Search

The vapor-liquid equilibrium of binary mixtures of xenon + S[F.sub.6] was measured at nine temperatures from 235.34 to 295.79 K and pressure up to 6.5 MPa. The mixture critical line is continuous between the critical points of the pure components, hence the system is classified as type 1 phase behavior in the scheme of van Konynenburg and Scott.

An experimental and theoretical study of the thermodynamic properties of semifluorinated alkanes is performed. The results are interpreted using the SAFT-VR equation of state and the results are able to predict the thermodynamic behavior of the perfluoroalkylalkanes.

A charge-transfer reactive (CTR) force field along with other force fields taken from literature is used to predict the thermophysical properties of polyhedral oligomeric silsesquioxanes (POSS). All the force fields tested performed satisfactorily, but results indicate that the literature force field Hybrid-COMPASS and CTR predict structural properties of systems that contain POSS monomers, through molecular simulation.

The infinite dilution properties of two water +n-alkane mixtures by using a statistical associating fluid theory (SAFT) are presented. Recent theoretical and simulation studies suggest an unexpected shift in the solubility of n-alkanes in near critical water.

A study of the thermodynamic functions of the liquid mixtures xenon + n-butane and xenon + isobutane is presented. It is shown that the mixtures have a similar behavior to that of other xenon + alkane mixtures.

Measurements of total vapor pressures for liquid mixtures of xenon + propane at 161.40, 182.34 and 195.49 K and of xenone + ethane at 161.40 and 182.34 K reveal negative deviations from Raoult's law at all temperatures. The thermodynamics of the two liquid mixtures are further discussed.

The statistical associating fluid theory for chain molecules with attractive potentials for variable range was used to investigate the fluid-phase equilibria of the methane + n-hexane system. The system was found to exhibit a type V phase behavior, in which the two components display partial miscibility. The region of liquid-liquid immiscibility was accurately predicted by using the Lorentz-Berthelot mixing rules.

A statistical associating fluid theory for potential of variable range is developed to model dipolar fluids (SAFT-VR+D) and the high-pressure phase diagram of hydrogen sulfide + n-alkane binary mixtures is examined. The SAFT-VR+D theory has explored and predicted the effect of temperature and the molecular weight of the hydrocarbon component on the solubility of [H.sub.2]S in the n-alkanes.

The equilibrium molecular dynamics simulation results for polyhedral oligomeric silsesquioxanes (POSS) monomers dissolved in liquid normal hexane at 300 K and at 400 K are reported.

The statistical associating fluid theory for potentials of variable range (SAFT-VR) equation of state is extended to account for quadrupolar interactions by using a Pade-approximation term derived from perturbation theory. The inclusion of the quadrupolar interaction into the SAFT-VR equation improves the predictive ability of the SAFT-VR EOS when studying the phase behavior of alkane + nitrogen systems and reduces the need to use effective parameters by explicitly including at the molecular level interactions due to the quadrupole moment.

Molecular-simulation results for POSS-POSS radial distribution functions (where POSS=polyhedral oligomeric silsesquioxane), potentials of mean force, and self-diffusion coefficients for POSS monomers dissolved in poly(dimethylsiloxane) are presented. Results demonstrate that it is possible to modulate the effective POSS-POSS interaction by increasing the temperature or by substituting by hydrogen atoms in the POSS monomer with methyl groups.

A new apparatus, based on the static and analytical method, is developed for the study of vapor-liquid or vapor-liquid-liquid equilibrium. The equilibrium vapor pressures of (xenon + methane) binary mixtures are measured and the results have been correlated by the Peng-Robinson equation of state.

Statistical associating fluid theory for potentials of variable attractive range (SAFT-VR) is used in studying high-pressure equilibria in perfluoro-(n)-alkane + (n)-alkane binary mixtures. These compounds undergo large positive deviations from Raoult's Law while the phase behavior changes from type II to type III. The perfluoro-n-alkane and n-alkane molecules are assumed to be attractive spherical segments tangentially bonded together to form chains. The critical lines predicted by SAFT-VR for the mixtures are in close agreement with experimental data.

