We had three papers published this week. The first was by Hao Wu and is entitled "Water Solubility and Dynamics of CO2 Capture Ionic Liquids Having Aprotic Heterocyclic Anions". It appears in Fluid Phase Equilibria, 368, 72-79 (2014). In this paper, Hao studied the solubility and dynamics of water with five different ionic liquids having a common tetrabutylphosphonium cation paired with different aprotic heterocyclic anions capable of reacting with CO2. He showed that the 2-cyanopyrrolide anion is the most hydrophobic of all those studied but that it becomes quite hydrophilic upon reaction with CO2. This changes the local structure about the anion as well as the dynamics of the system.
In the same issue of Fluid Phase Equilibria, Cassiano Aimoli published a paper entitled "Force Field Comparison and Thermodynamic Property Calculation of Supercritical CO2 and CH4 Using Molecular Simulation" (C. Aimoli, E. J. Maginn and C. R. A. Abreu, Fluid Phase Equilibria, 368, 80-90 (2014)). Seven different carbon dioxide force fields and two different methane force fields were evaluated for their ability to reproduce thermodynamic properties of the pure fluids over a wide range of temperatures and pressures. Those showing better accuracy when compared to experimental PVT results were used to calculate the volume expansivity, isothermal compressibility, isobaric and isochoric heat capacities, Joule–Thomson coefficient, and speed of sound. It was found that for CO2 the model of Zhang and Duan (J. Chem. Phys., 122, 214507 (2005)) overall performed the best, followed closely by the TraPPE model of Potoff and Siepmann (AIChE J., 37, 1676-1682 (2001)). The single-site "SAFT-gamma" model of Muller and co-workers (J. Phys. Chem. B, 115, 11154-11169 (2011)) did remarkably well, especially considering its simple form and the speed with which calculations can be performed. The models used for methane all performed about the same. A multi-state Bennet Acceptance Ratio (MBAR) method was used to improve the accuracy of the results and reduce computational time. You can view an audio slide presentation of the paper by following this link.
Finally, Surya Tiwari and Neeraj Rai published their paper entitled "Dynamics of Actinyl Ions in Water: A Molecular Dynamics Study" in Phys. Chem. Chem. Phys. In this work, they tested the ability of newly-developed potentials for actinyl ions obtained from high level quantum calculations to model the structural and dynamic properties of these ions in water. The systems investigated are AnO2n+ ions (n=1, 2 and An=U, Np, Pu and Am). Our simulations suggest that there are two distinct water exchange mechanisms for mono and dications. An associative interchange pathway is observed for water exchange involving dication actinyls, while in monocation actinyls the exchange occurs via a dissociative mechanism. The residence time of water molecules in the ﬁrst solvation shell depends on the water exchange mechanism. In the case of dications, a stiffer actinyl bond angle results in a longer residence time, while for monocations, a shorter water coordination distance leads to a longer residence time. The simulations predict much faster water exchange for UO2+2 than what is observed experimentally with NMR, but other properties are consistent with experiments.