The kinetics and energetics of transitions between water repellent and wettable soil conditions: a linear free energy analysis of the relationship between WDPT and MED/CST

✍ Scribed by Peter Douglas; Kathryn A. Mainwaring; Christopher P. Morley; Stefan H. Doerr

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 156 KB
• Volume: 21
• Category: Article
• ISSN: 0885-6087
• DOI: 10.1002/hyp.6747

✦ Synopsis

Abstract

Soil water repellency is commonly assessed using the Water Droplet Penetration Time (WDPT) and Molarity of Ethanol (MED) or Critical Surface Tension (CST) tests. The former is a kinetic measurement indicating the persistence of water repellency; the latter, a thermodynamic measurement, gives the initial severity of water repellency. This study aims to provide a theoretical framework to understand (i) what determines differences in persistence and initial severity of soil water repellency and (ii) correlations between such data. A linear free energy relationship between Solution Droplet Penetration Time (SDPT) and the difference between the surface tension of the droplet solution, γ~sol~, and the Critical Surface Tension of the soil, γ__c__~x~, has been derived.

Here: γ~sol~ is the surface tension of water (for WDPT) or the solution being used (SDPT); N is the number of adsorbed molecules per unit area; k the Boltzmann constant; T the temperature; A is 1/SDPT when γc~x~ = γ~sol~; and f is an experimentally determined parameter. WDPT, SDPT (ethanol/water and propan‐1‐ol/water), and CST measurements for a group of sandy soils support this analysis. The value of f obtained (4·2 ( ± 0·4)), suggests that the surface free energy contribution to the free energy of activation of wetting is given by the difference between the cohesive energy of the molecular film adsorbed on the soil grains and that of the wetting solution.

In this interpretation: γc~x~ is determined by the cohesive energy of the organic film adsorbed on the soil; WDPT is determined by the difference in cohesive energies between this adsorbed film and the droplet solution; and soil‐to‐soil variations in both WDPT and γc~x~ are due to organic films of different cohesive energies present on the soil particles. For aqueous solutions of simple linear alcohols and ketones, SDPT depends on γ~sol~ but is independent of the compound used to control γ~sol~. Copyright © 2007 John Wiley & Sons, Ltd.