We propose a general approach for determining the entropy and free energy of complex systems as a function of temperature and pressure. In this method the Fourier transform of the velocity autocorrelation function, obtained from a short ͑20 ps͒ molecular dynamics trajectory is used to obtain the vibrational density of states ͑DoS͒ which is then used to calculate the thermodynamic properties by applying quantum statistics assuming each mode is a harmonic oscillator. This approach is quite accurate for solids, but leads to significant errors for liquids where the DoS at zero frequency, S(0), remains finite. We show that this problem can be resolved for liquids by using a two phase model consisting of a solid phase for which the DoS goes to zero smoothly at zero frequency, as in a Debye solid; and a gas phase ͑highly fluidic͒, described as a gas of hard spheres. The gas phase component has a DoS that decreases monotonically from S(0) and can be characterized with two parameters: S(0) and 3N g , the total number of gas phase modes ͓3N g →0 for a solid and 3N g →3(NϪ1) for temperatures and pressures for which the system is a gas͔. To validate this two phase model for the thermodynamics of liquids, we applied it to pure Lennard-Jones systems for a range of reduced temperatures from 0.9 to 1.8 and reduced densities from 0.05 to 1.10. These conditions cover the gas, liquid, crystal, metastable, and unstable states in the phase diagram. Our results compare quite well with accurate Monte Carlo calculations of the phase diagram for classical Lennard-Jones particles throughout the entire phase diagram. Thus the two-phase thermodynamics approach provides an efficient means for extracting thermodynamic properties of liquids ͑and gases and solids͒.

There is increasing evidence that cyclic and linear carbonates, commonly used solvents in Li ion battery electrolytes, are unstable in the presence of superoxide and thus are not suitable for use in rechargeable Li-air batteries employing aprotic electrolytes. A detailed understanding of related decomposition mechanisms provides an important basis for the selection and design of stable electrolyte materials. In this article, we use density functional theory calculations with a Poisson-Boltzmann continuum solvent model to investigate the reactivity of several classes of aprotic solvents in nucleophilic substitution reactions with superoxide. We find that nucleophilic attack by O(2)(•-) at the O-alkyl carbon is a common mechanism of decomposition of organic carbonates, sulfonates, aliphatic carboxylic esters, lactones, phosphinates, phosphonates, phosphates, and sulfones. In contrast, nucleophilic reactions of O(2)(•-) with phenol esters of carboxylic acids and O-alkyl fluorinated aliphatic lactones proceed via attack at the carbonyl carbon. Chemical functionalities stable against nucleophilic substitution by superoxide include N-alkyl substituted amides, lactams, nitriles, and ethers. The results establish that solvent reactivity is strongly related to the basicity of the organic anion displaced in the reaction with superoxide. Theoretical calculations are complemented by cyclic voltammetry to study the electrochemical reversibility of the O(2)/O(2)(•-) couple containing tetrabutylammonium salt and GCMS measurements to monitor solvent stability in the presence of KO(2)(•) and a Li salt. These experimental methods provide efficient means for qualitatively screening solvent stability in Li-air batteries. A clear correlation between the computational and experimental results is established. The combination of theoretical and experimental techniques provides a powerful means for identifying and designing stable solvents for rechargeable Li-air batteries.

Abstract:We introduce the Cohesive Energy Density (CED) method, a multiple sampling Molecular Dynamics computer simulation procedure that may offer higher consistency in the estimation of Hildebrand and Hansen solubility parameters. The use of a multiple sampling technique, combined with a simple but consistent molecular force field and quantum mechanically determined atomic charges, allows for the precise determination of solubility parameters in a systematic way ( ϭ 0.4 hildebrands). The CED method yields first-principles Hildebrand parameter predictions in good agreement with experiment [root-mean-square (rms) ϭ 1.1 hildebrands]. We apply the CED method to model the Caltech electronic nose, an array of 20 polymer sensors. Sensors are built with conducting leads connected through thin-film polymers loaded with carbon black. Odorant detection relies on a change in electric resistivity of the polymer film as function of the amount of swelling caused by the odorant compound. The amount of swelling depends upon the chemical composition of the polymer and the odorant molecule. The pattern is unique, and unambiguously identifies the compound. Experimentally determined changes in relative resistivity of seven polymer sensors upon exposure to 24 solvent vapors were modeled with the CED estimated Hansen solubility components. Predictions of polymer sensor responses result in Pearson R 2 coefficients between 0.82 and 0.99.

The effect of molecular architecture of a surfactant, particularly the attachment position of benzene sulfonate on the hexadecane backbone, at the decane-water interface was investigated using atomistic MD simulations. We consider a series of surfactant isomers in the family of alkyl benzene sulfonates, denoted by m-C16, indicating a benzene sulfonate group attached to the mth carbon in a hexadecane backbone. The equilibrated model systems showed a well-defined interface between the decane and water phases. We find that surfactant 4-C16 has a more compact packing, in terms of the interfacial area and molecular alignment at the interface, than other surfactants simulated in this study. Furthermore, surfactant 4-C16 leads to the most stable interface by having the lowest interface formation energy. The interfacial thickness is the largest in the case of surfactant 4-C16, with the thickness decreasing when the benzene sulfonate is located farther from the attachment position of 4-C16 (the 4th carbon). The interfacial tension profile was calculated along the direction perpendicular to the interface using the Kirkwood-Buff theory. From the comparison of the interfacial tension obtained from the interfacial tension profile, we found that surfactant 4-C16 induces the lowest interfacial tension and that the interfacial tension increases with decreasing interfacial thickness as a function of the attachment position of benzene sulfonate. Such a relationship between the interfacial thickness and interfacial tension is rationalized in terms of the miscibility of the alkyl tail of surfactant m-C16 with decane by comparing the "effective" length of the alkyl tail with the average end-to-end length of decane. Among the surfactants, the effective length of the 4-C16 alkyl tail (9.53 ( 1.36 Å) was found to be closest to that of decane (9.97 ( 1.03 Å), which is consistent with the results from the density profile and the interfacial tension profile.

There is increasing experimental evidence that organic carbonate-based electrolytes are incompatible with the discharge products of the nonaqueous lithium−air (oxygen) battery. Theoretically, the lithium−air battery offers the highest gravimetric density for energy storage applications, promising to revolutionize electric vehicle transportation. Calculations suggest that propylene carbonate, ethylene carbonate, and dimethyl carbonate, commonly used electrolytes in Li-ion batteries, are easily decomposed by the superoxide ion via nucleophilic attack at the ethereal carbon atom. In the case of propylene carbonate, base-mediated proton abstraction from the methyl group has to be considered as an additional solvent decomposition pathway. The present study provides a mechanistic understanding of solvent instability to assist the design of stable electrolytes for Li−air energy storage systems.