Entropy scaling of viscosity for molecular models of molten salts.

Entropy scaling relates dynamic and thermodynamic properties by reducing the viscosity to a function of only the residual entropy. Molecular simulations are used to investigate the entropy scaling of the viscosity of three models of sodium chloride and five monovalent salts. Even though the correlation between the potential energy and the virial is weak, entropy scaling applies at liquid densities for all models and salts investigated. At lower densities, entropy scaling breaks down due to the formation of ion pairs and chains. Entropy scaling can be used to develop more extendable correlations for the dynamic properties of molten salts.

Molecular simulation of liquid-vapor coexistence for NaCl: Full-charge vs scaled-charge interaction models.

Scaled-charge models have been recently introduced for molecular simulations of electrolyte solutions and molten salts to attempt to implicitly represent polarizability. Although these models have been found to accurately predict electrolyte solution dynamic properties, they have not been tested for coexistence properties, such as the vapor pressure of the melt. In this work, we evaluate the vapor pressure of a scaled-charge sodium chloride (NaCl) force field and compare the results against experiments and a non-polarizable full-charge force field. The scaled-charge force field predicts a higher vapor pressure than found in experiments, due to its overprediction of the liquid-phase chemical potential. Reanalyzing the trajectories generated from the scaled-charge model with full charges improves the estimation of the liquid-phase chemical potential but not the vapor pressure.

Genetic Algorithm Driven Force Field Parameterization for Molten Alkali-Metal Carbonate and Hydroxide Salts.

Molten alkali-metal carbonates and hydroxides play important roles in the molten carbonate fuel cell and in Earth's geochemistry. Molecular simulations allow us to study these systems at extreme conditions without the need for difficult experimentation. Using a genetic algorithm to fit ab intio molecular dynamics-computed densities and radial distribution functions, as well as experimental enthalpies of formation, we derive new classical force fields able to accurately predict liquid chemical potentials. These fitting properties were chosen to ensure accurate liquid phase structure and energetics. Although the predicted dynamics is slow when compared to experiments, in general the trends in dynamic properties across different systems still hold true. In addition, these newly parametrized force fields can be extended to the molten carbonate-hydroxide mixtures by using standard combining rules.

System-Size Dependence of Electrolyte Activity Coefficients in Molecular Simulations.

A systematic study of the dependence of electrolyte activity coefficients on simulation system size has been undertaken. Using implicit-solvent simulations for which calculations with low statistical uncertainty are feasible, it was found that the chemical potential for a NaCl model depends strongly on simulation system size at concentrations up to about 0.3 mol/L; system-size effects at higher concentrations are much smaller. Similar trends were confirmed in systems with an explicit solvent. System-size effects on the chemical potential, when uncorrected, can lead to systematic errors in the activity coefficient greater than 10%. The rigorous method to correct for such system-size effects is to perform multiple simulations at each concentration and extrapolate to infinite system size. Unfortunately, this becomes impractical for explicit-solvent simulations at low concentrations, because of computational limitations that lead to large statistical uncertainties in the results. Somewhat counterintuitively, we find that lower systematic errors for the Henry's law reference chemical potential are obtained by using simulations at higher concentrations, for which system-size effects are much smaller, to obtain estimates for the reference chemical potential. This is the case even though at these higher concentrations deviations from the Debye-Hückel limiting law (or its empirical extensions) are greater than those at lower concentrations.

Bacterial communication ("quorum sensing") via ligands and receptors: a novel pharmacologic target for the design of antibiotic drugs.

The purpose of the present Perspectives is to present a synopsis of the literature on bacterial "quorum sensing" as a background for the proposal that interference with this communication system offers potential targets for the design of novel antibiotic drugs. Quorum sensing is the recently discovered chemical communication system among bacteria (both Gram-positive and -negative). It is vital for intra- and interbacterial gene regulation and for keeping bacterial colonies ("biofilms") intact, allowing resident bacteria to assume specialized roles that contribute to enhanced survival of the group. There are several processes involved in quorum sensing that are familiar to pharmacologists; i.e., specific signaling molecules bind to and activate receptors that transduce the quorum-sensing signal into intracellular second messenger responses. We highlight herein the similarity between quorum-sensing communication to ligand-receptor interactions, suggesting that inhibitor drugs could be designed using current standard pharmacologic principles. Such drugs would have novel mechanisms of action and might therefore be more effective against antibiotic-resistant strains of bacteria.