Experimental investigations for stearic acid crystallisation in mineral-oil mediated environment showed an interesting shift in shape of C-polymorph of a crystal, wherein, with gradual increase in the concentration of an additive, it changed from a perfect rhombus to broken rhombuses and ultimately to disordered dendritic structure. Here, we present the results of molecular dynamics investigations performed to highlight the molecular level details involved in the crystallisation process by replicating exact chemical surroundings. Simulations depicting the mother-liquor environment comprising of solvent (hexane)-solute (stearic acid) and additive showed a significant change in hydrogen-bonding network between natively present dimmers of stearic acid molecules due to intervention by additive. An actual layer-by-layer growth of crystal lattice is modeled by growing a pre-integrated frozen crystal lattice, inside mother-liquor environments of varied relative supersaturations and additive concentrations. With increase in the relative supersaturations, increase in the rate of deposition of stearic acid molecules on the surface of nucleated crystal lattice is observed, while a corresponding increase in the surfactant concentrations lead to decrease in deposition rate and in turn the rate of crystal growth. Lowering down of rate of deposition of newly entrant molecules on the nucleated crystal lattice can be reasoned to two simultaneously occurring phenomena, namely, decrease in the number of mobile solute molecules in the mother-liquor due to aggregation of surfactant-solute molecules and deposition of surfactant molecules on crystal lattice, in turn, altering the surface dynamics of growing crystal. Surfactant molecules were found to stack on the surface of crystal lattice with attachment of bulky polar group with carboxylic group of constituent lattice molecules, thus making the lattice surface underneath unavailable for ordered deposition of newly entrant stearic acid molecules. Based on obtained simulation results a detailed molecular-level mechanism explaining the shift in crystal habit is proposed. <br/> <br/>

Stearic acid crystal habit alteration in presence of a non-ionic surfactant: Molecular dynamics investigation

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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