Molecular-level understanding of gibbsite particle aggregation in water

Using molecular dynamics simulations, we investigate the molecular scale origin of crystal face selectiv-ity when one gibbsite particle attaches to another in water. A comparison of the free energy per unit sur -face area of particle-particle attachment indicates that particle attachment through edge surfaces, where the edge surfaces are either (1 0 0) or (1 1 0) crystal faces, is more energetically favorable compared to attachment between two basal surfaces (i.e., (0 0 1) crystal faces) or between the basal surface of one par-ticle and the edge surface of another. This result suggests that gibbsite crystals with low basal/edge sur -face area ratio will preferentially attach through edge surfaces, potentially helping the crystals grow laterally. However, for larger gibbsite particles (high basal/edge surface area ratio) the total free energy, not normalized by surface area, of particle attachment through the basal surfaces is lower (more nega-tive) than attachment through the edge surfaces, indicating that larger gibbsite particles will preferen-tially aggregate through basal surface attachments. The short-range electrostatic interactions including the interparticle hydrogen bonds from surface -OH groups drive particle attachment, and the dominant contribution to the free energy minimum is enthalpic rather than entropic. However, the enthalpy of basal-edge attachment is significantly offset by the entropy leading to a higher free energy (less negative) compared to that of basal-basal attachment. Study of the free energy for a few imperfect attachments of two particles indicates a higher free energy (i.e., less negative, less stable), compared to a perfect attachment (c) 2021 Published by Elsevier Inc.