IR Spectral Study of Mesoscale Process during Urea Crystallization from Aqueous Solution

Unravelling the mesoscale process and the dynamic heterogeneous structures that appear on the mesoscale in a crystallization system is important in designing and fabricating functional crystalline materials. Recent experimental observations show the existence of stable clusters and amorphous intermediates before the formation of a crystalline solid, which seems to contradict classical nucleation theory. Here we show by in situ infrared spectroscopy and theoretical calculation that the liquid/solid phase transformation of urea proceeds through the agglomeration of primary clusters. The phase transformation pathway of urea in solution has been identified, in which urea molecules initially aggregate into one-dimensional (1D) molecular chains, and then these 1D molecular chains assemble to 2D plane-like and 3D net-like clusters. Crystalline urea with P–421m symmetry can be formed when these 3D net-like clusters overcome a critical size. Both experimental and calculated results demonstrate that the liquid/solid phase transformation of urea in aqueous solution obeys the classical nucleation theory. Finally, a morphology diagram of urea is provided on the basis of relative chemical bonding energy density. This morphology diagram can be used to understand the multiple anisotropic geometries for how urea crystals in an aqueous solution system can be laid out. Our results demonstrate the concept of identifying a particular mesoscale process in a urea crystallization system by both in situ vibration spectroscopy observations and chemical bonding calculations.