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

The effective specific air‐water interfacial area 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 values at different S w . For the range of experimental conditions employed in this study, the retardation factor for SDBS ranged from R t = 1.00 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 ). 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.