Mechanisms of Crystal Plasticization by Lattice Water

Purpose: Water of crystallization has been observed to increase plasticity, decrease crystal hardness, and improve powder compressibility and tabletability of organic crystals. This work is aimed at gaining a molecular level insight into this observation. Method: We systematically analyzed crystal structures of five stoichiometric hydrate systems, using several complementary techniques of analysis, including energy framework, water environment, overall packing change, hydrate stability, and slip plane identification. Results: The plasticizing effect by lattice water is always accompanied by an introduction of more facile slip planes, lower packing efficiency, and lower density in all hydrate systems examined in this work. Three distinct mechanisms include 1) changing the distribution of intermolecular interactions without significantly changing the packing of molecules to introduce more facile slip planes; 2) changing packing feature into a flat layered structure so that more facile slip planes are introduced; 3) reducing the interlayer interaction energies and increasing the anisotropy. Conclusion: Although the specific mechanisms for these five systems differ, all five hydrates are featured with more facile slip planes, lower packing efficiency, and lower density.