Flexible conformation in organic matter coatings: An hypothesis about soilwater repellency

Some soils develop severe water repellency several years or decades followi ng oil contamination. We previously reported that soil water repellency is completely eliminated by extraction with amphiphilic solvents, but barely r educed by extraction with nonpolar solvents. We report here on solvent-indu ced reversible soil water repellency. Our results indicate that: (i) water repellency is completely eliminated following extraction with amphiphilic s olvent, but partially restored following subsequent exposure to nonpolar, n on-H-bonding solvent; (ii) extraction with nonpolar, non-a-bonding solvent generates water repellency in readily wettable control wettable soils, but not in pristine wettable soils, and (iii) repeated sequential extractions a lternating between amphiphilic and nonpolar, non-H-bonding solvent increase extractable material and reduce the magnitude of solvent-induced soil wate r repellency with time. We attribute reversible soil water repellency to solvent-induced changes in the conformation of causative agents of soil water repellency. Recent lite rature reports on the structural flexibility of "insoluble" organic macromo lecules are discussed for supporting evidence. We propose that exposure to nonpolar, non-H-bonding solvents induces stretching of surface-exposed, non polar moieties (i.e. alkyl chains), whereas exposure to polar, H-bonding so lvents induces their coiling. These solvent-induced conformational changes are retained upon solvent removal. Our results indicate that the wettabilit y of oil-contaminated soils depends on both the interfacial conformation an d the fractional coverage of their surface-exposed nonpolar moieties.