Colloid Science in Supercritical Water

Abstract

Water at high temperature and high pressure near the gas/liquid critical point (T_c = 374°C, P_c = 22.1 MPa) exhibits properties that are remarkably different from those of ambient water. For example, the dielectric constant decreases from 78 at 25°C and 0.1 MPa to 2 at 400°C and 25 MPa, the value of which is comparable to those of hydrocarbons. Accordingly, water and hydrocarbons become freely mixable, while electrolytes precipitate out. Under such extreme conditions, dispersion stability of colloidal particles should be very different because DLVO forces depend not only on the surface properties of the particles but also on the properties of the dispersion medium. In this article, properties of colloidal dispersions in water at high temperatures and high pressures are reviewed. Anomalous long–range repulsion that appears between silica surfaces in ethanol near the gas/liquid critical point (T_c = 241°C, P_c = 6.1 MPa) will also be reviewed.