Solubility effects on surfactant-induced unsaturated flow through porous media

It has been demonstrated that local surface tension depression due to the presence of an insoluble surfactant, myristyl alcohol, can result in capillary pressure gradients that cause significant flow in unsaturated porous media. We hypothesized that the effects of a soluble surfactant, butanol, would differ from those of myristyl alcohol despite the fact that both give a similar reduction in the surface tension of water. We investigated this hypothesis through a series of horizontal column experiments and unsaturated flow modeling. We found that both surfactants induced water flow from regions of high surfactant concentration to regions of low concentration but that the shapes of the resultant moisture content profiles were fundamentally different. Because surface tension depression from myristyl alcohol was confined to the original source zone, we were able to simulate the system using a standard unsaturated flow model by assigning separate sets of hydraulic functions to the initially clean and source zones. Modeling showed that the redistribution of water due to the initial surfactant-induced capillary pressure gradient created moisture content induced capillary pressure gradients which acted to balance the system over time. The moisture content profile within the butanol system indicated a propagation of the zone of reduced surface tension. Because the surface tension of water is a function of solute concentration, surfactant-induced capillary pressure gradients are expected to remain and influence the flow system as long as concentration gradients exist.