Water repellency of volcanic ash soils from Ecuadorian paramo: effect of water content and characteristics of hydrophobic organic matter

Water repellency of volcanic ash soils from the Ecuadorian paramo was studied by a combination of extraction and analysis of water-repellent products, Molarity Ethanol Droplet values, water contact-angle measurements by capillary rise, and N-2 adsorption isotherms. The undried samples studied are hydrophilic, but exhibit water repellency after moderate drying (48 hours at 30degreesC). The advancing water contact-angle measured by capillary rise varies from 78degrees to 89degrees. These water contact-angles decrease strongly after extraction of organic materials by an isopropanol-water mixture. Elemental analysis, infrared spectra and gas chromatography-mass spectrometry analyses were used to characterize the extracts. The results show that long-chain fatty acids and more complex non-polar alkyl components (waxes) are the main water-repellent materials. The deposition of such extracted materials onto hydrophilic sand leads to the rapid increase of water contact-angle until values close to those measured on the soil samples are achieved. Assuming a coating of the mineral surface by organic hydrophobic products and using Cassie's law, the water contact-angle of extracted materials was computed. The values ranged from 100degrees to 157degrees. Nitrogen specific surface areas of the soils studied were very small, indicating a low adsorbent-adsorbate interaction on hydrophobic surfaces. These results partially validate the hypothesis of water-repellent materials that occur as coatings at least after a drying process.