Hydronium Behavior at the Air–Water Interface with a Polarizable Multistate Empirical Valence Bond Model

Molecular dynamics simulations were carried out to understand the propensity of the hydronium ion for the air–water interface with a polarizable multistate empirical valence bond (MS-EVB) model. Reasonable agreement with experiment for radial distribution functions and very good agreement for hydronium diffusion were found for the model. The polarizable MS-EVB model had no free energy minimum at the air–water interface. However, when polarizability on the hydronium ion alone was removed, a free energy of around −1.5 kcal/mol was calculated at the air–water interface. This discrepancy was found to be due to the behavior of water molecules in the first solvation shell of a hydronium ion. These water molecules contained a moderate amount of hydronium character, resulting in the delocalization of the hydronium ion. For the system with polarizable hydronium ions, this delocalization was the same at the interface as in the bulk, but for the system without polarizable hydronium ions, the delocalization increased as the hydronium approached the air–water interface. This delocalization results in a stabilization of the hydronium charge and moves it more toward the bulk, increasing its propensity for the air–water interface when hydronium ion polarizability is removed

Computational Observation of Pockets of Enhanced Water Concentration at the 1‑Octanol/Water Interface

Molecular dynamics simulations with polarizable potentials were carried out to investigate the 1-octanol–water interface in which a significant amount of water migrated into the 1-octanol phase. A region of enhanced water concentration, around three times the average concentration in water saturated 1-octanol, was present 18 Å from the Gibbs dividing surface into the 1-octanol phase. This coincided with two layers of 1-octanol molecules, forming a somewhat ordered bilayer with the first layer having its hydroxyl group pointed toward the water phase. The second layer of 1-octanol had hydroxy groups pointed in the opposite direction on average. A consequence of this was a region of high alkyl concentration and reduced polarity, as has been previously observed. Water structure in the octanol phase contracted as it approached the 1-octanol phase, opposite what was observed at the <i>n</i>-octane–water interface with polarizable potentials. In contrast, 1-octanol hydroxy structure expanded as it came in contact with water

Molecular Mechanisms for the Lithiation of Ruthenium Oxide Nanoplates as Lithium-Ion Battery Anode Materials: An Experimentally Motivated Computational Study

First-principles computational studies were used to calculate discharge curves for lithium in RuO₂ and to understand the molecular mechanism of lithium sorption into crystalline bulk RuO₂. These studies were complemented by experiments to provide new insights into the molecular mechanisms for the first and subsequent discharges of RuO₂ anodes in lithium ion batteries. RuO₂ nanoplates show slow fading of capacity over multiple cycles, retaining 76% of their original capacity after 20 cycles. The calculated discharge curves for lithium in RuO₂ lattice show qualitative agreement with experimental discharge curves for RuO₂ nanoplates. The molecular level analysis shows that an intercalation mechanism is operational until a 1:1 Li:Ru ratio is reached, which is followed by a conversion mechanism into Ru metal and Li₂O. Furthermore, in agreement with experiment, the computations predict superstoichiometric capacity of RuO₂, i.e., accommodation of lithium well beyond the stoichiometric limit of 4:1 Li:Ru ratio, and show that the additional lithium atoms reside at the interface of the Ru metal and Li₂O. This shows that the extra capacity can be explained without invoking electrolyte or solvent–electrolyte interface effects

Transferable Potentials for Phase Equilibria–United Atom Description of Five- and Six-Membered Cyclic Alkanes and Ethers

While the transferable potentials for phase equilibria–united atom (TraPPE–UA) force field has generally been successful at providing parameters that are highly transferable between different molecules, the polarity and polarizability of a given functional group can be significantly perturbed in small cyclic structures, which limits the transferability of parameters obtained for linear molecules. This has motivated us to develop a version of the TraPPE–UA force field specifically for five- and six-membered cyclic alkanes and ethers. The Lennard-Jones parameters for the methylene group obtained from cyclic alkanes are transferred to the ethers for each ring size, and those for the oxygen atom are common to all compounds for a given ring size. However, the partial charges are molecule specific and parametrized using liquid-phase dielectric constants. This model yields accurate saturated liquid densities and vapor pressures, critical temperatures and densities, normal boiling points, heat capacities, and isothermal compressibilities for the following molecules: cyclopentane, tetrahydrofuran, 1,3-dioxolane, cyclohexane, oxane, 1,4-dioxane, 1,3-dioxane, and 1,3,5-trioxane. The azeotropic behavior and separation factor for the binary mixtures of 1,3-dioxolane/cyclohexane and ethanol/1,4-dioxane are qualitively reproduced

Transferable Potentials for Phase Equilibria–United Atom Description of Five- and Six-Membered Cyclic Alkanes and Ethers

While the transferable potentials for phase equilibria–united atom (TraPPE–UA) force field has generally been successful at providing parameters that are highly transferable between different molecules, the polarity and polarizability of a given functional group can be significantly perturbed in small cyclic structures, which limits the transferability of parameters obtained for linear molecules. This has motivated us to develop a version of the TraPPE–UA force field specifically for five- and six-membered cyclic alkanes and ethers. The Lennard-Jones parameters for the methylene group obtained from cyclic alkanes are transferred to the ethers for each ring size, and those for the oxygen atom are common to all compounds for a given ring size. However, the partial charges are molecule specific and parametrized using liquid-phase dielectric constants. This model yields accurate saturated liquid densities and vapor pressures, critical temperatures and densities, normal boiling points, heat capacities, and isothermal compressibilities for the following molecules: cyclopentane, tetrahydrofuran, 1,3-dioxolane, cyclohexane, oxane, 1,4-dioxane, 1,3-dioxane, and 1,3,5-trioxane. The azeotropic behavior and separation factor for the binary mixtures of 1,3-dioxolane/cyclohexane and ethanol/1,4-dioxane are qualitively reproduced