Scientists Show Strontium's Swimming Skills

Oct 27, 2009
Snapshot from the simulations with the polarizable potential. Water molecules are in blue. The Cl- ions are in green and the corresponding Sr2+ ions are in red.

(PhysOrg.com) -- Recently, a trio from Pacific Northwest National Laboratory and Louisiana Tech University showed that strontium ions congregate on water's surface. Their computer simulation and careful calculations finally demonstrated why experiments and conventional wisdom clashed about the behavior of this type of ion, a divalent cation or one with two electrons missing.

Understanding ions behavior where air and water meet is vital to predicting and eventually controlling , whether those reactions form clouds or produce fuels. However, this understanding has to be accurate. When measurements from experiments do not agree with the current understanding, well, that's where theoretical chemists with good modeling skills come in.

For years, the conventional wisdom was that cations fled from the surface of water or the aqueous interface. But, experiments with x-ray relectivity contradicted this view. So, PNNL postdoctoral fellow Dr. Xiuquan Sun, Dr. Liem Dang, also of PNNL, and Dr. Collin Wick of Louisiana Tech University decided to see if they could figure out what was happening.

They chose to study ions, which come from strontium chloride (SrCl2) dissolved in water. Strontium is a divalent cation, with two positive charges. It behaves differently than sodium or other ions with only a single positive charge.

For the simulation, they used a molecular dynamics approach. This approach is a computer simulation that allows the tiny, fast-moving ions and water molecules to interact based on the laws of physics. The investigators record these interactions, which happen within a split second. Then, they can calculate the density of the electrons at the surface and the structures that are forming there and see how close the calculations come to what was measured during the x-ray reflectivity experiments.

One parameter the team checked was if the model's representation of the shape of the electron cloud around the ion mattered. Electrons are the fast-moving negative particles that zoom around an ion.

In one series of simulations, the researchers considered that the electron cloud might not be a perfect sphere around the ion and solvents. Other ions and molecules might distort the cloud. This was called the polarizable potential model. In another series of simulations, they did not include the electron cloud around the ion and solvent. This was the nonpolarizable potential model.

The team found that the polarizable approach provided better results, more closely matching the data from the x-ray reflectivity experiments. So, using the polarizable model, they calculated where the electrons congregated and the resulting structures created on the surface of the water. They found that the strontium ions strongly prefer to congregate on the surface, just as the experiments showed.

However, the results from the approach were far from perfect. At low concentrations of strontium , the researchers find good agreement with the experiments, but at higher concentrations, the molecular models overestimate the surface concentration by around a factor of 2. So, the team is refining the models to get better agreement at higher concentrations.

More information: Sun X, CD Wick, and LX Dang. 2009. "Computational Studies of Aqueous Interfaces of SrCl2 Salt Solutions." Journal of Physical Chemistry B. DOI: 10.1021/jp9079525

Provided by Pacific Northwest National Laboratory (news : web)

Explore further: World's fastest manufacture of battery electrodes

add to favorites email to friend print save as pdf

Related Stories

Hot and Cold Moves of Cyanide and Water

Sep 08, 2009

(PhysOrg.com) -- Scientists have long known that molecules dance about as the temperature rises, but now researchers know the exact steps that water takes with a certain molecule. Results with small, electrically ...

Hot and cold moves of cyanide and water

Sep 03, 2009

Scientists have long known that molecules dance about as the temperature rises, but now researchers know the exact steps that water takes with a certain molecule. Results with small, electrically charged cyanide ...

Artificial Cells

Nov 10, 2005

Do cells always have to be developed from organic carbon-containing compounds? When resourceful scientists stretch their imaginations, they quickly find an answer to this question. This is demonstrated by the work of Achim ...

Chemists probe combustion process

Jan 04, 2006

Chemists at the U.S. Department of Energy's Brookhaven National Laboratory, working with colleagues at Stony Brook University, have developed a unique experimental technique to measure the flow of energy inside a molecule ...

Recommended for you

World's fastest manufacture of battery electrodes

13 minutes ago

New world record: Scientists at the Karlsruhe Institute of Technology (KIT) increased the manufacturing speed of electrode foils coated batch-wise by a factor of three – to 100 meters per minute. This was ...

Waste, an alternative source of energy to petroleum

14 minutes ago

The group led by Martín Olazar, researcher in the UPV/EHU-University of the Basque Country's Department of Chemical Engineering, is studying the development of sustainable refineries where it is possible ...

Researchers developing new thermal interface materials

55 minutes ago

In the microelectronics world, the military and private sectors alike need solutions to technologic challenges. Dr. Mustafa Akbulut, assistant professor of chemical engineering, and two students lead a project ...

New insights on carbonic acid in water

15 hours ago

Though it garners few public headlines, carbonic acid, the hydrated form of carbon dioxide, is critical to both the health of the atmosphere and the human body. However, because it exists for only a fraction ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

Graeme
not rated yet Oct 29, 2009
So does that mean that divalent ions can be concentrated by skimming the surface of the solution? Others in sea water would be calcium and magnesium.