Calculation of van der Waals forces in adhering systems

The existing theories of van der Waals attractive forces between two media separated by a vacuum or a third medium can be adapted to the case of true adhesion by introducing an adjustable parameter z₀. The consistency of the new model leads to the introduction of an effective dielectric constant. A study of the influence of the frequency range of the dielectric spectrum on van der Waals forces shows that when there is adhesion or when the intermediate layer is thin, only the u.v. component of the dielectric spectrum plays a role.The invariance of van der Waals forces to changes in the i.r. component of the dielectric spectrum allows the work function of electrons and accordingly the always and simultaneously present electrostatic adhesion forces, which are of the same order of magnitude, to be varied without changing the magnitude of the van der Waals adhesion forces.     

van der Waals interaction energies between non-planar bodies

The van der Waals interaction energies between non-planar geometries are obtained without the assumption that the distance between two non-planar bodies is much smaller than radii of the non-planar bodies. Based on atomto-body van der Waals energies, we calculate body-to-body van der Waals interaction energies for several non-planar geometries. Using the continuum approach, we discuss the van der Waals interactions in two-dimensional carbon nanotubes and C₆₀ molecules. Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University     

Electronic-Vibrational Spectroscopy of Large van der Waals Molecules

In this paper we focus on the electronic-vibrational spectra of tetracene · A₁ (A = Ar, Kr, Xe) heteroclusters, which provide information on the intermolecular nuclear motion of a rare-gas atom on the microsurface of a large aromatic molecule. Calculations of the energy levels and nuclear Franck-Con-don factors for the excitation of the parallel and perpendicular intermolecular vibrations in the electronic S₀ → S₁ excitation of heteroclusters were performed. The experimental energetic and intensity data for the perpendicular intermolecular nuclear excitation are well accounted for by our calculations. This study provides some information regarding the potential surfaces of large heteroclusters in electronically excited states, which are important for the elucidation of the spectroscopy, nuclear motion, and isomerization dynamics of large aromatic molecule · (rare gas)n heteroclusters.     

The Wave Function of a van der Waals Resonance

The wave function of the resonant state describing rotational predissociation in a standard model of the HeH₂ van der Waals complex is calculated. This wave function offers an opportunity to use for the resonance width a formula given by Humblet and Rosenfeld     

Discrete structure of van der Waals domains in globular proteins

Most globular proteins are divisible by domains, distinct substructuresof the globule. The notion of hierarchy of the domains was introducedearlier via van der Waals energy profiles that allow one tosubdivide the proteins into domains (subdomains). The questionremains open as to what is the possible structural connectionof the energy profiles. The recent discovery of the loop-n-lockelements in the globular proteins suggests such a structuralconnection. A direct comparison of the segmentation by van derWaals energy criteria with the maps of the locked loops of nearlystandard size reveals a striking correlation: domains in generalappear to consist of one to several such loops. In addition,it was demonstrated that a variety of subdivisions of the sameprotein into domains is just a regrouping of the loop-n-lockelements.     

Role of van der Waals force in latex film formation

Capillary pressure force and direct surface tension force are known to be sufficient though probably not necessary to drive the compaction stage of latex film formation. There is abundant evidence that van der Waals force can draw particles together progressively more around the perimeters of interparticle contacts, but their role in compaction remains unanswered. With the powerful technique of high-resolution cryogenic scanning electron microscopy (cryo-SEM), together with fast-freezing and freeze-drying followed by controlled annealing at temperatures below and around the nominal glass transition temperature, we have documented the role of van der Waals force in film formation in the water-free condition, i.e., with capillary pressure and surface tension forces absent. Results of imaging the freeze-dried and annealed coatings are fully consistent with the hypothesis that van der Waals force alone can compact a latex coating. The rate at which particles flatten and thus the coating compacts by annealing increases, as expected, with temperature and time. The results of rewetting tests of coatings annealed at various temperatures demonstrate that compacted coating is not necessarily coalesced, and that even full compaction of solid particles can be elastic, hence reversible, rather than viscoelastic or viscoplastic. Some of the results suggest that soluble ionic surfactant and oligomeric and grafted polymeric stabilizers at particle surfaces, collapse to undetectable dimensions during freeze-drying, yet reswell to detectable size during rewetting, if they have not dissolved into polymer particles during annealing. Presented at 2006 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, November 1–3, 2006 in New Orleans, LA.     

A modified van der Waals type equation of state

A cubic equation of the van der Waals type is presented with the critical compressibility factor taken as substance dependent. Input requirements are Pitzer's acentric factor and the critical temperature and pressure. Parameter b is treated as independent of temperature, and the temperature dependence of parameter a is given by an expression similar to that proposed by Soave. A test on vapour pressure is presented for more than one hundred substances and on liquid density for 65 substances and comparisons are given with results of Soave's and Peng-Robinson's equations. Representation of vapour pressure, particularly at low pressures, and of the liquid density has been improved. The equation is restricted to non-associating compounds.     

