Do surface active substances from water repellent soils aid wetting?

Abstract

SummarySoil 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 (γSL) or reduction of the liquid‐vapour interfacial energy (γLV), or both. The reduction of γSL 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 γLV 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 (γLV 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 γLV, electrical conductivity, and monovalent/divalent cation ratio as the soil extracts, was much faster. Ethanol solutions are usually used as an agent to reduce γLV 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 γLV, but to also to changes in either or both γSL and the solid‐vapour interfacial energy (γSV). These results stand in sharp contrast to well‐accepted logical paradigms.