Physicochemical controls on adsorbed water film thickness in unsaturated geological media

Abstract

Adsorbed water films commonly coat mineral surfaces in unsaturated soils and rocks, reducing flow and transport rates. Therefore, it is important to understand how adsorbed film thickness depends on matric potential, surface chemistry, and solution chemistry. Here the problem of adsorbed water film thickness is examined by combining capillary scaling with the Derjaguin‐Landau‐Verwey‐Overbeek (DLVO) theory. Novel aspects of this analysis include determining capillary influences on film thicknesses and incorporating solution chemistry–dependent electrostatic potential at air‐water interfaces. Capillary analysis of monodisperse packings of spherical grains provided estimated ranges of matric potentials where adsorbed films are stable and showed that pendular rings within drained porous media retain most of the “residual” water except under very low matric potentials. Within drained pores, capillary contributions to thinning of adsorbed films on spherical grains are shown to be small, such that DLVO calculations for flat surfaces are suitable approximations. Hamaker constants of common soil minerals were obtained to determine ranges of the dispersion component to matric potential–dependent film thickness. The pressure component associated with electrical double‐layer forces was estimated using the compression and linear superposition approximations. The pH‐dependent electrical double‐layer pressure component is the dominant contribution to film thicknesses at intermediate values of matric potential, especially in lower ionic strength solutions (<10 mol m−3) on surfaces with higher‐magnitude electrostatic potentials (more negative than ≈−50 mV). Adsorbed water films are predicted to usually range in thickness from ≈1 to 20 nm in drained pores and fractures of unsaturated environments.