Role of Divalent Fatty Acid Salts in Soil Water Repellency

Abstract

The effect of pH and counterions on the persistence of soil water repellency was examined. Repellency persistence increased at elevated pH in the presence of Ca2+ but not Na+, increased at high cation concentrations, and decreased with increasing temperature. These results are explained by the behavior of monolayers of long‐chain fatty acids on solid and liquid substrates. Other hydrophobic film‐forming compounds, including long‐chain alkanes, long‐chain amines, and hydrophobins, cannot account for the observations. A conceptual model for repellency in soils was developed: (i) fatty acid headgroups are complexed with cations at positively charged surfaces and via divalent cation bridges at negatively charged surfaces, with chains facing outward, rendering the surface hydrophobic; (ii) these complexed fatty acid molecules have a relatively slow rate of reorientation when contacted by water; and (iii) there are relatively few easily hydrated hydrophilic moieties at the surface. The factors that determine whether a hydrophobic film will form at soil surfaces, and the stability and longevity of that film, include the abundance and nature of fatty acids and other components at particle surfaces, the mineralogy of the substrate, and the chemistry of the soil solution. This model explains many observed inconsistencies of repellent soils, including spatial and temporal variability, development on soils of differing acidities, changes in repellency upon heating, and more. In accordance with the model, we suggest that the molarity of ethanol droplet test is an indirect measure of the fatty acid abundance and strength of attachment at surfaces, and not of the initial contact angle.