Estimation of Air-Water Interfacial Area in Unsaturated Porous Medium: A New Experimental Approach

Abstract

Air-water interfacial area (a 0 ) was estimated using a surfactant (surface-active agent) adsorption concept in unsaturated porous medium by a new experimental technique. Sodium dodecylbenzene sulfonate (SDBS) was selected as anionic surfactant and a glass bead of size 425–600μm (d 50 =0.50mm) was chosen as the porous medium. Sorption of surfactant onto the solid surfaces was evaluated by miscible displacement under water saturated conditions using nonreactive NaCl and reactive SDBS, and was found to be zero for the glass bead surfaces. The number of surfactant monomers adsorbed onto the air-water interface per unit area was determined by Gibbs isotherm. The main experiment was done in a column composed of several rings where the medium was made unsaturated step by step from the saturated condition to allow the surfactant monomers to be adsorbed at the air-water interface in a regular fashion. After the column reached equilibrium and became homogeneous, the rings were dismantled and the total amount of surfactant in each ring as well as the surfactant concentration in the aqueous phase was determined by the two-phase Hyamine 1622 titration method. Finally, based on these estimated parameters, the air-water interfacial area was determined and the values obtained were found to support the general concept of decreasing a 0 with increasing water saturation, S w . The surface area of the solid (75cm2/cm3), estimated from extrapolation of the experimental data (a 0 ~S w ) at S w =0, was found to be close to the geometrically calculated area (76cm2/cm3), which proves the validity of the method.