A method for determining air–water interfacial area in variably saturated porous media

Abstract

The air–water interfacial area ( a 0) was estimated using the surfactant adsorption concept in unsaturated porous media. A column composed of several stainless steel rings was used to carry out saturated/unsaturated experiments in three glass beads media (0.25, 0.50 and 0.75 mm) using sodium dodecylbenzene sulfonate (SDBS) and NaCl as the reactive and nonreactive tracers, respectively. SDBS sorption onto the glass bead surfaces was determined by miscible displacement and was found to be zero. The medium was made unsaturated step by step with recycled surfactant solution from saturated conditions to achieve homogeneous surfactant concentration in the liquid phase. This also helped to form a stable air–water interface and pressure–saturation relationship inside the soil column. After establishing equilibrium in the system, the rings were dismantled and total surfactant mass from each ring was extracted and analyzed by the two-phase Hyamine 1622 titration method. The number of surfactant monomers adsorbed onto the air–water interface per unit area was determined by Gibbs isotherm. The air–water interfacial area was then estimated based on the amount of SDBS onto the air–water interface divided by the number of SDBS monomers per unit area. The results obtained by this technique for different porous media revealed that the estimated a 0 followed the general concept of a decreasing trend with increasing S w and grain size. Surface areas of the solid, obtained by extrapolating the curves from the experimental data ( a 0∼ S w) at S w=0, were also found closer to the areas calculated by the geometrical method.