Do surface active substances from water repellent soils aid wetting?

Soil water repellency is usually unstable, as exemplified by the common method of quantifying repellency degree - the water drop penetration time (WDPT) test. Dynamic penetration and infiltration of water into repellent soils is generally attributed to either reduction of the solid-liquid interfacial energy (γ_SL) or reduction of the liquid-vapour interfacial energy (γ_LV), or both. The reduction of γ_SL can result from conformation changes, hydration, or rearrangement of organic molecules coating soil particle surfaces as a result of contact with water, while the reduction of γ_LV can result from dissolution of soil-borne surface active organic compounds into the water drop. The purpose of this study was to explicitly test the role of the second mechanism in dynamic wetting processes in unstably repellent soils, by examining the drop penetration time (DPT) of water extracts from repellent soils obtained after varying extraction times and at different soil : water ratios. It was indeed found that soil extracts had lower surface tensions (γ_LV approx. 51-54 mN m^-1) than distilled water. However, DPT of the soil extracts in water repellent soils was generally the same or greater than that of water. Salt solutions with the same electrical conductivity and monovalent/divalent cation ratio as the soil extracts, but lacking surface active organic substances, had the same DPT as did the extracts. In contrast, DPT of ethanol solutions prepared with the same γ_LV, electrical conductivity, and monovalent/divalent cation ratio as the soil extracts, was much faster. Ethanol solutions are usually used as an agent to reduce γ_LV and as such, to reduce DPT. It is concluded that the surface-active, soil-derived organic substances in aqueous soil extracts do not contribute to wetting dynamics, and as such, this mechanism for explaining kinetics of water penetration into water repellent soils is rejected. It is also concluded that the rapid penetration of ethanol solutions must be due not only to changes in γ_LV, but to also to changes in either or both γ_SL and the solid-vapour interfacial energy (γ_SV). These results stand in sharp contrast to well-accepted logical paradigms.