Interactions of Urea Surfaces with Water as Relative Humidity Obtained from Dynamic Vapor Sorption Experiments, In Situ Single-Particle Raman Spectroscopy, and Ab Initio Calculations

Abstract

Urea is a critical nitrogen carrier molecule that is abundant in the environment due to its anthropogenic activity to enhance crop growth. The intrinsic link between its high solubility and volatilization, resulting in the reactive nitrogen species and CO2, especially under excessive relative humidity (RH) conditions, suggests that the urea hydrolysis-initiated decomposition reaction can be affecting the global nitrogen balance. Fundamental analysis of water as RH adsorption on urea particle surfaces was performed using a combination of dynamic vapor sorption (DVS) experiments, in situ single-particle Raman spectroscopy, and ab initio calculations. The DVS data acquired exhibited three RH adsorption regimes with urea with 74% RH dramatically changing adsorbate–urea interactions from monolayer- to multilayer-induced deliquescence. Several empirical kinetic models were utilized to understand the RH interaction with urea surfaces, and the Guggenheim–Anderson–de Boer model provided a good description of the adsorption at <60% RH values, while a Van Campen model was used to fit the data acquired during the urea crystal deliquescence. The experimental water sorption rate using the Van Campen model showed a gradual rise from 0.02 mg/min at 80% to 0.08 mg/min at 95% in agreement with Van Campen’s model of increasing trend, albeit at higher rates ranging from 0.03 mg/min at 80% to 0.1 mg/min at 95%. In situ Raman spectroscopy combined with optical images of a single particle showed that the urea 1009 cm–1 peak full width at half-maximum can provide in-depth information on the transient phenomena taking place on the urea particle surface as well as in the partially liquefied environment. Finally, density functional theory results suggested that Wulff’s reconstruction of a single urea crystal depended on the presence of higher crystalline planes; particularly, the (111) facet became significant together with (101) and (110), while in the presence of bulk H2O, (101) became the dominant facet. The results presented in this work will allow for a better understanding of urea–water vapor interactions in the environment at the molecular level including its potential for aerosol formation in the regions of high agricultural activity. Further, this study will allow us to better understand urea partitioning into soil pore volume where high RH is prevalent. Finally, it will provide a microscopic, single-particle understanding of the urea transformations in moist environments and pave the way for a new, nitrogen-efficient material fertilizer design.