Applicability of interfacial theories of surface tension to water-repellent soils

Most methods used to characterize the magnitude of soil water repellency consist of direct or indirect measurements of the initial advancing contact angle (θ) at the solid-liquid-vapor interface. Aqueous ethanol solutions (AETS) are commonly used as testing liquids having different liquid-vapor surface tensions (γ_LV); however, θ measurements using AETS have rarely been performed on water-repellent soils (WRS). Measurements of θ in this study were conducted using both the Wilhelmy plate method (WPM) and the CRM (weight-gain capillary rise method) for three natural and four hydrophobized WRS (water-repellent soils). The values of the Young equation (solid-vapor and solid-liquid surface tension) were calculated, and correlated with the Goods-Girifalco interaction parameter, Φ. The factor Φ was found to be a linear function of the solid-liquid surface tension: Φ = 1 - 0.011γ_SL, with no significant differences between soils. This relation was then used to formulate an ESIT (empirical equation of state of interfacial tension), suggesting that from one universal constant, θ can be predicted as function of γ_SV. The applicability of the ESIT approach to WRS was found to be inferior, in contrast to its successful use for ideal solid polymers. Nevertheless, it was found that for a water-WRS system, Φ was ∼0.6 rather than 1.0 as previously assumed. Applying Φ = 0.6 was successfully used in predicting γ_SV as well as the hydrophilic domain of θ vs. γ_LV for water and AETS.