Nanoparticles hold great promise for a diverse array of materials applications, ranging from electronic circuits to bulk materials with novel mechanical properties to biological materials. Many applications involve colloidal nanoparticles, whose effective use in nanotechnology hinges on their selective assembly or their stabilization against aggregation. Various methods have been used to stabilize colloidal nanoparticles; however all involve dispersant molecules such as surfactants or polyelectrolytes. Not only do these dispersants alter the chemistry and physics of nanoparticle systems, but since they occupy a significant mass fraction of a suspension, they produce a tremendous waste stream during processing. An improved understanding of the forces between “bare” colloidal nanoparticles could lead to new and environmentally beneficial strategies for engineering colloidal nanoparticle suspensions.

Historically, the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory has been used to describe electrostatic and van der Waals interactions in colloidal systems. However, the assumptions of DLVO theory do not apply to nanoparticles. Further, recent studies suggest that forces that are not taken into account by DLVO theory, such as solvation and depletion, could be important in colloidal nanoparticle systems. From a theoretical point of view, it is possible to simulate colloidal nanoparticles using molecular dynamics (MD). These studies can yield atomic-scale detail that is not currently accessible with experimental methods and they can be used to resolve the origins and magnitudes of forces between colloidal nanoparticles.

We use parallel MD to simulate two solid nanoparticles immersed in a liquid solvent. In these studies, we are interested in the interplay between solvation and van der Waals forces. We consider the influence of nanoparticle size, shape, and surface roughness, as well as solvent type (Lennard-Jones vs. n-decane) and solvent-solid interaction (“solvophobic” vs. “solvophilic”). We find that solvation forces can be comparable to van der Waals attraction and, thus, they can play an important role in determining the stability of colloidal suspensions. Surface roughness causes nanoparticles to rotate so they approach one another via paths of minimum free energy. For example, this rotation causes crystalline (icosahedral) nanoparticles to approach one another via alternating face-face and vertex-vertex conformations, suggesting that solvation forces can control nanoparticle alignment during assembly. Finally, our simulations of solvophobic nanoparticles in n- decane yield insight into how the drying transition is influenced by the relative sizes of the solvent molecules and nanoparticles.