Space and Physics

A Single Ion Can Have A Dramatic Effect On Water


Stephen Luntz

Freelance Writer

clockApr 11 2016, 19:50 UTC
915 A Single Ion Can Have A Dramatic Effect On Water
Ions can “twist” and “stiffen” water molecules, and can do so to hundreds of thousands more molecules than shown here. LPB/EPFL

For something so ubiquitous and so essential, it is amazing how much we don't know about water. The latest discovery is that a single ion, for example of sodium or chlorine from table salt, can alter the bonds of a million water molecules – 10,000 times more than previously suspected.


Water sounds simple – one oxygen atom and two hydrogen atoms time-share electrons so that the oxygen end of the molecule becomes negative and the two hydrogens positive. In the presence of a positively or negatively charged ion, however, things get more complicated.

The ion's influence perturbs the hydrogen bonds in a process lead author Dr. Sylvie Roke of the Ecole Polytechnique Federale de Lausanne calls “twisting,” making molecules “stiffer.”

This effect was already known, but not the reach. “Until now it was not possible to see beyond a hundred molecules,” Roke said in a statement. This covered a sphere three molecules in radius. “Our measurements show that water is much more sensitive to ions than we thought."

In Science Advances, Roke reports that “electrolytes induce orientational order” at concentrations as low as one ten-thousandth of a mole per liter. This is surprising since, as the paper notes, “The electrostatic potential around an ion decays with 1/R (with R being the distance away from the ion) and is damped by the high dielectric constant of water and the presence of other ions.”


This effect was measured in three ways: The surface tension of pure water was compared to that of an extremely dilute electrolyte solution, ultrafast optical measurements were conducted to explore the arrangement of water molecules, and interactions were modeled by computer.

To measure surface tension, Roke said, “we simply dipped a thin metal plate into the water and pulled gently using a tensiometer to determine the water's resistance. We observed that the presence of a few ions makes it easier to pull the plate out.”

The research has resolved an old question. “This strange effect had already been observed in 1941, but it remained unexplained until now,” Roke said. “Through our multiscale analysis we were able to link it to ion-induced stiffening of the bulk hydrogen bond network: a stiffer bulk results in a comparatively more flexible surface."


The effect's strength is determined by the water molecules, not the intruders. Roke's team dissolved 21 different salts, including examples without common elements such as magnesium sulphate (MgSO4) and calcium chloride (CaCl2). Responses didn't vary.

On the other hand, a comparison was made between light water containing hydrogen atoms without neutrons and heavy water, which incorporates deuterium, whose nucleus has a proton and neutron. Heavy water required six times as many ions as light water to produce the same perturbation.

It needs to be said, Roke stressed, "Our research has nothing to do with water memory or homeopathy. To prove the role of water in homeopathy, another million-billion-billion water molecules would have to be affected to even come close.”

Space and Physics
  • surface tension,

  • ions,

  • water molecules,

  • hydrogen bonds