We have developed a general predictive method for vapor pressures and enthalpies of vaporization based on the calculation of the solvation free energy that consists of three components; the electrostatic, dispersion, and cavity formation contributions. The electrostatic contribution is determined using the quantum mechanical COSMO solvation model. Thermodynamic perturbation theory for hard-core molecules is used for the cavity term, and the dispersion term is modeled using a mean field term proportional to the density and molecular surface area. The proposed model uses one set of van der Waals atomic radii to describe molecular shape, two universal interaction parameters for the electrostatic interaction, one set of atom-specific dispersion coefficients, one universal parameter to scale the atomic exposed surface area, and a single universal parameter for the ratio of the hard-core to atomic radii. The model parameters have been determined using 371 pure substances of varying molecular structure, functionality, and size. The average accuracy of the model for vapor pressures and enthalpies of vaporization at the normal boiling temperature is found to be 76% and 4.81 kJ/mol, respectively, with temperature-independent parameters. The average error in the normal boiling temperature is found to be 16 K for species whose boiling points range from 191 to 610 K.

The solvation free energies of amino acids and their side-chain analogues in water and cyclohexane are calculated by using Monte Carlo simulation. The molecular interactions are described by the OPLS-AA force field for the amino acids and the TIP4P model for water, and the free energies are determined by using the Bennett acceptance method. Results for the side-chain analogues in cyclohexane and in water are used to evaluate the performance of the force field for the van der Waals and the electrostatic interactions, respectively. Comparison of the calculated hydration free energies for the amino acid analogues and the full amino acids allows assessment of the additivity of the side chain contributions on the number of hydrating water molecules. The hydration free energies of neutral amino acids can be reasonably approximated by adding the contributions of their side chains to that of the hydration of glycine. However, significant nonadditivity in the free energy is found for the zwitterionic form of amino acids with polar side chains. In serine and threonine, intramolecular hydrogen bonds are formed between the polar side chains and backbone groups, leading to weaker solvation than for glycine. In contrast, such nonadditivity is not observed in tyrosine, in which the hydroxyl group is farther separated from, and therefore cannot form an intramolecular hydrogen bond with, the backbone. For histidine we find that a water molecule can form a bridge when the intramolecular hydrogen bond between the polar group and the backbone is broken.

No abstract

Thermodynamic, structural, and dynamic properties of polymethylene melt surfaces are studied by molecular dynamics simulations using both an explicit atom and a united atom model. N-tridecane (C13H28) melt films with a thickness of about 30 Å are studied by NVT-MD simulation method at the temperatures from 300 K to 450 K. We obtain stable surface properties such as surface tension, density profile, order parameter, and diffusivity upon performing the simulation on these films for 1 or 2 ns. When compared with experiment, simulations give a reasonable agreement for the surface tension with error of ca. 20%. It is observed that the density of chain-end group (methyl) is enhanced near the free surface, while it is depleted in the region below the surface. The interfacial thickness of the density transition region defined as liquid density divided by maximum density gradient is estimated to be about 5 Å at room temperature. In this interfacial region, a slight preference for chain segments to orient along the direction parallel to the surface is observed with practically no difference in the chain conformation from the bulk value. The molecular diffusivity along the film surface is enhanced by a factor of ca. 3 compared with the diffusivity along the surface normal in the interfacial region. Both the explicit atom and the united atom model show almost the same thermodynamic and structural properties near the surface.