Unique diffusion behavior observed in supercritical ethanol

We have systematically investigated the diffusion behavior of silica nanoparticles within supercritical ethanol, in terms of solvent properties by varying temperature (T) and pressure (P), to elucidate how the inhomogeneous solvent structures and density fluctuations in the solvent affect the diffusion behavior of solute particles. Results show that at a constant pressure, the diffusion coefficient (D) of the particles increases with increasing temperature, reaches the maximum (D^[max]) within the gaslike supercritical fluid (slightly below the ridge), and finally decreases abruptly at very low fluid density when temperature is increased further. Results reveal that D is appreciably larger than the theoretical prediction (Einstein-Stokes relationship) in the vicinity of the critical density (ρc) of the solvent. We interestingly observed that D becomes maximum (D^[max]) at a particular thermodynamic condition (Ti,Pi), which is expressed by the empirical formula T_[ri]=P^[0.16]_[ri] (for Tri>1, Pri>1). Here, Tri=Ti/Tc and P_[ri]=Pi/Pc; Tc and Pc are the temperature and the pressure at critical point, respectively. Results further reveal that D^[max] increases significantly with decreasing solvent density within the gaslike supercritical fluid where the changes in viscosities are negligible. These findings are unique, novel, and intriguing. We suggest that the enhancement of the diffusion coefficient in the vicinity of the critical density and the abrupt decrease in the diffusion coefficient in very low density gaslike fluid are associated with the change in the solvent-solvent and solute-solvent direct correlation function (related to the effective interaction potential) upon density change when the fluid crosses the ridge of density fluctuations and within the gaslike fluid