Adsorption behavior of cationic dyes on citric acid esterifying wheat straw: kinetic and thermodynamic profile

The kinetic and thermodynamic behaviors of cationic dye adsorption onto citric acid esterifying wheat straw (EWS) from aqueous solution were investigated. Two cationic dyes, methylene blue (MB) and crystal violet (CV) were selected as adsorbates. The kinetic and thermodynamic parameters of dye adsorption were examined with a batch system by changing various experimental factors (e.g. initial pH, EWS dosage, dye concentration, contact time, temperature). The MB and CV removal ratios came up to the maximum value beyond pH 4. The 2.0 g/L or up of EWS could almost completely remove MB and CV from 250 mg/L of dye solution. The adsorption percentages of MB and CV kept above 95% over a range from 50 to 350 mg/L of dye concentration when 2.0 g/L of EWS was used. The isothermal data followed the Langmuir model. The adsorption processes could be described by the pseudosecond-order kinetic model. The dual linear plots of intraparticle diffusion indicated that two intraparticle diffusion steps occurred in the dye adsorption processes. The thermodynamic study indicated that the adsorptions of dyes were spontaneous and endothermic. High temperatures favored the adsorption processes.     

Kinetic and thermodynamic studies of adsorption of Cu2+ and Pb2+ onto amidoximated bacterial cellulose

Removal of Cu²⁺ and Pb²⁺ from aqueous solutions by adsorption onto amidoximated bacterial cellulose (Am-BC) was investigated. The effects of pH, initial concentration, contact time and temperature were studied in batch experiments. The pseudo-first and pseudo-second orders and intraparticle diffusion equation were used to evaluate the kinetic data and the constants were determined. The experimental data fits well to the pseudo-second order kinetic model, which indicates that the chemical adsorption is the rate-determining step, instead of mass transfer. The equilibrium adsorption data were described by the Langmuir, Freundlich, and Temkin isotherms. The Am-BC showed a better fit to the Langmuir isotherm. The separation factor (R _L ) revealed the favorable nature of the isotherm. The thermodynamic parameters (ΔH _ads⁰, ΔS _ads⁰, ΔG _ads⁰) for Cu²⁺ and Pb²⁺ adsorption onto Am-BC were also determined from the temperature dependence. The values of enthalpy and entropy indicated that this process was spontaneous and exothermic. The experimental studies indicate that Am-BC would be a potential effective adsorbent to remove the metal ions from wastewater.     

A new concept for augmented van der Waals equations of state

A novel approach to perturbed equations of state for simple fluids is presented and its advantages over the traditional perturbed hard sphere equations are demonstrated by its application to several model fluids. The approach is based on a short range Yukawa reference which incorporates, in addition to repulsive interactions, also attractive interactions at short separations. The considered models of common interest are the Sutherland, Lennard-Jones, and EXP6 fluids. It is shown that using the proposed approach the reference system captures a good deal of properties of the studied fluids and that an accurate equation of state can be obtained using only the crude mean field (augmented van der Waals) approach.     

Continuum modeling of van der Waals interactions between carbon onion layers

A continuum formulation of the van der Waals interactions between carbon onion layers in form of pressure/radius relations is obtained. The relations are derived analytically considering the doubly-curved geometry of carbon onion layers. For finite layer radii it is shown that the van der Waals induced pressures on opposing faces of two adjacent onion layers are not equal and depend on both layer radii. Equilibrium configurations of different double-layer onion models are calculated using the theorem of stationarity of the total potential energy. The results are compared with those of simplified models which do not consider any curvature effects. It is shown that curvature effects have no significant influence on the equilibrium interlayer distances, but they significantly change the results for the radial displacements and, consequently, for the membrane forces in the layers. Comparison with Monte Carlo simulations of C60 in C180 and C60 in C240 shows that the derived van der Waals model represents the radial displacements and equilibrium interlayer distances better than the simplified models. Thus, whenever a good representation of the membrane forces is important, e.g., in stability analysis, the new approach should be used.     

Repulsive van der Waals Forces. II. Mechanism of Hydrophobic Chromatography

Abstract

When two materials with different interfacial free energies are immersed in a liquid with an interfacial free energy intermediate between those of the two materials, the net van der Waals forces between these two materials are repulsive. Thus by lowering the interfacial free energy of the liquid medium, solutes or particles previously adsorbed onto low energy surfaces can be readily eluted from such surfaces. This is demonstrated by the coupling to and subsequent elution from Octyl Sepharose and Phenyl Sepharose of serum and other proteins. The elution of all proteins commenced when the surface tension of the eluting liquid was decreased to a point just below that of the protein in question. The eluted serum proteins successively emerged from the column in the exact decreasing order of their own interfacial free energies. In hydrophobic chromatography, coupling is favored when the van der Waals forces between solutes (or particles) and ligand are attractive (and maximum, frequently through the admixture of salts); elution is brought about by causing the van der Waals interaction between each solute (or particle) and ligand successively to become repulsive (by gradually lowering the surface tension of the eluting liquid).     

