Characterizing time-dependent contact angles for sands hydrophobized with oleic and stearic acids

Capillary barriers (CBs) represent useful, low-cost systems for limiting water infiltration and controlling seepage at solid waste landfills in semiarid and arid regions. The application of CBs in wet regions can be problematic due to loss of water-impermeable properties under high-frequency precipitation. A potential solution is to alter soil grain surfaces to become water repellent by mixing or coating the soil cover material with hydrophobic agents (HAs). In this study, hydrophobic CBs comprised of sands mixed with environmentally friendly HAs (oleic acid [OA] and stearic acid [SA]) were studied. Water repellency (WR) characteris- tics for hydrophobized sand samples with different HA contents and representing different coating methods (mixing in and solvent aided) were measured. Initial contact angles (α _i) for OA-coated samples sharply increased with increasing HA content and reached peak values of 97 to 101° at 0.75 to 1.0 g HA kg ^-1 sand, whereatier α _i gradually decreased. Measured α _i values for SA-coated samples increased sharply to 90° and then gradually reached a maximum of 108° at 6.0 g kg ^-1 HA content. Each test sample exhibited a decrease in contact angle (a) with time (t) and reached an apparent equilibrium atier around 1200 s. The time dependence of a was expressed by an exponential function, α = α _i exp (-At), where A is the coefficient of temporal change in WR (s ^-1). While the A values for the solvent-aided OA-coated samples were relatively constant (between 3 × 10 ^-4 and 6 × 10 ^-4 s ^-1), A values for the mixing-in OA-coated samples fluctuated. Generally, the solvent-aided coating method yielded less time dependency of a and higher WR persistence.