Drop infiltration dynamics of anionic, cationic, and nonionic surfactants into hydrophobic soils: effect of the particle size distribution

Surfactants have been long used to aid water infiltration into hydrophobic soils since it can reduce the surface tension of water and consequently, the contact angle (CA) form at the solid-liquid-air interface. The degree of soil hydrophobicity is commonly engaged with direct or indirect measurements of the apparent initial advancing CA which is not necessarily correlated with infiltration characteristics of aqueous surfactant solutions. The main objective of this study was to quantify the dynamics of surfactant drop penetration into hydrophobic soils. Three surfactants were examined: anionic (SDS), cationic (CTAB), and nonionic (TX100) at aqueous concentrations of 0.4, 0.8, 1, and 2 C/CMC (where C is the bulk concentration and CMC is critical micelle concentration). Sand with the particle size distribution of 100-210, 425-500, and 600-700 μm was hydrophobized using Leonardite (IHSS). Each run was initiated by placing a 30 μL droplet on the soil surface that was packed into quartz cuvette (2.5×2.5×4 cm). The droplet infiltration dynamics were monitored by an optical goniometer (OCA 20, DataPhysics, Germany), specifically, the drop height, drop base diameter, and CA as a function of time. Notable differences between droplet infiltration characteristics of the three surfactants could be observed. For a given particle fraction, the TX100 and SDS, at concentrations above and below the CMC, the CA and drop height decreased while the drop base diameter increased, suggesting that spreading took place during infiltration. For the CTAB, a significant lag-phase could be observed for all quantities, ranging from 100 to 1000. Following this phase, the drop height and CA showed a relatively gradual decrease while the base diameter exhibited minor changes, suggesting minor changes in solution spreading on the soil surface. Additional observation and interpretation on infiltration characteristics of aqueous surfactants solution will be presented and their implications for enhanced infiltration rate in hydrophobic soils will be discussed.