Isobaric vapor-liquid equilibrium for binary systems containing benzothiophene

Isobaric vapor-liquid equilibrium (VLE) of the following systems was measured with a recirculation still: benzothiophene + dodecane at 99.6 kPa, benzothiophene + 1-dodecene at 100.1 kPa, and benzothiophene + 1-octanol at 100 kPa. All systems studied exhibit positive deviation from Raoult's law. A minimum temperature azeotrope was found in the systems benzothiophene + dodecane (x₁ = 0.491, P = 99.6 kPa, T = 484.72 K) and benzothiophene + 1-dodecene (x₁ = 0.185, P = 100.1 kPa, T = 484.45 K). No azeotropic behavior was found in benzothiophene + 1-octanol system at 100 kPa. The experimental results were correlated with the Wilson model and compared to COSMO-SAC predictive model. Liquid and vapor phase compositions were determined with gas chromatography. All measured data sets passed the thermodynamic consistency tests. The activity coefficients at infinite dilution are also presented.     

A new method for approximate estimates of the dispersion interaction between two molecules

A new method to evaluate the dispersion interaction between two weakly interacting closed shell molecules is presented. The method is based on a second-order sum-over-states perturbation method, where the nominator is approximated by products of one electron integrals.     

Coating for preventing nonspecific adhesion mediated biofouling in salty systems: Effect of the electrostatic and van der waals interactions

Development of anti-biofouling coating has attracted immense attention for reducing the massively detrimental effects of biofouling in systems ranging from ship hulls and surgical instruments to catheters, implants, and stents. In this paper, we propose a model to quantify the role of electrostatic and van der Waals (vdW) forces in dictating the efficacy of dielectric coating for preventing the nonspecific adhesion mediated biofouling in salty systems. The model considers a generic charged lipid-bilayer encapsulated vesicle-like structure representing the bio-organism. Also, we consider the fouling caused by the nonspecific adhesion of the bio-organism on the substrate, without accounting for the explicit structures (e.g., pili, appendages) or conditions (e.g., surface adhesins secreted by the organisms) involved in the adhesion of specific microorganism. The model is tested by considering the properties of actual coating materials and biofouling causing microorganisms (bacteria, fungi, algae). Results show that while the electrostatic-vdW effect can be significant in anti-biofouling action for cases where the salt concentration is relatively low (e.g., saline solution for surgical instruments), it might not be effective for marine environment where the salt concentration is much higher. The findings, therefore, point to a hitherto unexplored driving mechanism of anti-biofouling action of the coating. Such an identification will also enable the appropriate choices of the coating materials (e.g., possible dielectric material with volume charge) and other system parameters (e.g., salinity of the solution for storing the surgical instruments) that will significantly improve the efficiency of the coatings in preventing the nonspecific adhesion mediated biofouling.     

Fluid phase stability and equilibrium calculations in binary mixtures: Part I: Theoretical development for non-azeotropic mixtures

A framework is proposed for the solution of fluid phase equilibrium (P–T flash) for binary mixtures described by equations of state of general form. The framework is based on decomposing the phase equilibrium problem into sub-problems with more convenient and tractable mathematical and numerical properties. Systematic procedures are used to identify the mapping of the problem in the density and composition space, referred to as the density–composition pattern, at specified temperature and pressure. A series of stability tests is then carried out to explore the existence or non-existence of phases. Once the existence of a phase has been determined, the limits of stability and physical bounds on the problem are used to define the search area for that phase in the density–composition pattern. Finally, all available information from this detailed analysis is used for the solution of phase equilibrium between the phases identified in order to find the stable state at the specified conditions. The features of the proposed approach are exposed in detail through an algorithm for the fluid phase equilibria of the augmented van der Waals equation of state applied to non-azeotropic mixtures.     

Determination of van der Waals forces in monolayer films of lipids & biopolymers. Equation of state for two-dimensional films

Amphiphile molecules are characterized by the dual property arising from the interactions between the apolar [alkyl] and the polar part and the surrounding solvent, i.e., water. In assemblies which amphiphiles form in diverse systems, e.g., micelles, soap bubbles, monolayers or bilayers at interfaces, the attractive forces are attributed to the van der Waals forces. It is not easy to estimate the magnitude of van der Waals forces in some of these systems by any direct method.The magnitude of van der Waals forces in spread monolayers of lipids and biopolymers has been reported to be estimated from experimental data. The magnitude of these forces has been estimated by using an equation of state of a very general form, as delineated herein. In the current literature no such attempt has been reported in the analyses of these monolayers spread on aqueous surfaces. These analyses suggest that the predominant surface forces arise from van der Waals interactions, if the magnitude of electrostatic charge repulsions is weak. The equation-of-state as derived indicates that it is useful in providing information about the molecular interaction in monolayers, for both lipids and biopolymers.     

