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<p>The surface tension of dilute salt water is a fundamental property that is crucial to understanding the complexity of many aqueous phase processes. Small ions are known
to be repelled from the air-water surface leading to an increase in the surface tension
in accordance with the Gibbs adsorption isotherm. The Jones-Ray effect refers to
the observation that at extremely low salt concentration the surface tension decreases
in apparent contradiction with thermodynamics. Determining the mechanism that
is responsible for this Jones-Ray effect is important for theoretically predicting the
distribution of ions near surfaces. Here we show that this surface tension decrease
can be explained by surfactant impurities in water that create a substantial negative
electrostatic potential at the air-water interface. This potential strongly attracts positive cations in water to the interface lowering the surface tension and thus explaining
the signature of the Jones-Ray effect. At higher salt concentrations, this electrostatic
potential is screened by the added salt reducing the magnitude of this effect. The
effect of surface curvature on this behavior is also examined and the implications
for unexplained bubble phenomena is discussed. This work suggests that the purity
standards for water may be inadequate and that the interactions between ions with
background impurities are important to incorporate into our understanding of the
driving forces that give rise to the speciation of ions at interfaces. </p>
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