Assembly Rules and Dehydration Mechanism of an Unconventional Hydrate: On the Complexity of the Hydrates of Creatine Phosphate Sodium

Unconventional hydrates with differently bound water molecules and related mazy intermolecular interaction networks need systematic investigation. The assembly rules, interaction analysis, and dehydration behavior can be dramatically more complicated when compared with those of common hydrates. In this work, creatine phosphate sodium (CPS) was selected as a model compound representing unconventional hydrates. The packing mode and the role of water molecules related to the dehydration mechanism were explored by the combination of experimental (diffraction, thermal, microscopy) and quantum methods. It was observed that in the structure of CPS heptahydrate and heminonahydrate, channel, isolated-site, and ion-associated water molecules exist simultaneously, constructing the framework via the coordination bond and hydrogen bond. The binding energy of some ion-coordinated water molecules is lower than that of hydrogen-bonded ones while that of some isolated-site water molecules is lower when compared with that of channel ones, which is counterintuitive. Moreover, the diverse types and locations of water molecules, complicated H2O center dot center dot center dot H2O interactions, and the trade-off between CPS center dot center dot center dot H2O and H2O center dot center dot center dot H2O lead to multistage, variable, and binding-energy-independent dehydration behavior. This work sheds light on the novel structure and variable dehydration mechanism of complicated hydrate systems and inspires the particularity and further investigation of unconventional hydrates.

Automated summary

In this study, the structure and dehydration mechanism of creatine phosphate sodium hydrates were explored using experimental and quantum methods. The existence of channel, isolated-site, and ion-associated water molecules was observed, which constructed the framework through coordination bonds and hydrogen bonds. Surprisingly, the binding energy of some ion-coordinated water molecules was lower than that of hydrogen-bonded ones, and some isolated-site water molecules had lower binding energy compared to channel ones. The multiple stages, variable dehydration behavior with no correlation to binding energy, were attributed to the diverse types and locations of water molecules and the complex interactions between CPS and water molecules. This work provides insights into the novel structure and variable dehydration mechanism of complicated hydrate systems and stimulates further investigation on unconventional hydrates.