from confirming the findings of XRPD, reveal the changes in hydrogen bonding pattern upon transformation of the dihydrate to the anhydrous compound. DSC thermograms obtained in two subsequent heating and cooling runs suggest a reversibility and heating rate independence in breaking and re-forming of hydrogen bonds between oxalic acid moieties and water molecules in temperature range t* similar to 60-76 degrees C. The latter is also confirmed by solid state H-1 NMR spectra. Similarly to alpha-oxalic acid dihydrate, disruption of WaMB in SOM occurs in that temperature range, but does not reappear immediately in the second heating run and it is heating-rate dependent, which points to a relevance of structural relaxation processes in SOM. Simultaneous monitoring of spectral signatures changes and detection of the water release temperature suggests that temperature-dependent FT-IR spectroscopy might be a helpful method to monitor WaMB disruption and formation in SOM. (C) 2016 Elsevier B.V. All rights reserved.