Charged Surface-Active Impurities at Nanomolar Concentration Induce Jones–Ray Effect

Abstract

The electrolyte surface tension exhibits a characteristic minimum around a salt concentration of 1 mM for all ion types, known as the Jones-Ray effect. We show that a consistent description of the experimental surface tension of salts, bases, and acids is possible by assuming charged impurities in the water with a surface affinity typical for surfactants. Comparison with experimental data yields an impurity concentration in the nanomolar range, well below the typical experimental detection limit. Our modeling reveals salt-screening enhanced impurity adsorption as the mechanism behind the Jones-Ray effect: for very low salt concentration added salt screens the electrostatic repulsion between impurities at the surface, which dramatically increases impurity adsorption and thereby reduces the surface tension.

Metal Ion Interactions with Crude Oil Components: Specificity of Ca2+ Binding to Naphthenic Acid at an Oil/Water Interface

“…Thus, Figure 2d shows that the interfacial tension reduction for <0.1 mol/L Ca 2+ is~1.4 mN/m, similar to values obtained elsewhere [60]. Suggested reasons for this effect have been debated, and have included cation [60], anion [62][63][64] or impurity [65] adsorption, or electrostatic effects influencing water structure at the interface [66,67]. The curve fitted through the experimental data in Figure 2d is of the form γ = γ 0 − Aγ 0.5 + Bγ, where A and B are empirical constants (0.158 (mN/m) 0.5 and 0.217, respectively) and γ 0 is the interfacial tension of the n-heptane/water interface (48.4 mN/m).…”

Intentionally Added Ionic Surfactants Induce Jones-Ray Effect at Air-Water Interface

“…For the DTAB concentration at c b DTAB = 10 µM, the minimum is distinct and located at approximately c b NaCl = 100 mM. By using the mean-field theory for the surface tension of the air-electrolyte interface [22], we quantitatively explain the experimental data. This theory is based on the Poisson-Boltzmann equation with the ion-specific adsorption potentials such as…”

“…This scenario is supported by the negative zeta potential of bubbles in water [21] and the repulsive disjoining pressure of wetting films on silica [1]. However, this scenario contradicts with the surface tension of NaOH solutions because the surface tension of NaOH solutions is larger than that of NaCl solutions, meaning Email address: uematsu@chem.kyushu-univ.jp (Yuki Uematsu) that OH − is more surface-inactive than Cl − [2,3,22]. Although other explanations were suggested [23,24], the experimental fact that the concentration and depth of the minima depend on independent measurements [6][7][8][9][10][11][12][13] implies uncontrolled hidden parameters in the experiments.…”

“…Although other explanations were suggested [23,24], the experimental fact that the concentration and depth of the minima depend on independent measurements [6][7][8][9][10][11][12][13] implies uncontrolled hidden parameters in the experiments. Recently, one of the authors constructed the theory of the surface tension of electrolytes, and the Jones-Ray effect is explained by the traces of charged impurities in the water [22,25]. In this scenario, assuming that the charged impurities have an affinity similar to typical ionic surfactants, namely sodium dodecylsulfate, the sufficient concentration of the impurities is a few nanomolar, which cannot be detected by conductivity measurements.…”

“…However, this accuracy level requires a special apparatus, and such a tremendous precision has been difficult to achieve so far. According to the impurity scenario [22], intentionally added ionic surfactants make the surface tension minimum deeper and the concentration of the minimum larger. Although the surface tension of electrolytes in the presence of ionic surfactants has been studied to date, these studies have not focused on the Jones-Ray effect [26][27][28].…”