Influence of a Nonionic Surfactant on the Water Retention Properties of Unsaturated Soils

Surfactants are widely used in household products, industrial processes and as adjuvants to improve the delivery and effectiveness of agrochemicals. Due to their amphiphilic nature, surfactants tend to accumulate at gas-liquid and solid-liquid interfaces, and thus, have the potential to influence water flow and retention in unsaturated soils. The objective of this study was to investigate the effects of a nonionic surfactant, Triton X-100, on the interfacial properties and capillary pressure-water content relationships of F-70 Ottawa sand and Appling soil. In the presence of surfactant, soil water contents decreased incrementally as the surfactant concentration was increased from 0 g L⁻¹ up to the critical micelle concentration (CMC) of Triton X-100 (0.15 g L⁻¹). Over the same surfactant concentration range, the surface tension of water decreased from 7.2 × 10⁻² J m⁻² to 3.2 × 10⁻² J m⁻² while solid-liquid contact angle decreased from 40° to 10°. No further changes in interfacial properties or soil water characteristics were observed at surfactant concentrations above the CMC. The experimental results were used to develop and evaluate alternative scaling approaches to describe concentration dependent changes in soil water characteristics based on the van Genuchten model. A scaling factor that incorporated both surface tension and content angle relationships provided accurate predictions of soil water retention curves over a range of surfactant concentrations. A simplified form of the scaling factor also was developed, on the basis of a single fitting parameter without the need for surface tension and contact angle data. Although further validation of the simplified scaling factor will be required, this approach offers an efficient means to describe the effects of concentration dependent changes in interfacial properties on soil water characteristics.