Influence of Polar Protic Solvents on Urea Morphology: A Combination of Experimental and Molecular Modeling

Abstract

The modification of crystal shapes is used intensively in designing particles with desired properties to ensure the efficiency of the manufacturing processes. In this work, urea crystals grew in various polar protic solvents by cooling crystallization that produced α-form urea. The aspect ratios of the crystals were found to be in the range from 14.77 to 1.44, grown in the following solvents: water > methanol > ethanol > isopentanol > isobutanol > pentanol > butanol > propanol > isopropanol. An attempt to establish a correlation between the nonbonded energy, solvent molecule size, binding energy, and the change in morphology revealed that the aforementioned factors could not accurately predict the aspect ratios of the urea crystals. The solvent-surface binding energy showed the {111} and {001} capping facets recorded the highest strength (most negative energy), while the {110} facet was the lowest. The inhibition of the solvent molecules on the {001} and polar {111} facets stopped the growth along the c-axis by stopping the formation of synthon A and synthon B, while the growth along the a-axis was halted by stopping the formation of synthon B. The transformation from elongated cuboid to prismoidal shape of urea was governed by the {111}/{110} energy ratio, i.e., it must be <1.78.