Molecular Paradigm Dependent Nucleation in Urea Aqueous Solution

During the nucleation process in urea aqueous solution, rigid urea molecules are linked by hydrogen bonding without any changes in molecular symmetry, which makes it difficult to track the nucleation by identifying the symmetric variation of urea in a solution system. In this work, combining in situ Raman and infrared spectroscopy was found to be powerful to observe the fine variations of different groups such as CO, CN, NH₂, and OH in urea molecules at the nucleation stage. The dehydration of the hydrated urea and the aggregation between urea molecules were experimentally confirmed in the nucleation. According to the evolution of vibration bands of these typical groups, three states can be clearly distinguished, i.e., hydrated monomers, prenucleation clusters, and crystalline nuclei. In the initial period of nucleation, hydrated urea monomers and clusters coexist in the solution. With an increasing the size of urea aggregations, prenucleation clusters are formed, which then transform into crystalline nuclei within a very short time. During this period, a rapid structural adjustment occurs, which can be identified by the dramatic variations of both wavenumbers and vibration intensity of constituent groups in urea molecules. Finally, crystalline urea nuclei are formed and grow, during which the recorded Raman and attenuated total reflection-infrared spectral signals remain unchanged. Our present work will deepen the understanding of the nucleation of rigid molecules nearly without symmetric variations in a solution system.