This work reports densities, speeds of sound, and viscosities of binary mixtures of n-alkylcyclohexanes (propyl- to dodecylcyclohexane) in n-hexadecane as a function of temperature. Isentropic bulk moduli for these mixtures were calculated from these speed of sound and density data. Mixture densities increased with increasing alkylcyclohexane concentration. As the alkyl-chain length on the alkylcyclohexane increased, the excess molar volume decreased, with n-propylcyclohexane and n-dodecylcyclohexane mixtures having positive and negative excess molar volumes, respectively. Molecular dynamics simulations accurately predict densities and isentropic bulk moduli of n-propylcyclohexane and n-dodecylcyclohexane mixtures, and suggest that the differences in excess molar volumes for different alkyl-chain lengths are related to changes in molecular packing. The speed of sound as a function of mole fraction was modeled using a second-order polynomial, and viscosities were modeled using the McAllister three-body equation. Excess speeds of sound and excess molar Gibbs energies of activation for viscous flow at 293.15 K were not statistically different from zero, which suggest ideal behavior. Many of these mixtures have densities similar to those of petroleum-based diesel and jet fuel and viscosities comparable to diesel fuel. The isentropic bulk modulus of jet fuel is best matched by mixtures of n-propylcyclohexane, while that of diesel fuel is matched by mixtures of n-decylcyclohexane or n-dodecylcyclohexane.

Molecular simulation is a powerful computational tool for a broad range of applications including the examination of materials properties and accelerating drug discovery. At the heart of molecular simulation is the analytic potential energy function. These functions span the range of complexity from very simple functions used to model generic phenomena to complex functions designed to model chemical reactions. The complexity of the mathematical function impacts the computational speed and is typically linked to the accuracy of the results obtained from simulations that utilize the function. One approach to improving accuracy is to simply add more parameters and additional complexity to the analytic function. This approach is typically used in non-reactive force fields where the functional form is not derived from quantum mechanical principles. The form of other types of potentials, such as the bond-order potentials, is based on quantum mechanics and has led to varying levels of accuracy and transferability. When selecting a potential energy function for use in molecular simulations, the accuracy, transferability, and computational speed must all be considered. In this focused review, some of the more commonly used potential energy functions for molecular simulations are reviewed with an eye toward presenting their general forms, strengths, and weaknesses.

Because of the complexity of petroleum-based fuels, researchers typically use simplified mixtures, known as surrogates, to study combustion behavior and to attempt to identify how physical properties are related to combustion. The process of determining the surrogate composition to yield a desired set of thermophysical properties can be a complicated and time-consuming task. As a result, the use of computer simulations to narrow the number of possible surrogate compositions is beginning to be explored. Herein, molecular dynamics (MD) simulations are used to model binary mixtures of n-hexadecane with either benzene, toluene, n-ethylbenzene, n-propylbenzene, or n-butylbenzene. Calculated densities are in quantitative agreement with experimental values. With the exception of the mixtures containing benzene, simulated excess molar volumes are also in very good agreement with measured values. Isentropic bulk moduli are in qualitative agreement with experiment, and reproduce interesting trends observed in the experimental data. Specifically, minima in the bulk moduli at intermediate compositions of several of the alkylbenzenes are correctly reproduced. In addition, the structures of the fluids are also examined. For mixtures of n-hexadecane with alkylbenzenes with longer chains, the orientation of the aromatic rings is not substantially impacted by composition. In contrast, increasing n-hexadecane content increases the ratio of parallel to perpendicular arrangements of benzene and toluene molecules. In those mixtures, this change in orientation of the aromatic rings could be responsible for the minima observed in the bulk moduli data. These results show that MD simulations can assist in development of fuel surrogates, both by predicting thermophysical properties and by providing insight into how molecular structure and composition affect those properties.

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The conformation of single molecules of dialkyl poly para phenylene ethynylenes (PPEs), electro-active polymers, is studied in solutions using molecular dynamics simulations. The conformation of conjugated polymers affects their electro-optical properties and therefore is critical to their current and potential uses, though only limited theoretical knowledge is available regarding the factors that control their configuration. The present study investigates the affects of molecular parameters including molecular weight of the polymer and chemical structure of the side chains of PPEs in different solvents on the conformation of the polymers. The PPEs are modeled atomistically where the solvents are modeled both implicitly and explicitly. The study finds that PPEs assume extended configuration which is affected by the length of the polymer backbone and the nature and length of substituting side chains. While the polymer remains extended, local dynamics is retained and no long range correlations are observed within the backbone. The results are compared with scattering experiments.