Isotope effect reveals non-cooperative water dynamics in salt solutions

May 28, 2018, University of Amsterdam

Water molecules surrounding ions behave in a much less cooperative way than they do in bulk water. This follows from a study on the isotope-dependent dielectric response of salt solutions, which has just been published in Physical Review Letters by researchers from the Amsterdam research institutes HIMS and AMOLF. Their results lead to an update of Nobel-laureate Onsager's 40-year old theory for the response of salt solutions to electric fields, and enables a reliable determination of hydration numbers that play a key role in chemistry and biophysics.

Solutions of ions in water are ubiquitous in physics, chemistry and biology, and the complicated way in which ions influence the hydrogen-bond network of water has been the subject of intensive experimental and computational research. Ions in interact with the water surrounding them, thereby reducing the dielectric polarizability of an ionic solution as compared to that of neat water. This reduction arises from two effects: the rotational immobilization of water molecules directly binding to the ions (static contribution), and the long-range dynamical response of water molecules to the moving ions (kinetic contribution).

The separation of these two contributions to the reduction of the dielectric polarizability has always been a challenge in research on ionic solutions. Currently, the standard approach is to use the theoretical expression for the kinetic contribution derived by Onsager and his co-worker Hubbard, which was published in a paper in 1977 that has become a classic in the field.

Putting the model to the test

Researchers from the University of Amsterdam's Van 't Hoff Institute for Molecular Sciences and the Amsterdam-based AMOLF research institute have now developed an experimental method to determine the kinetic contribution. In this method, they compare the dielectric polarizability of a solution of ions in ordinary water (H2O) with the response of the same ions dissolved in heavy water (D2O). As the static contribution to the reduction of the polarizability will be the same in H2O and D2O, this comparison provides direct information on the kinetic contribution. In this way they could test Hubbard and Onsager's model, and found that the experimentally observed kinetic contribution is much less than theoretically predicted. The researchers explain the discrepancy from a reduced cooperativity of the motion of water molecules surrounding ions—in contrast to the theory that assumes a uniform degree of cooperativity for all molecules, independent of their surroundings.

The team proposes a simple modification of Hubbard and Onsager's theory to take the locally reduced cooperativity into account. This modification makes it possible to determine ionic hydration numbers in a reliable manner, and avoids the unphysical (negative!) hydration numbers that are sometimes obtained when using the original Hubbard-Onsager theory. The results will thus be of practical use, since hydration numbers are commonly used in chemistry and biophysics to characterize salt solutions.

Explore further: Straightforward technique allows for accurate computer simulations of calcium signaling

More information: Roberto Cota et al. Evidence for Reduced Hydrogen-Bond Cooperativity in Ionic Solvation Shells from Isotope-Dependent Dielectric Relaxation, Physical Review Letters (2018). DOI: 10.1103/PhysRevLett.120.216001

Related Stories

Study of salts in water causing stir

February 2, 2018

New insight into science that seems, on its surface, exceedingly simple—what happens when you add salt to water—could ultimately lead to a better understanding of biochemical processes in cells and perhaps advance sources ...

Giant charge reversal observed for the first time

July 12, 2017

Charged surfaces submerged in an electrolyte solution can sometimes become oppositely charged. This nonintuitive phenomenon, known as charge inversion, happens when excess counter ions adsorb, or adhere, to the surface. It ...

Recommended for you

The secret to measuring the energy of an antineutrino

June 18, 2018

Scientists study tiny particles called neutrinos to learn about how our universe evolved. These particles, well-known for being tough to detect, could tell the story of how matter won out over antimatter a fraction of a second ...

Quantum transfer at the push of a button

June 15, 2018

In new quantum information technologies, fragile quantum states have to be transferred between distant quantum bits. Researchers at ETH have now realized such a quantum transmission between two solid-state qubits at the push ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.