Equilibrium interfacial tension and the influence of extreme dilutions of uni-univalent salts: An expression of the “Jones–Ray effect”

There are inevitable low levels of salt content in qualified waters which can alter accurate estimations of water interfacial tension with organic immiscible liquids. In this study, the equilibrium interfacial tension variation of extremely diluted (0.001 mmol · dm−3 to 0.050 mmol · dm−3) aqueous solutions of NaCl, KCl and KI, and toluene at different temperatures of (20.0 to 50.0) °C was studied. Measurements were performed by the drop-weight method. Interfacial tension becomes reduced by (0.41 to 0.86) mN · m−1 at the highest salt concentrations used and different temperatures. A linear decrease in this parameter was also observed with increasing temperature. The order of this salt effect on the interfacial tension follows: KI > KCl ≈ NaCl at all temperatures except 50.0 °C where KCl has a stronger effect than NaCl. Assuming that inorganic ions do not penetrate the non-polar organic phase, the Petersen–Saykally model for surface tension which predicts the descending trend of variations and is based on the well-known “Jones–Ray effect” was used for modelling the data provided. Good agreement was found with three adjustable parameters for each salt and temperature.

► Extremely diluted uni-univalent salts cause a reduction in toluene–water interfacial tension.
► Adsorption of anions like Cl− and I− is important for the interfacial tension variations.
► At temperatures less than 50.0 °C relative effects follow the order: KI > KCl ≈ NaCl.
► The data can be nicely explained by the “Jones–Ray effect” model of Petersen–Saykally.
► The ions adsorption parameters show consistent values at different temperatures.