  The Jones-Ray effect is an anomalous minimum in the surface tension of
aqueous electrolytes at millimolar salt concentrations. We experimentally
demonstrated that intentionally added ionic surfactants induce the Jones-Ray
effect. The one-dimensional Poisson-Boltzmann theory, including the effect of
surfactant adsorption and salt depletion, excellently agrees with the obtained
experimental data. All the parameters of the ion-specific surface affinities
used in the theory are consistent with previous experiments. These results
strongly suggest that the Jones-Ray effect observed so far has been induced by
the inevitable contamination of the air-water interfaces.
