The Jones-Ray Effect Is Not Caused by Surface-Active Impurities

Pure aqueous electrolyte solutions display a minimum in surface tension at concentrations of 2 ± 1 mM. This effect has been a source of controversy since it was first reported by Jones and Ray in the 1930s. The Jones-Ray effect has frequently been dismissed as an artifact linked to the presence of surface-active impurities. Herein we systematically consider the effect of surface-active impurities by purposely adding nanomolar concentrations of surfactants to dilute electrolyte solutions. Trace amounts of surfactant are indeed found to decrease the surface tension and influence the surface chemistry. However, surfactants can be removed by repeated aspiration and stirring cycles, which eventually deplete the surfactant from solution, creating a pristine surface. Upon following this cleaning procedure, a reduction in the surface tension by millimolar concentrations of salt is still observed. Consequently, we demonstrate that the Jones-Ray effect is not caused by surface-active impurities.