Direct experimental validation of the Jones–Ray effect

In 1937, Jones and Ray observed a minimum in the surface tension of electrolyte solutions at ∼ 1 mM concentrations. According to the Gibbs adsorption equation, a decreasing surface tension corresponds to an enhanced ion concentration at the interface, which is contrary to textbook descriptions based on Onsanger–Samaras theory. The ‘Jones–Ray Effect’ has since been essentially dismissed as an artifact of the indirect experimental method used, and remains today as a curiosity. Here, we present direct experimental confirmation of the enhanced anion concentration around 1 mM of alkali iodide solutions using resonance enhanced femtosecond second harmonic generation, and the extraction of a large and negative value for the surface excess free energy (−6.2 ± 0.2 kcal/mol).