Water Film Thickness in Unsaturated Porous Media: Effect of Pore Size, Pore Solution Chemistry, and Mineral Type

Abstract

AbstractThe thickness of the water film on minerals is key for a quantitative understanding of mass transport and reaction in porous media under air entrapment. How the film thickness changes with pore structure, ionic strength, pH, and type of mineral, however, is not fully understood. Here, we present a model to predict the film thickness in porous media under air entrapment. The model numerically solves the Poisson–Boltzmann equation combined with the electrical triple‐layer model to consider the overlapping of electric double layers (EDLs) in the film, where EDLs formed at the mineral/water and air/water interfaces interact with each other. Based on the model, we estimated the film thickness for silica, montmorillonite, calcite, and goethite for various ionic strengths and pH. With decreasing film thickness, the overlapping of EDLs reduces the surface charges but increases the electric potentials of silica/water and air/water interfaces. The predicted film thickness for silica generally agreed with the measured value. The film on calcite and goethite is significantly thinner than that on silica and montmorillonite because the surface charges for calcite and goethite have a sign opposite to that of the air/water interface. Our model shows that the pore radius, ionic strength, pH, and sign of surface charge are the primary factors controlling the film thickness. When the ionic strength and pH are similar to those of rainwater, river water, and groundwater, the typical film thicknesses for silica in silt, sand, and gravel were predicted to be <14 nm, 14–45 nm, and >23 nm, respectively.