Advanced #73

For some salts there is a surface tension minimum -- the Jones-Ray effect

Certain salt solutions show a surface tension minimum at low concentrations.

Scientific Explanation

When salt is added to water, the surface tension normally rises monotonically with concentration — ions are repelled from the surface into the interior, and the stronger ordering at the surface increases the tension. The Jones-Ray effect describes a surprising deviation from this expected behavior: at very low salt concentrations (around 1 millimolar), the surface tension briefly dips below that of pure water before rising at higher concentrations as expected.

This minimum was first observed experimentally in 1937 by Grinnell Jones and William Ray and remained controversial for decades. Recent measurements and simulations have confirmed the effect and proposed various explanations. One hypothesis suggests that at very low concentrations, individual ions disrupt the surface layer and break up the local hydrogen bond order, slightly reducing surface tension. Only at higher concentrations does the ionic exclusion from the surface dominate, and the tension rises.

The exact explanation remains a subject of active research and touches on fundamental questions about ion distribution at interfaces.

Jones-Ray Effect: Surface Tension Minimum Line chart showing surface tension of a salt solution versus concentration. At very low salt concentrations (around 1 millimolar), the surface tension dips below that of pure water before rising at higher concentrations. This dip is the Jones-Ray effect. Surface Tension γ Salt Concentration (log scale) 0 10⁻³ M 10⁻² M 10⁻¹ M 1 M Pure water Minimum (Jones-Ray) Expected (monotonic rise) Jones-Ray Effect — Surface Tension Minimum
The Jones-Ray effect: an unexpected surface tension minimum at very low salt concentrations.

Everyday Relevance

Although the Jones-Ray effect occurs at very low concentrations, it is relevant for understanding interfacial processes in nature. In sea spray, cloud droplets, and biological membranes, salt concentrations exist where such effects can play a role. Understanding ion distribution at water surfaces is also important for atmospheric chemistry, where reactions on the surface of salt aerosols influence ozone depletion and aerosol formation.