Van der Waals attraction between Stöber silica particles in a binary solvent system

Hydrophilic Stöber silica particles are stable in ethanol, but flocculation may be induced by the addition of sufficient cyclohexane. Low-shear rheological measurements indicated non-Newtonian behaviour beyond the critical cyclohexane concentration. The thickness and composition of the solvation layer around the particles were calculated from the adsorption excess isotherm on the basis of a multilayer adsorption model. The composition dependences of the Hamaker constants of the dispersion medium and the adsorption layer were obtained from optical dispersion measurements. A single-sheet, hard-sphere model predicted a weak van der Waals attraction in the ethanolic regime, but a strong attraction in the cyclohexane-rich region, in good accordance with the rheological properties of the dispersions and visual observations.     

Finite extensibility and orientational effects in rubbers. A physical interpretation of the ‘van der Waals equation’ for the elastic force

We propose a physical interpretation of the so-called van der Waals equation of state for rubbers, which gives a relation between the force and the deformation. On a phenomenological basis this equation takes the finite extensibility and a non-defined interaction into account. Here the fininte extensibility is discussed for the dilute case (no entanglements) and the highly entangled limit. The intramolecular interactions are described by orientational effects. The resulting equation of state for the force shows the same features as the van der Waals equation.Dedicated to Prof. H.-G. Kilian on the occasion of his 60th birthday.     

A rapid method for molecular shape comparison of medium-sized molecules. Conformational calculations on sandalwood odor,IV

Summary A fast method of a surface comparison of two or more molecules to be matched is presented. The Van der Waals surfaces of molecules are described by points calculated as the intersection of grid lines with the molecular surface. The mean surface of various molecules with the same biological activity can be constructed. It is used for further comparisons with similar molecules lacking this activity. Deviations of any molecular surface from the mean surface can be mapped onto the surface. The method was tested on a distinct group of sandalwood odor molecules and it was shown that such matching and comparison procedures are useful in the investigation of odor structure-activity relationships proposed as CAFD (computer aided fragrance design).Dedicated to Prof. Dr. W. Fleischhacker, on occasion of his 60th anniversary     

Thermodynamic modeling of saturated liquid compositions and densities for asymmetric binary systems composed of carbon dioxide,alkanes and alkanols

The present study mainly focuses on the phase behavior modeling of asymmetric binary mixtures. Capability of different mixing rules and volume shift in the prediction of solubility and saturated liquid density has been investigated. Different binary systems of (alkane + alkanol), (alkane + alkane), (carbon dioxide + alkanol), and (carbon dioxide + alkane) are considered. The composition and the density of saturated liquid phase at equilibrium condition are the properties of interest. Considering composition and saturated liquid density of different binary systems, three main objectives are investigated. First, three different mixing rules (one-parameter, two parameters and Wong–Sandler) coupled with Peng–Robinson equation of state were used to predict the equilibrium properties. The Wong–Sandler mixing rule was utilized with the non-random two-liquid (NRTL) model. Binary interaction coefficients and NRTL model parameters were optimized using the Levenberg–Marquardt algorithm. Second, to improve the density prediction, the volume translation technique was applied. Finally, Two different approaches were considered to tune the equation of state; regression of experimental equilibrium compositions and densities separately and spontaneously. The modeling results show that there is no superior mixing rule which can predict the equilibrium properties for different systems. Two-parameter and Wong–Sandler mixing rule show promoting results compared to one-parameter mixing rule. Wong–Sandler mixing rule in spite of its improvement in the prediction of saturated liquid compositions is unable to predict the liquid densities with sufficient accuracy.     

Van der Waals interactions in aromatic systems: structure and energetics of dimers and trimers of pyridine.

Full geometry optimizations at the dispersion-corrected DFT-BLYP level of theory were carried out for dimers and trimers of pyridine. The DFT-D interaction energies were checked against results from single-point SCS-MP2/aug-cc-pVTZ calculations. Three stacked structures and a planar H-bonded dimer were found to be very close in energy (interaction energies in the range from -3.4 to -4.0 kcal mol(-1)). Two T-shaped geometries are higher lying, by about 1 kcal mol(-1), which is explained by the more favorable electrostatic interactions in the stacked and H-bonded arrangements. The DFT-D approach has proved to be a reliable and efficient tool to explore the conformational space of aromatic van der Waals complexes and furthermore provides interaction energies with errors of less than 10-20 % of DeltaE. Comparisons with previous results obtained by using only partially optimized model geometries strongly indicate that unconstrained optimizations are mandatory in such weakly bonded low-symmetry systems.     

