Contribution of Binary Organic Layers to Soil Water Repellency: A Molecular Level Perspective

Soil water repellency (SWR) is an extensively occurring phenomenon on natural and agricultural soils with a severe impact on soil water relations and thus crop yields and ecosystem productivity. It is caused by long chain amphiphilic compounds that originate from plant cuticular waxes. However, the severity of SWR varies with soil physical properties and the concentration of the compounds closely associated with producing hydrophobic coatings on soil surfaces. The induction of SWR by hexadecane, isopropyl tetradecanoate, and palmitic acid (PA), as pure (individual) coatings and as coatings composed of binary mixtures, was investigated by applying a range of loadings on acid-washed sand (AWS) (300-500 μm diameter) and AWS with 5% kaolinite. Molarity of ethanol droplet (MED) tests were conducted to assess the severity of SWR. Palmitic acid was very effective at inducing SWR at loadings of >0.5 × 10^-6 mol g^-1. Hexadecane and isopropyl tetradecanoate had no effect on SWR when applied as single component coatings. However, when hexadecane was combined with palmitic acid, it enhanced the SWR effect of palmitic acid. In comparison, isopropyl tetradecanoate was found to partially mitigate the SWR caused by palmitic acid. The experimental measurements of SWR were complemented by fully atomistic molecular dynamics simulations that suggested variations of SWR could be explained through molecular level interactions, packing on different soil mineral surfaces and the surface characteristics of the mineral surfaces. In addition, H-donor interactions of PA were found to be instrumental in intermolecular and surface interactions. Furthermore, cohesion and packing of hydrocarbon chains were found to be important parameters favoring surface adhesion, which in turn led to the formation of hydrophobic molecular coatings. The finding that ester derivatives of long chain fatty acids do not induce water repellency suggests that the introduction of chemical or biological processes that promote esterification of fatty acids could be a mechanism for reducing soil water repellency in agricultural soils.