Rapid expansion of supercritical solutions (RESS) is a promising technique for separation and mixing of nanopartricles. Further development and tuning of RESS require fundamental understanding of interparticle forces in supercritical fluids. We present a molecular simulation study of interactions of silica nanoparticles of different sizes in supercritical carbon dioxide (T = 318K, 60 atm < P < 200 atm), focusing on the dependence of the interparticle force on particle size, carbon dioxide pressure, and particle surface modification.

The potentials for fluid-fluid and solid-fluid interactions are fitted to experimental data on (a) PVT data of bulk CO2 at similar temperatures and pressures (b) experimental sorption isotherms at cryogenic and critical conditions. We employ two different approaches to the modeling of interparticle forces. The forces between very small particles (2.2nm) are determined directly: two nanoparticles are placed in a bath of supercritical CO2. To calculate the forces between the larger particles (20-60nm in diameter), we first study the disjoining pressure between planar silica surfaces, which form a classical slit-shaped pore. Then the disjoining pressure between similar spherical surfaces is derived from the planar disjoining pressure via the Derjaguin approximation.

Our simulations show that the interactions of particles with supercritical CO2 result in notable effective attraction between the particles. The attraction is most prominent for strongly hydroxylated particles at the lower limit of the considered pressure range. The effective attraction is substantially weaker for dehydroxylated particles. The interparticle attraction also weakens as the pressure increases from 68 to 100 atm, but barely changes when the pressure of CO2 increases further to 200atm.