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Title of Talk:
Green Engineering of Dispersed Nanoparticles: Measuring and Modeling Nanoparticle Forces

Abstract of Talk:
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 now possible to simulate colloidal nanoparticles using large-scale, parallel 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. The bulk solvent is simulated as more than 100,000 Lennard-Jones (LJ) or n-decane molecules. We study four different types of nanoparticles: small (1.6 nm diameter) and large (6.0 nm diameter) rough, spherical nanoparticles, as well as cubic and icosahedral crystals. To investigate the influence of surface roughness, the nanoparticles are rotated so that they contact from a different angles and have different contacting surfaces. The nanoparticles can be either solvophilic (solvent loving) or solvophobic (solvent fearing).

Solvation forces for solvophilic and solvophobic nanoparticles have been calculated for the different nanoparticle systems. In all the solvophilic nanoparticle systems, the solvation forces oscillate between attraction and repulsion. The oscillatory behavior is caused by the solvent's ordering near the surface. This effect is particularly evident for the cubic nanoparticles, which exhibit the strongest solvation forces. A comparison of solvation forces and van der Waals forces indicates that solvation forces can be comparable to van der Waals forces. This indicates that solvation forces may be beneficial in preventing nanoparticles from aggregating and that stable nanoparticle dispersions may be achieved in suitable nanoparticle-solvent systems. We find that solvophilic solvation forces can be highly sensitive to the relative orientation of the nanoparticles and that these forces can cause the nanoparticles to rotate in solution to minimize their free energy. This effect is especially pronounced for the icosahedral nanoparticles, which alternate their relative orientation as they approach each other. In this case solvation forces may be utilized to align nanoparticles for applications in self-assembly.

We find that solvation forces for solvophobic nanoparticles are always attractive. In this case, solvent molecules are repelled from the interparticle region and the density there is lower than the bulk density. The solvophobic solvation forces for nanoparticles in n-decane are particularly interesting, as they exhibit a region of uniform attraction followed by a sudden jump of the forces to zero. These attractive forces can be greater than the van der Waals forces. They arise from an interesting ordering of n-decane around the nanoparticles, in which the molecules orient themselves normal to the particle surfaces, to increase van der Waals attraction between the decane molecules and minimize contact with the nanoparticles. In this orientation, solvent is repelled from the interparticle gap until the gap exceeds twice the end-to-end distance of a decane molecule, at which point solvent enters the interparticle region and assumes a bulk-like structure.

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