| The correlation between the wettability of soil materials and soil chemical composition and surface energy is examined. Wettability, as measured by the contact angle, is a governing parameter of capillary flow in the subsurface. The contact angle is determined by the intermolecular forces at the intersection of a drop with a surface and surrounding fluid. It was previously assumed that although most soil materials are water wet, similar to quartz sands, some organic-carbon containing materials may be hydrophobic. However, this work determines that some common subsurface media may have radically different wetting behavior depending on the past wetting history even in the absence of surface active agents. The wettability of four organic carbon containing natural soil materials (Ann Arbor II sand, Lachine shale, Garfield shale, and Waynesburg Coal), three carbon containing mineral materials (graphite, dolomite, and calcite), and two pure mineral materials (quartz and talc) was measured by bottle tests and sessile contact angle measurements. The liquid systems examined included air-water, perchloroethylene-water, and carbon tetrachloride-water. The wettability of soil materials is shown to depend on the order of fluid contact with the surface such that some soils (talc and organic carbon-containing materials) are non-water wet if contacted by a non-aqueous phase first. This relatively unexplored phenomenon is termed wettability hysteresis and may indicate that the wettability of some soils will differ depending on the saturation of the system. The effect of the surface composition on the contact angle and degree of hysteresis was quantified by measuring the surface energy components of the surface. It is shown that oxygen indices are a better predictor of contact angle and wettability hysteresis than carbon content. The surface energy components (dispersion, polar, and total) of each solid are determined by the van Oss-Chaudhury-Good method and the polar surface energy component is shown to determine wettability hysteresis. Evidence that a water film forms when water is the first fluid to contact the surface, causing wettability hysteresis, is provided from multiple approaches. The estimation of contact angle and wettability hysteresis based on the polar surface energy component is presented. |