We have developed a multi-input E( n ) equivariant graph convolution-based model designed for the prediction of chemical properties that result from the interaction of heterogeneous molecular structures. By incorporating spatial features and constraining the functions learned from these features to be equivariant to E( n ) symmetries, the interactional-equivariant graph neural network (IEGNN) can efficiently learn from the 3D structure of multiple molecules. To verify the IEGNN's capability to learn interactional properties, we tested the model's performance on three molecular data sets, two of which are curated in this study and made publicly available for future interactional model benchmarking. To enable the loading of these data sets, an open-source data structure based on the PyTorch Geometric library for batch loading multigraph data points is also created. Finally, the IEGNN's performance on a data set consisting of an unknown interactional relationship (the frictional properties resulting between monolayers with variable composition) is examined. The IEGNN model developed was found to have the lowest mean absolute percent error for the predicted tribological properties of four of the six data sets when compared to previous methods.

The structural properties of two- and three-component gel-phase bilayers were studied using molecular dynamics simulations. The bilayers contain distearoylphosphatidylcholine (DSPC) phospholipids mixed with alcohols and/or fatty acids of varying tail lengths, with carbon chain lengths of 12, 16, and 24 studied. Changes in both headgroup chemistry and tail length are found to affect the balance between steric repulsion and van der Waals attraction within the bilayers, manifesting in different bilayer structural properties. Lipid components are found to be located at different depths within the bilayer depending on both chain length and headgroup chemistry. The highest bilayer ordering and lowest area per tail are found in systems with medium-length tails. While longer tails can enhance van der Waals attractions, the increased tail-length asymmetry is found to induce disorder and reduce tail packing. Bulkier headgroups further increase steric repulsion, as reflected in increased component offsets and reduced tail packing. These findings help explain how bilayer composition affects the structure of gel-phase bilayers.

The structural properties of two-component gel-phase bilayers of distearylphosphatidylcholine (DSPC) and alcohol molecules with different compositions and chain lengths (12-24 carbons long) are studied via molecular dynamics simulations. Several bilayer properties, including area per lipid, tilt angle, chain interdigitation, and headgroup offset, are studied for each system and compared, revealing important structural implications depending upon headgroup size and chain length. While tail tilt is the primary mechanism for single-component bilayers to balance tail attraction and headgroup repulsion, our results demonstrate that the lipid mixtures studied adjust this balance via an offset between the depths of the different molecular species in the bilayer; this behavior is found to depend both on composition and on the length of alcohol molecules relative to the length of DSPC tails. It is shown that the structural properties of bilayers with asymmetric tail lengths depend strongly on the bilayer composition, while the composition has less influence on mixed-component bilayers with nearly symmetric tail lengths. These findings are explained on the basis of the interdigitation between bilayer leaflets and how interdigitation is related to other structural properties.

The vapor pressure of four liquid perfluoroalkylalkanes (CF3(CF2)n(CH2)mCH3; n = 3, m = 4,5,7; n = 5, m = 5) was measured as a function of temperature between 278 and 328 K. Molar enthalpies of vaporization were calculated from the experimental data, and the results were compared with data from the literature for the corresponding alkanes and perfluoroalkanes. The heterosegmented statistical associating fluid theory was used to interpret the results at the molecular level both with and without the explicit inclusion of the dipolar nature of the molecules. Additionally, ab initio calculations were performed for all perfluoroalkylalkanes studied to determine the dipole moment to be used in the theoretical calculations. We demonstrate that the inclusion of a dipolar term is essential for describing the vapor-liquid equilibria of perfluoroalkylalkanes. It is also shown that vapor-liquid equilibria in these compounds result from a subtle balance between dipolar interactions, which decrease the vapor pressure, and the relatively weak dispersive interactions between the hydrogenated and fluorinated segments.

In nature, processive and non-processive cellulase enzymes deconstruct cellulose to soluble sugars. From structural studies, the consensus is that processive cellulases exhibit tunnels lined with aromatic and polar residues, whereas non-processive cellulases exhibit open clefts with fewer ligand contacts. To gain additional insight into the differences between processive and non-processive cellulases, we examine the glycoside hydrolase family 7 (GH7) cellobiohydrolase, Cel7A, and the endoglucanase, Cel7B, from Trichoderma reesei with molecular simulation. We compare properties related to processivity and compute the binding affinity changes for mutation of four aromatic residues lining the Cel7A active site tunnel and Cel7B cleft to alanine. For the wild-type enzymes, dissimilar behavior is observed at nearly every glucopyranose-binding site from -7 to +2, except in the -2 site, suggesting that the structural differences directly around the catalytic center and at the active site tunnel entrances and exits may all contribute to processivity in GH7s. Interestingly, the -2 site is similar in both enzymes, likely due to the significant conformational change needed in the cellodextrin ligand near this site for catalysis. Moreover, aromatic residue mutations in the Cel7A and Cel7B active sites display only small differences in binding affinity, but the ligand flexibility and enzyme-ligand interactions are only locally affected in Cel7A, whereas the entire ligand is significantly affected when any aromatic residue is mutated in Cel7B.

