Determination of effective air-water interfacial area in partially saturated porous media using surfactant adsorption

The effective specific air-water interfacial area ((a) over bar(i)) in a sand-packed column was measured at several water saturations (S-W) using a surface-reactive tracer (sodium dodecylbenzene sulfonate (SDBS)) and a nonreactive tracer (bromide). Miscible displacement experiments were conducted under steady water flow conditions to quantify the retardation of SDBS resulting from its adsorption onto the air-water interface in a sand-packed column. A consistent trend of increased retardation of SDBS compared with the nonreactive tracer, bromide, was observed with decreasing S-W. The data for air-water surface tension measured at various SDBS concentrations were interpreted using the Gibbs model to estimate the required adsorption parameters. The retardation factors (R-t) for SDBS breakthrough curves were then used in combination with the estimated SDBS adsorption coefficient to calculate the (a) over bar(i) values at different S-W. For the range of experimental conditions employed in this study, the retardation factor for SDBS ranged from R-t = 1.07 at S-W = 1.00 (R-t > 1 due to SDBS sorption on sand) to R-t = 3.44 at S-W = 0.29 (which corresponds to (a) over bar(i) = 46 cm(2)/cm(3)). These values are in agreement with theoretical predictions and recently published data. Improvements needed to overcome the experimental limitations of the presented method are also discussed.