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The structural and energetic properties of a group of selected amides, of well-known importance for the design of efficient clathrate inhibitors, are calculated with Hartree-Fock and density functional theory, B3LYP, theoretical levels, and a 6-311++g** basis set in the gas phase and a water solution. The conformational behavior of the molecules is studied through the scanning of the torsional potential energy surfaces and by the analysis of the differences in the energetic and structural properties between the isomers. The properties of the amides in water solution are determined within a self-consistent reaction field approach with a polarizable continuum model that allows the calculation of the different contributions to the free energy of solvation. The calculated barriers to rotation are in good agreement with the available experimental data and the comparison of the gas and water results shows the strong effect of the solute polarization. The properties of different amide-water complexes are calculated and compared with available experimental information.

This work continues the systematic study of global phase diagrams in binary systems relevant for the description of natural gases using the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state (EOS). Reservoirs frequently contain non-hydrocarbon compounds such as CO2, N2, or H2S that have strong effects upon the thermodynamic properties of natural gases. Thus, we study the behavior of their binary mixtures with n-alkanes over wide pressure and temperature ranges to get a deeper insight into the intermolecular pair interactions reflected in the phase behavior of the mixtures. These systems show complex patterns within the van Konynenburg classification, and the theoretical model accurately reproduces them. In the case of H2S containing mixtures (H2S is a compound that has autoassociation hydrogen-bonding ability), we analyze several association models to obtain the most reliable approach by comparing the predicted association degree of H2S to available experimental data. A conclusion from this series of papers is that PC-SAFT is a reliable and accurate model to predict the behavior of the different types of binary mixtures involved in complex multicomponent natural gases, with the ability to describe the different patterns of fluid-phase behavior presented by the studied systems over wide pressure and temperature ranges.

In this work, we extend the previously reported study of global phase diagrams in binary mixtures relevant for the description of natural gases using the perturbed-chain statistical associating fluid theory (PC-SAFT) model to systems containing n-alkanes + other nonlinear hydrocarbons. These mixtures have branched, cyclic, and aromatic compounds and provide a deeper insight into the effect upon their thermodynamic behavior by introducing additional structural complexity into the mixtures. The studied systems show different patterns of phase equilibria according to the van Konynenburg classification. The PC-SAFT model usually describes properly the previously reported n-alkane + n-alkane binary mixtures, including their varied and complex phase behavior. The results reported in this work contribute to a better understanding of the binary mixtures fluid-phase behavior and show the ability of the PC-SAFT model to describe the thermodynamic properties of these systems and to model complex intermolecular effects, and they can be useful for the modeling and understanding of natural gas properties in industrial applications.

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