The Effect of van der Waals and Viscous Forces on Aerosol Coagulation

An analysis is carried out to determine the combined effect of van der Waals and viscous fluid forces on coagulation of spherical aerosol particles in the free molecular, transition, and continuum regimes. The effect of viscous forces is taken into account by modifying the particle diffusion coefficient. An asymptotic solution is substituted for the classical formulation of viscous forces. The results of free molecular and continuum regimes are then extended to the transition size range by an interpolation formula.     

Reduced van der Waals equation of state and critical properties of rubbers

A reduced equation of state describing the deformation mode as simple elongation is derived for van der Waals networks, thus manifesting general relationships for molecular networks also embracing their stability characteristics and the occurrence of a phase transition.     

On the Importance of Gradient-Corrected Correlation for van der Waals Density Functionals

The van der Waals density functional (vdW-DF) family of exchange?Ccorrelation functionals is a promising step towards accounting for van der Waals interactions in density functional theory. This approach consists of a nonlocal correlation term in addition to semilocal generalized gradient approximation exchange and local density approximation correlation. It has proven useful for describing vdW bonded complexes but unfortunately deteriorates the prediction of solid-state properties such as bulk lattice parameters and cohesive energies, as compared to the underlying GGA functional. By considering a broad range of different condensed matter systems including weakly interacting complexes as well as strongly bonded molecules and bulk solids, we show that inclusion of gradient-corrected correlations in vdW-DF-type calculations may not only improve the accuracy for vdW bonded systems, but also amend vdW-DF deficiencies in predicting structural properties of solids. Based on this insight we construct a prototype vdW-DF which demonstrates high accuracy in describing the dispersive interactions responsible for benzene adsorption on the noble Au(111) surface.     

推广的van der Waals 压缩因子及其对超临界流体的应用

利用实验测得的液体的热压力系数,修正原始van der Waals模型中的排斥体积,得到了一个适用于高密度流体的推广的van der Waals压缩因子。将它与Redlich和Kwong提出的吸引项相结合,建立了一个流体状态方程。用某些有代表性的超临界流体的pVT数据检验,结果表明,其对体积、压力和逸度系数的计算准确度可与CS-RK方程相媲美。     

Gas solubilities in ionic liquids using a generic van der Waals equation of state

A family of modified van der Waals equations of state (vdW EOS) is extremely useful for many industrial applications. For example, the generic Redlich-Kwong (RK) EOS or its modification by Soave (SRK EOS) and Peng-Robinson (PR EOS) are still of popular use in industry to the present day. These two most popular (“cubic”) EOSs are based on modifications [1/(V² + bV), or 1/(V² + 2bV − b²)] of the volume dependence on the attractive part of the original van der Waals EOS [1/V²] and also modifications of the temperature dependence of the attractive “a(T)” parameter of the original EOS (constant a). It is extremely rare in actual EOS applications to use the volume dependence of the original van der Waals EOS. In the present phase equilibrium calculations, we employ such a generic vdW EOS, P = RT/(V − b) − a(T)/V², with our well-tested mixing rule for multi-component mixtures. Using the same form of the “a(T)” parameter and the mixing rule, it has been found that all generic RK, PR, and vdW EOSs can present the phase behaviors (temperature-pressure-composition diagrams) equally well. It is shown that experimental gas solubility data (CO₂, CF₃-CFH₂, SO₂, and NH₃) in room-temperature ionic liquids are well correlated with the present EOS model, and also that the phase behaviors such as LLE (liquid-liquid separations) are satisfactorily predicted.     

Molecular Formation upon Ionization of van der Waals Clusters and Implication to Astrochemistry

The presence and formation of a large variety of organic molecules in the interstellar medium is evident from both astronomical data of absorption and emission bands at different regions of the spectrum. Specifically, polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium (ISM). The mechanism for their formation in ISM‘s low temperature environment is, as of yet, a mystery nonetheless. Understanding the mechanism of formation of complex molecules such as PAHs and nitrogen-based PAHs (PANH) in the ISM is a long-standing challenge which has been drawing a growing attention for the past several decades. In this review we wish to emphasize two things: Firstly, the essential role quantum chemistry can play in the study of astrochemical reactions. Secondly, we wish to demonstrate that said variety of possibilities for chemical reaction, starting upon ionization of van der Waals clusters. The potential for different chemical reactions to occur within a cluster environment arises from the fact that such processes can take place at low temperatures as the systems pose large amounts of energy upon ionization. Moreover, the spectator molecules in the cluster can provide a dissipation route for energy by detachment from the cluster, thus the system can stabilize efficiently even at low densities. The spectator molecules can also change the potential energy surface, by which it will pose a catalytic effect for certain reactions. We will demonstrate this by presenting Ab Initio Molecular Dynamic results on ionization of small acetylene clusters.