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Surfactant Impurities Can Explain the Jones-Ray Effect

preprint
revised on 06.02.2018 and posted on 06.02.2018 by Timothy Duignan, Marcel Baer, Christopher Mundy

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.

Funding

Department of Energy, Pacific Northwest National Laboratory

History

Email Address of Submitting Author

timothy.duignan@pnnl.gov

Email Address(es) for Other Author(s)

marcel.baer@pnnl.gov Chris.mundy@pnnl.gov

Institution

Pacific Northwest National Laboratory

Country

United States of America

ORCID For Submitting Author

0000-0003-3772-8057

Declaration of Conflict of Interest

None

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in The Journal of Chemical Physics

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