Generalized relativistic effective core potential calculations of the adiabatic potential curve and spectroscopic constants for the ground electronic state of the Ca2 molecule

The potential curve, dissociation energy, equilibrium internuclear distance, and spectroscopic constants for the ground state of the Ca₂ molecule are calculated with the help of the generalized relativistic effective core potential method, which allows one to exclude the inner core electrons from the calculations and to take the relativistic effects into account effectively. Extensive generalized correlation basis sets were constructed and used. The scalar relativistic coupled cluster method with corrections for high‐order cluster amplitudes is used for the correlation treatment. The results are analyzed and compared with the experimental data and corresponding all‐electron results. © 2013 Wiley Periodicals, Inc.     

A proposed equation of correlation for the study of thermodynamic properties (density,viscosity, surface tension and refractive index) of liquid binary mixtures

The literature on the physicochemical properties of liquid binary mixtures shows that most such systems exhibit nonlinear behavior. As a result, rigorous data and equations capable of affording a reliable estimate of the behavior of such mixtures are needed.     

A partition function‐based weighting scheme in force field parameter development using ab initio calculation results in global configurational space

In force field parameter development using ab initio potential energy surfaces (PES) as target data, an important but often neglected matter is the lack of a weighting scheme with optimal discrimination power to fit the target data. Here, we developed a novel partition function‐based weighting scheme, which not only fits the target potential energies exponentially like the general Boltzmann weighting method, but also reduces the effect of fitting errors leading to overfitting. The van der Waals (vdW) parameters of benzene and propane were reparameterized by using the new weighting scheme to fit the high‐level ab initio PESs probed by a water molecule in global configurational space. The molecular simulation results indicate that the newly derived parameters are capable of reproducing experimental properties in a broader range of temperatures, which supports the partition function‐based weighting scheme. Our simulation results also suggest that structural properties are more sensitive to vdW parameters than partial atomic charge parameters in these systems although the electrostatic interactions are still important in energetic properties. As no prerequisite conditions are required, the partition function‐based weighting method may be applied in developing any types of force field parameters. © 2013 Wiley Periodicals, Inc.     

A new continuous dilution vapor liquid equilibrium apparatus: The activity coefficients of benzene and water in cyclohexane at 50°C

A semi-automated apparatus to measure vapor pressure differences between a reference solvent and dilute solutions as a function of concentration at constant temperature is described. Application to the benzene-cyclohexane and benzene-cyclohexane-water systems at high cyclohexane concentrations and low water concentrations is reported. At 50°C in the two component mixture we find ln_B=ln(f _B/X _B f _B ⁰ )=0.408–0.89X_B for 10^–3 B <3×10^–2 and in the three component system, ln_W=6.79±0.04 for 10^–6 W <2×10^–5 and 10^–3 B <3×10^–2.Taken in part from a PhD. thesis, University of Tennessee, 1983.     

Two‐component calculations of spin–orbit effects for a van der Waals molecule Rn2

Electronic structures of the weakly bound Rn₂ were calculated by the two‐component Møller–Plesset second‐order perturbation and coupled‐cluster methods with relativistic effective core potentials including spin–orbit operators. The calculated spin–orbit effects are small, but depend strongly on the size of basis sets and the amount of electron correlations. Magnitudes of spin–orbit effects on D_e (0.7–3.0 meV) and R_e (−0.4∼−2.2 Å) of Rn₂ are comparable to previously reported values based on configuration interaction calculations. A two‐component approach seems to be a promising tool to investigate spin–orbit effects for the weak‐bonded systems containing heavy elements. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 139–143, 1999     

Revision of the volumetric method for measurements of liquid–liquid equilibria in binary systems

A theoretical analysis of the accuracy of the volumetric method for the determination of liquid–liquid equilibrium was carried out. The results show that, under certain conditions, this method can be used to investigate systems showing relatively small mutual solubilities. Relations were derived to estimate standard deviations of the equilibrium compositions determined by the volumetric method.

In the experimental part of the work, an apparatus for measurements of mutual solubilities of liquids was constructed. A procedure that enabled us to determine precisely volumes of liquid phases was developed. This procedure and apparatus present the advantage that relatively small amounts of samples are required (approximately 2 × 20 ml). Theoretical conclusions concerning the applicability of the volumetric method were checked by measuring mutual solubilities at 303.15 K in systems methylcyclohexane + N,N-dimethylformamide, 1-butanol + water and dimethyl phthalate + water. Further, the method was used to measure systematically the liquid–liquid equilibrium in systems ethyl acetate + ethylene glycol and phenyl acetate + ethylene glycol at temperatures from 293 to 323 K. Data for these systems were acquired by means of other methods as well and a good agreement was observed on comparison.