A Surfactants Walk to Work: Modes of Action of Citrate Controlling (10-10) and (000-1) Zinc Oxide Surface Growth from Solution
T Milek and D Zahn, ZEITSCHRIFT FUR ANORGANISCHE UND ALLGEMEINE CHEMIE, 642, 902-905 (2016).
Molecular dynamics simulations unravel the association of citrate to (10-10) and (000-1) growth fronts of zinc oxide and demonstrate an unexpected mobility of the surfactant. Citrate association to perfectly planar (10-10) and (000-1) ZnO-ethanol interfaces was found to be favored over dissociation by 1.5-2 eV hence suggesting strongly bound, immobilized surfactants. However, intramolecular stress prevents binding of all three carboxyl groups to planar surfaces and the typical arrangement is that of two carboxyl-Zn contacts (including two salt bridges each) whilst the remaining carboxyl group is dangling into the solvent. As a consequence, the surfactant exhibits a "walking" mechanism to move along the surface by exchanging the role of its carboxyl groups. This finding has strong implications for the role of citrate during crystal growth as illustrated by a recently developed simulation scheme based on hundreds of individual Zn2+ and OH-ion association steps. In particular, for the (10-10) surface which grows via formation of ridges embedded by 10-10 faces these simulations show how citrate ions move towards steps and bind to the growth front by additional 4 eV per surfactant molecule.
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