Distribution of sorbed water in the pore network of mudstones assessed from physisorption measurements

Abstract

Abstract Nitrogen- and carbon dioxide physisorption isotherms were measured on three mudstone samples which were each moisture equilibrated to five different relative humidities (11, 32, 56, 75 and 97% RH at 303 K). Moisture equilibration was realized in desiccators using the deliquescent points of saturated salt solutions. Systematic testing and reproducibility tests indicate no pore structure alteration and validate the experimental approach. Traditional physisorption theories were employed for analysing the experimental data. The data reveals a decreasing trend in sorption capacity, Gurvich total pore volume, in the specific surface area from the Brunauer-Emmett-Teller theory and in the limiting micropore volume from the Dubinin-Astakhov theory. A combination of Barrett-Joyner-Hallenda and Dubinin-Astakhov theory was applied to compute cumulative pores size distributions disclosing that preadsorbed water is located in pore throats or narrow mesopores. However, no direct correlation between pore size and relative humidity is observed for relative humidities below 97%, suggesting no singular pore size control with respect to sorption mechanism. This contradicts the capillary condensation theory of water in which pores fill sequentially but supports the assumption that water sorption in mudstones is rather surface chemistry controlled. Discrepancies between the loss of Gurvich total pore volume caused by the adsorption of water and the volume of the adsorbed water itself were identified and denote that some of the preadsorbed water is located in parts of the pore system which are not accessible by nitrogen. Water sorption into the interlayers of expandable clay minerals may provide an explanation for the observed volume imbalances, as calculated interlayer water volumes, are within in the same order of magnitude as the volume imbalance. Carbon dioxide physisorption isotherms of moisture equilibrated mudstones show the same decreasing trend in sorption capacity and limiting micropore volume as the nitrogen physisorption isotherms.