Soil-water contact angle as affected by the aqueous electrolyte concentration

Electrolyte concentration changes the surface tension of the water-air interface, which is expected to affect the solid-water contact angle. In the present study, we focused on examining the effects of aqueous electrolyte concentration on the contact angle of soil samples with different hydrophobicities using sodium chloride (NaCl) and calcium chloride (CaCl ₂) solutions. Japanese Andisol and silica sand were hydrophobized using stearic acid to obtain different hydrophobicities. The contact angle of all the samples increased with increasing electrolyte concentration. This was attributed to the increasing surface tension of the solutions. The response of contact angle to the electrolyte concentration was positively correlated with soil surface free energy fitting into a linear relationship in Andisol, but not in silica sand. Surface tension of electrolyte solutions increases linearly with electrolyte concentration up to high salinities, and the contact angle can be expected to increase with increasing electrolyte concentration in a corresponding pattern. The relationship observed in the present study was non-linear, where more prominent increase of contact angle was observed at low electrolyte concentrations. The response of contact angle to the increasing electrolyte concentration was almost negligible at concentrations higher than 0.06 mol L ⁻¹, [ionic strength (I): 0.06 mol L ⁻¹] for NaCl, and 0.1 mol L ⁻¹, (I : 0.3 mol L ⁻¹) for CaCl ₂ solutions. The lowered solid-liquid interfacial free energy and its effects on contact angle with increasing adsorption of ions at the interface was considered to be the reason for decreasing slopes of the curves. Initial increase in contact angle with increasing ionic strength of CaCl ₂ was lower than that of NaCl. This might be because the adsorption of ions on soil surfaces, and the consequent effects on lowering the free energy of the solid-liquid interface and the contact angle, are more pronounced for calcium ions (Ca ²⁺) than for sodium ions (Na ⁺).