Small-Angle Neutron Scattering Study of Deuterated Propane Adsorption in Silica Aerogel

Abstract

The small angle neutron scattering intensities from fully deuterated propane (C3D8) absorbed in a silica aerogel were measured at two fixed temperatures (102 and 147 °C) and different pressures (P) increasing up to 16 MPa as well as at two different bulk loading densities (0.428 and 0.483 g/cm3) and temperatures (T) decreasing from 102 to 40 °C. They are numerically analyzed assuming that samples behave as made up of three homogeneous phases. The first phase is formed by the silica scaffold, the second phase (liquid film) by the C3D8 molecules at the silica surface, and the third one (confined fluid) by the C3D8 molecules filling up the remaining pore volume. Thus, the absorbed fluid is split into a liquid film and a confined fluid. The thickness δ of the liquid film as well as the scattering-length densities of the two fluid phases are obtained analyzing the Porod invariants and the oscillations observed in the Porod plots of the observed intensities. The resulting δ's vary in the range 25–45 Å. The density of the liquid film is about 0.3 g/cm3 and is nearly insensitive to P and T in contrast to the density of the confined fluid that, however, never exceeds 0.2 g/cm3. The excess densities are evaluated and compared to those of carbon dioxide absorbed in the same aerogel. They satisfactorily agree with the available values obtained by isotherm adsorption measurements.