Measurements on the Thermodynamic Properties of Porewater in Sandstone by Vapor Pressure Method

Abstract

The thermodynamic properties of porewater in sandstone were obtained as a function of water content and temperature by vapor pressure method. The activity, the relative partial molar Gibbs free energy (Δ HH2O), the relative partial molar enthalpy (Δ SH2O) and the relative partial molar entropy (ΔSH2O) of the porewater were determined at 298.15 K. The activity decreased with decreasing water content in the region where water content is about 1.5%, and drastically decreased in the region where water content is about 0.5%. No clear dependencies of Δ H 2O and Δ SH2O on water content were found. Synthesizing the correlations of the activity, Δ GH2O, Δ H H2O and Δ S H2O with water content and the enthalpy and entropy of fusion for ice, the porewater at the surface of the solid was found to have a low chemical potential compared with free water, and dissimilar with the structure of ice. Correlating the activity and Δ GH2 O with water content, the specific surface area of sandstone and the porosity, the porewater is deduced to be weakly affected up to a distance of approximately 3 nm from the surface of the solid. However, as the porewater approaches a distance of approximately 1.1nm from the solid surface, the effect from the solid surface abruptly becomes strong. This is deduced to be the effects of hydrogen bond at the solid surface and hydration to cations corresponding to the CEC.