Interactions of silica nanoparticles in supercritical carbon dioxide

We report molecular simulation studies on the interaction forces between silica nanoparticles in supercritical carbon dioxide at 318 K. Our goal is to find a better understanding of the interparticle solvation forces during rapid expansion of supercritical solutions. The parameters for interatomic potentials of fluid-fluid and solid-fluid interactions are obtained by fitting our simulations to (i) experimental bulk CO2 phase diagram at a given temperature and pressure and (ii) CO2 sorption isotherms on silica at normal boiling and critical temperatures. Our simulations show that the interaction forces between particles and supercritical CO2 at near-critical pressure of p=69 atm (i.e., slightly below critical condition) reaches a minimum at distances of 0.5-0.8 nm between the outer surfaces of the particles and practically vanishes at distances of approximately 3 nm. The attraction is most prominent for densely hydroxylated particle surfaces that interact strongly with CO2 via hydrogen bonds. The effective attraction between silica and CO2 is significantly weaker for dehydroxylated particles. We also compared fluid sorption and interparticle forces between supercritical CO2 and subcritical nitrogen vapor, and our results showed qualitative similarities, suggesting that the CO2 configuration between the particles resembles a liquidlike junction.