Uncovering the Mode of Action of Solvent and Additive Controlled Crystallization of Urea Crystal: A Molecular-Scale Study

Abstract

We report a computational approach to elucidate the role of external growth parameters, additives, solvents, and their concentration to predict the steady-state crystal growth morphology. The developed growth rate expression relates the kinetic and thermodynamic aspects of the adsorption of these species on flat faces of crystals. Searches for stable solvent–surface interfacial structures are performed by ab initio evolutionary algorithm to accurately determine the adsorption energies of solvent at crystal surfaces. The calculations of solute–surface, solvent–surface, and additive–surface energetics have been obtained using periodic first-principles dispersion corrected density functional theory. The approach has been successfully applied to the case of urea crystal to study the aqueous growth morphology with and without addition of additive such as biuret. Different step configurations along 2-D lattice vector and molecular orientations are examined before obtaining the rate-determining adsorption energies for solute. In the absence of biuret, we predict needle-like shapes of urea crystal from aqueous solution as functions of supersaturation and temperature, which are in good agreement with the experimental data. The results show that the growth of the fast-growing (001) face is significantly retarded when biuret is present. On the other hand, addition of biuret hardly affects the growth of (110) face. The adsorption energy and, hence, surface coverage of biuret molecules at (001), (111), and (1̅1̅1̅) faces are significantly higher than that of (110) face. On (001), (111), and (1̅1̅1̅) faces, the biuret molecule formed stronger lateral bonds with the surface; on the (110) face instead, the biuret molecule has smaller adsorption energy as compared to water at the lattice sites exposed on this face.