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.

Automated summary

The study reveals that particle attachment through edge surfaces (such as (1 0 0) or (1 1 0) crystal faces) is energetically favorable, promoting lateral growth of gibbsite crystals. However, larger particles preferentially aggregate through basal surface attachments, as the total free energy for attachment through basal surfaces is lower.