Gibbs ensemble simulations of vapour—liquid phase equilibria of cyclic alkanes

Interfacial Properties of Cyclic Hydrocarbons: A Monte Carlo Study

The Monte Carlo technique is used to study the vapor-liquid interface of cyclopentane, cyclohexane, and benzene. The OPLS and TraPPE potential fields are compared in the temperature range from 298.15 to 348.15 K (273.15-298.15 K for C5H10). A new method for the treatment of the long-range interactions in inhomogeneous simulations is used. When this new method is employed, the obtained values of saturated liquid density and of enthalpy of vaporization are equal to those obtained using the bulk isothermal-isobaric Monte Carlo technique. The values of surface tension become independent of the cutoff distance and they are significantly larger than those when only simple spherical truncation of intermolecular interactions is used.

Interactions of liquid crystal-forming molecules with phospholipid bilayers studied by molecular dynamics simulations

Recent experiments have shown that liquid crystals can be used to image mammalian cell membranes and to amplify structural reorganization in phospholipid-laden liquid crystal-aqueous interfaces. In this work, molecular dynamics simulations were employed to explore the interactions between commonly used liquid crystal-forming molecules and phospholipid bilayers. In particular, umbrella sampling was used to obtain the potential of mean force of 4-cyano-4'-pentylbiphenyl (5CB) and 4'-(3,4-difluor-phenyl)-4-pentyl-bicylohexyl (5CF) molecules partitioning into a dipalmitoylphosphatidylcholine bilayer. In addition, results of simulations are presented for systems consisting of a fully hydrated bilayer with 5CB or 5CF molecules at the lowest (4.5 mol %) and highest (20 mol %) concentrations used in recent laboratory experiments. It is found that mesogens preferentially partition from the aqueous phase into the membrane; the potential of mean force exhibits highly favorable free energy differences for partitioning (-18 k(B)T for 5CB and -26 k(B)T for 5CF). The location and orientation of mesogens associated with the most stable free energies in umbrella sampling simulations of dilute systems were found to be consistent with those observed in liquid-crystal-rich bilayers. It is found that the presence of mesogens in the bilayer enhances the order of lipid acyl tails, and changes the spatial and orientational arrangement of lipid headgroup atoms. These effects are more pronounced at higher liquid-crystal concentrations. In comparing the behavior of 5CB and 5CF, a stronger spatial correlation (i.e., possibly leading to aggregation) is observed between 5CB molecules within a bilayer than between 5CF molecules. Also, the range of molecular orientations and positions along the bilayer normal is larger for 5CB molecules. At the same time, 5CF molecules were found to bind more strongly to lipid headgroups, thereby slowing the lateral motion of lipid molecules.

Molecular simulations of the adsorption of cycloalkanes in MFI-type silica

A new force field for the simulation of the adsorption of cycloalkanes in nanoporous silica affords a significant improvement over any previously employed force field. The simulated isotherms reproduce the most salient features in the experimental isotherms extremely well. The study of cyclo-pentane, -hexane, and -heptane adsorption in MFI-type silica indicates an inflection for cyclopentane but not for cyclohexane at intermediate pressure. If corroborated by experiments, such an inflection point would afford an excellent calibration point for further force field developments. At low pressures, mixture isotherms of cyclohexane and n-hexane show a temperature dependence on the selectivity in accordance with recent results by J. P. Fox and S. P. Bates, J. Phys. Chem., 2004, 108, 17136. This dependence is caused by a difference in temperature dependence of the Henry coefficient for both molecules. At high pressures entropy effects due to packing always favor the sorption of n-hexane. Furthermore, the influence of the flexibility of the zeolite framework on the adsorption of these rather bulky molecules is investigated. It is found that this influence of the flexibility on the adsorption of cyclohexane is as small as with n-alkanes.