The role of superheated water on shielding and mediating hydrogen bonding in N,N '-1,2-ethanediyl-bis(6-hydroxy-hexanamide) crystallization

The role of superheated water on hydrogen bonding in amide crystallization was examined by the use of low molecular weight N,N '-1,2-ethanediyl-bis(6-hydroxy-hexanamide) model crystals that form hydrogen bonded sheets stacked by van der Waals forces. Thermodynamic, structural and conformational studies reveal that despite a low temperature transition, which originates in a reversible change from cooperative configurational flip-flop to conformational hydroxylic hydrogen bonding, a delicate balance between conformational disorder/order in the aliphatic segments and hydrogen bonding efficiency between the hydrogen bonded moieties validates the role of N,N '-1,2-ethanediyl-bis(6-hydroxy-hexanamide) as a model compound representing the crystalline domains in polyamides. Crystallization from the superheated state of water results in the formation of a thermodynamically metastable phase, where water molecules in the vicinity of the amide moieties shield the interchain hydrogen bonding. Because of this shielding water molecules erase the conformational limitations of the planar amide motifs during crystallization, and trans methylene conformations along the entire molecules coexist with strong intermolecular hydroxylic hydrogen bonding. By annealing or sequential temperature cycles the metastable crystals tend to transform irreversibly into thermodynamically more stable crystals. During the transformation the water molecules migrate from the amide moieties. Since the highly efficient hydrogen bonding between highly ordered hydroxylic end groups decreases the effect of thermally triggered rotating gauche conformers, less effect of the crankshaft type of motion in the methylene segments requires more energy to pursue a solid state crystalline transition. Conclusively, physically bound water molecules near the amide motifs shield intermolecular hydrogen bonding and mediate the formation of stabilizing hydroxylic hydrogen bonds.