Hydrophobic coating- and surface active solvent-mediated self-assembly of charged gold and silver nanoparticles at water-air and water-oil interfaces.

We report self-assembly of charge-stabilized gold and silver nanoparticles at water-air and water-oil interfaces, via manipulation of the interactions between the interfaces and the adsorbing nanoparticles. Nanoparticle adsorption from bulk colloids to an interface is an energy-favored, but finite sorption barrier-restrained (kinetics-controlled) process. Consequently, to successfully mediate self-assembly of nanoparticles, the finite sorption barrier should be decreased. That can be accomplished by manipulating its three controlling forces: the repulsive electrostatic force, the repulsive van der Waals force, and the attractive hydrophobic force between the interface and the adsorbing nanoparticles. It was found that hydrophobic coatings change nanoparticle hydrophobicity and greatly increase the attractive hydrophobic force. Surface active organic solvents (methanol, ethanol, isopropanol, and acetone) decrease the attractive hydrophobic force to some extent. However, they decrease the repulsive electrostatic force to a larger extent, via a "charge dilution" mechanism, due to their positive adsorption at the charged water-air and water-oil interfaces. Hydrophobic coatings and organic solvents consequently decrease the sorption barrier, facilitate nanoparticles overcoming the sorption barrier, and mediate the self-assembly of nanoparticles.