(435c) Density Functional Theory for the Structure and Solvent Capacity of Nanoparticle-Organic Hybrid Materials

Nanoparticle-organic hybrid materials (NOHMs) are a promising new class of materials with unique physicochemical and transport properties. Unlike most nanoparticle systems which aggregate irreversibly due to strong van der Waals attraction, these surface functionalized nanostructures can relax to an equilibrium state and show liquid-like behavior in the absence of solvent.

In a pure NOHM system, the nanoparticle cores are distributed in a homogeneous disordered structure governed by the requirement that the tethered oligomers must fill the interparticle space. We formulate a density functional theory for a system of hard cores with attached bead-spring oligomers in the absence of other solvent molecules. The oligomers are assumed to relax rapidly compared with the cores. The equilibrium concentration field of the oligomers around a core is obtained by minimizing the oligomer's free energy subject to the constraints that the field surrounding a core is normalized and the fluid number density (consisting of oligomers from the test particle and its neighbors) is independent of position outside of the excluded volume of the cores. This constraint leads to an oligomer-configurational entropy penalty for large spaces between the core particles. The static structure factor of the cores is shown to approach zero as the wave number goes to zero. This indicates a deficit of one neighboring core around a test core resulting from the fact that a core carries its own share of fluid.

Nanoparticle ionic materials (a subclass of NOHMs) with amines in the oligomeric chains are being tested experimentally as solvents for CO₂ capture. The low vapor pressure and good thermal stability of these fluids make them promising candidates for this application. We have developed a coarse-grained theory for the salvation properties of a NOHMs fluid, in which a Flory-Huggins interaction parameter describes the chemical affinity of the oligomers for the solute and the space between the cores is uniformly filled by a mixture of oligomers and solute.