Direct experimental validation of the Jones-Ray effect

In 1937, Jones and Ray observed a minimum in the surface tension of electrolyte solutions at similar to1 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). (C) 2004 Elsevier B.V. All rights reserved.