Role of Different Solvents and Tailor-Made Additives in Asymmetry in Growth Rates along the Opposite Ends of the Polar Axis: The Riddle of α-Resorcinol

Abstract

Understanding the mechanism of unidirectional growth of polar acentric materials along the polar axis has been an important issue so far. To this end, we computationally examine the role of various tailor-made additives in aqueous, acetic acid, and acetone solutions to elucidate the underlying mechanism of asymmetry in the growth of α-resorcinol crystals. A simulated annealing method followed by periodic density functional theory using the B3LYP method augmented with an empirical dispersion term has been employed to determine the solute–surface, solvent–surface, and additive–surface interfaces. The adsorption energies of different molecular auxiliaries in various step configurations on oxygen-rich {01̅1̅} and hydrogen-rich {011} faces have been determined to examine different adsorption pathways before obtaining the rate of growth. Considering that crystallization is a thermally activated process that involves the kinetic and thermodynamic aspects of the adsorption of different molecular auxiliaries at the interface, the rate of growth along the opposite ends of the polar axis has been calculated to determine growth anisotropy. We have constructed eight different step configurations (¢1–¢8) on the (011) and (01̅1̅) faces to determine the adsorption pathways of the resorcinol molecule. It has been found that the adsorption of different auxiliaries in ¢1, ¢4, and ¢1 step configurations would determine the growth rate of oxygen-rich faces from aqueous, acetic acid, and acetone solutions, respectively. Our results show that the oxygen-rich faces grew 9 and 6 times faster than the hydrogen-rich faces from aqueous and acetone solutions, respectively. However, the growth rate of the oxygen-rich faces is nearly 3 orders of magnitude lower than that of the hydrogen-rich faces during acetic acid-mediated crystallization. Our study suggests that the solvent adsorption energy toward the oxygen-rich faces is not strong enough to explain the experimentally observed growth disruption of the oxygen-rich faces from acetic acid solution. We find that the strongly adsorbed acetic acid on the oxygen-rich faces rotates one of the hydroxyl groups of the surface molecule from anti to the syn position, which causes an appreciable delay in their growth. Moreover, adsorption of different auxiliaries in the ¢6 step configuration would determine the growth rate of hydrogen-rich faces from aqueous, acetic acid, and acetone solutions. The adsorption energies of water, acetic acid, acetone, and different conformers/tailor-made additives in various step configurations have also been evaluated to determine the effect of these auxiliaries on the unidirectional growth along the polar axis. Our results show that the asymmetry in growth decreases with increasing saturation temperatures and supersaturation, which is consistent with the experimental results. Increased concentrations of β- and γ-phase conformers of resorcinol and orcinol lead to increased asymmetry from aqueous growth. However, asymmetry from aqueous growth decreases with increasing concentrations of 2-methylresorcinol and pyrogallol. Our findings clearly show that the growth asymmetry increased from 9 to 1 in aqueous solution containing 20% pyrogallol at a temperature of 25 °C, which is in good agreement with the experimental observation. The asymmetry in growth from aqueous and acetone solutions decreases with increasing supersaturation. On the other hand, the asymmetric growth induced by acetic acid increases with increasing supersaturation, which clearly suggests that the solvent interaction on oxygen-rich faces is higher than that on hydrogen-rich faces.