RESUMO
While the electrostatic potential and the counterion distribution produced by interfaces with idealized geometries can be well-described by analytical models, the same does not hold true for the interaction between surfaces with different and arbitrary geometries. Besides, the geometry of a charged interface may also affect the counterion adsorption, potentially modulating the electrostatic potential and the solvent organization close to the interfaces, demanding molecular details to be taken into account. The complex electrostatics of a sodium dodecyl sulfate micelle in the presence of monolayers of the same surfactant at the water-vapor interface was assessed by a set of molecular dynamics simulations. The electrostatic potential was evaluated numerically, and its total magnitude was decomposed into contributions arising from each species comprising the system. The counterion adsorption was stronger at the flat interfaces due to the more favorable formation of sodium bridges, where the same counterion is bounded to two or more anionic heads, while water reorientation was more pronounced near the micelle. These opposing effects counteracted each other so that the overall electrostatic potential changes were similar for both interfaces. The increase in the counterion concentration between the micelle and the interface originates a double layer mediated repulsion amounting to a free energy barrier of at least 14 kJ/mol, preventing the micelle to get closer to the monolayers. It is noteworthy that the hydrophobic regions had electrostatic potential contributions as large as those arising from the hydrophilic regions, mostly due to the orderly orientation of the terminal methyl groups.
RESUMO
Close-packed arrays of ZrO2 nanocrystals (NCs) have been self-assembled from a colloidal solution in a withdrawal dip coating process. A benzyl alcohol route was used to obtain NCs of narrowly controlled size, and then the capping layer was replaced by oleate using solvothermal treatment. The oleate solubility was explored in chloroform, hexane and toluene to prepare thin films of NCs using a dip coating process. From TEM images, the final structures show that increasing the solvent polarity improved self-assembly to prepare mono- and multi-layer superlattices, during solvent evaporation in a short time. The entangled organic chain in the NC surface offsets the limitations of the faceted NCs, improving the assembly quality, allowing the NC assembly to approach the formation of a hard sphere model, resulting in a FCC close-packed structure. Furthermore, the low interaction of chloroform with the capping layer reduces the shrinkage effect during the solvent evaporation preserving the array in the final self-assembled structure. Molecular dynamics simulations with soft potentials supported the conclusion that hexane interacts with the organic capping ligand, increasing the apparent radius of each NC and stabilizing the colloidal suspension, whereas chloroform is partially removed from the capping layer during the aggregation process, forming an array of nanoparticles.