We carry out Monte Carlo and molecular dynamics simulations of actinides (U, Pu, Np, Am) to better understand their thermodynamic and transport properties in solutions. In recent work, we developed a simple two-body potential for the interaction of uranyl ions (UO22+) in water. The figure below shows potential energy curves for various configurations of water with the uranyl ion.
The points are MP2 calculations and the lines are fits to a simple Lennard-Jones + Coulomb potential. We find that this simple potential does a remarkably good job matching energies obtained form the quantum calculations. Using this potential, we computed solvation free energies, dynanics and the structure of water about the uranyl ion. The spatial distribution functions shown below show how water organizes in the first and second solvation shells around the uranyl ion for four different water models. This was work carried our by Dr. Neeraj Rai and Surya Tiwari.
The spatial distribution functions on the right show the most likely positions of water molecules in the first, meso, and second solvation shells of uranyl ion. Subfigure labels 1, 2, and 3 show the front, front section, and top section views, respectively, of SDFs while labels a, b, c, and d show results for SPC/Fw, TIP3P, TIP4P, and TIP5P water models, respectively. The cyan, maroon, and purple surfaces show the most likely position water molecules in the first, meso, and second solvation shells, respectively. The white dashed regular pentagons are guides to the eye.
For more information, see: Neeraj Rai, Surya Prakash Tiwari and Edward J. Maginn, “Force Field Development for Actinyl Ions via Quantum Mechanical Calculations: An Approach to Account for Many Body Solvation Effects”, Journal of Physical Chemistry B, 2012, 116, 10885-10897. http://pubs.acs.org/doi/abs/10.1021/jp3028275
Once a force field is in hand, the thermodynamic and transport properties of the system can be interrogated. Below is a movie showing the exchange mechanism of water molecules around an uranyl ion. (Only the neighboring water molecules are shown for clarity).
This material is based upon work supported as part of the Materials Science of Actinides, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Of- fice of Basic Energy Sciences under Award Number DE-SC0001089. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U. S. Department of Energy under Contract No. DE-AC02-05CH11231.