A statistical associating fluid theory for potential of variable range has been recently developed to model dipolar fluids (SAFT-VR+D) [Zhao and McCabe, J. Chem. Phys. 2006, 125, 104504]. The SAFT-VR+D equation explicitly accounts for dipolar interactions and their effect on the thermodynamics and structure of a fluid by using the generalized mean spherical approximation (GMSA) to describe a reference fluid of dipolar square-well segments. In this work, we apply the SAFT-VR+D approach to real mixtures of dipolar fluids. In particular, we examine the high-pressure phase diagram of hydrogen sulfide+n-alkane binary mixtures. Hydrogen sulfide is modeled as an associating spherical molecule with four off-center sites to mimic hydrogen bonding and an embedded dipole moment (micro) to describe the polarity of H2S. The n-alkane molecules are modeled as spherical segments tangentially bonded together to form chains of length m, as in the original SAFT-VR approach. By using simple Lorentz-Berthelot combining rules, the theoretical predictions from the SAFT-VR+D equation are found to be in excellent overall agreement with experimental data. In particular, the theory is able to accurately describe the different types of phase behavior observed for these mixtures as the molecular weight of the alkane is varied: type III phase behavior, according to the scheme of classification by Scott and Konynenburg, for the H2S+methane system, type IIA (with the presence of azeotropy) for the H2S+ethane and+propane mixtures; and type I phase behavior for mixtures of H2S and longer n-alkanes up to n-decane. The theory is also able to predict in a qualitative manner the solubility of hydrogen sulfide in heavy n-alkanes.

The phase behavior of nitrogen + n-alkanes is studied within the framework of the statistical associating fluid theory for potentials of variable range (SAFT-VR). The effect of the quadrupole moment of nitrogen on the phase behavior is considered through an extension of the SAFT-VR equation that includes an additional contribution to the Helmholtz free energy due to quadrupolar interactions. A significant improvement in the description of the phase diagram of the binary mixtures of nitrogen with different n-alkanes is obtained with the new approach when compared to predictions from the original SAFT-VR EOS (i.e., without the quadrupolar term). The experimental value for the quadrupole moment of nitrogen is used in the new equation; thus, no additional parameters are employed. Given the nonideal nature of the binary mixtures, a binary interaction parameter is needed to describe the full-phase diagram and high-pressure critical lines of these systems; however, this can be fitted to a single system and successfully used to predict the phase behavior of other binary mixtures without additional fitting. Furthermore, only a single, transferable, cross-energy parameter is required when the quadrupolar term is considered, whereas a cross-range parameter is also needed with the original SAFT-VR approach. The inclusion of the quadrupolar term in the equation of state therefore reduces the need to use effective parameters by explicitly including at the molecular level interactions due to the quadrupole moment.

The SAFT-VRX equation of state combines the SAFT-VR equation with a crossover function that smoothly transforms the classical equation into a nonanalytical form close to the critical point. By a combinination of the accuracy of the SAFT-VR approach away from the critical region with the asymptotic scaling behavior seen at the critical point of real fluids, the SAFT-VRX equation can accurately describe the global fluid phase diagram. In previous work, we demonstrated that the SAFT-VRX equation very accurately describes the pvT and phase behavior of both nonassociating and associating pure fluids, with a minimum of fitting to experimental data. Here, we present a generalized SAFT-VRX equation of state for binary mixtures that is found to accurately predict the vapor-liquid equilibrium and pvT behavior of the systems studied. In particular, we examine binary mixtures of n-alkanes and carbon dioxide + n-alkanes. The SAFT-VRX equation accurately describes not only the gas-liquid critical locus for these systems but also the vapor-liquid equilibrium phase diagrams and thermal properties in single-phase regions.