Calculations of van der Waals forces in thin liquid films using Lifshitz' theory

For the study of thin, free liquid films (soap films) van der Waals dispersion forces were calculated from Lifshitz' theory for some three-layer models with film thicknesses between 5 and 150 nm. The complete expression as given by [2.] was used to calculate the force, the energy, and the second derivative of the energy after the thickness, as a function of film thickness. The second derivative of the energy after the thickness is needed in light scattering of soap films. The calculations are based on the dielectric data of [14.]. Some effects of the hydrocarbon layers on and electrolyte in the aqueous layer are considered. In order to make the results readily usable, the calculations are presented in the form of accurate empirical equations.     

Dewetting induced by complete versus nonretarded van der Waals forces

Spontaneous rupture of some polymer films upon heating is commonplace. The very criterion for this instability is the system free energy, G(L), possessing a negative curvature. In films that are apolar with h < or = 100 nm van der Waals (vdW) interactions usually constitute a major contribution to G(L) for which the approximate form G(L) = -A/12piL(2) (where A is the Hamaker constant), ignoring retardation, has been widely used. In this work, we investigate the limits to this approximation by calculating the complete vdW interactions for popular polymer film systems in dewetting experiments including air-polystyrene-SiO2-Si, air-polystyrene-poly(methyl methacrylate)-Si, and air-poly(methyl methacrylate)-polystyrene-Si based on the theory of Dzyaloshinskii, Lifshitz, and Pitaevskii (DLP). We found that retardation effects could produce significant modifications to G(L) even when the thickness of the polymer and/or the interlayer is only 1-2 nm, contrary to conventional presumption.     

Direct numerical simulation of aerosol coagulation with van der Waals forces

The van der Waals interaction energy has been computed for a large number of silver and water particle pairs in the range of 1 to 15 nm radius by first calculating the exact, retarded Hamaker factor for semi-infinite bodies and then applying a geometric correction for sphericity. The enhancement of particle collision rates has been deduced from these calculations. These values for the collisional enhancement were used in a direct numerical simulation of the coagulation of silver and water aerosols. Despite the size dependence of the van der Waals enhancement factors, the aerosol size distributions quickly attained nearly self-preserving forms.     

van der Waals forces between nanoclusters: importance of many-body effects

van der Waals interactions between nanoclusters have been calculated with a self-consistent, coupled dipole method. The method accounts for all many-body (MB) effects. Comparison is made between the exact potential energy, V, and the values obtained with two alternative methods: the sum of two-body interactions and the sum of two-body and three-body interactions. For all cases considered, the three-body term alone does not accurately represent the MB contributions to V. MB contributions are especially large for shape-anisotropic clusters.     

Unified treatment of chemical and van der Waals forces via symmetry-adapted perturbation expansion

We propose a symmetry-adapted perturbation theory (SAPT) expansion of the intermolecular interaction energy which in a finite order provides the correct values of the constants determining the asymptotics of the interaction energy (the van der Waals constants) and is convergent when the energy of the interacting system is submerged in the continuum of Pauli-forbidden states-the situation common when at least one of the monomers has more than two electrons. These desirable features are achieved by splitting the intermolecular electron-nucleus attraction terms of the Hamiltonian into regular (long-range) and singular (short-range) parts. In the perturbation theory development, the regular part is treated as in the conventional polarization theory, which guarantees the correct asymptotics of the interaction energy, while the singular part is weakened sufficiently by an application of permutational symmetry projectors so that a convergent perturbation series is obtained. The convergence is demonstrated numerically, for both the chemical and van der Waals minima, by performing high-order calculations of the interaction energy of the ground-state lithium and hydrogen atoms-the simplest system for which the physical ground state is submerged in the Pauli-forbidden continuum. The obtained expansion enables a systematic extension of SAPT calculations beyond second order with respect to the intermolecular interaction operator.     

Augmented van der Waals equations of state: SAFT-VR versus Yukawa based van der Waals equation

Performance of the SAFT-VR equation of state developed for the hard sphere based simple fluids, namely the square-well, Sutherland and Yukawa fluids, is examined by comparing its results with simulation data and an augmented van der Waals (vdW) equation based on a Yukawa (Y) reference. Its shown that both for the equilibrium vapor-liquid data and data along selected isotherms in the liquid and supercritical fluid phases the vdW(Y) equation provides better results, particularly when going to lower temperatures.     

Cyclic tetraselenadiynes: rigid cycles with long-range van der Waals forces between chalcogen centers

The synthesis of cyclic tetraselenadiynes could be achieved by a stepwise approach. Key steps were the reaction of the lithium salt of trimethylsilylacetylene (1) with alpha,omega-diselenocyanatoalkanes 2(m) (m = 2-5). By treating the bis-lithium salt of the resulting alpha,omega-diselenaalkadiynes 4(m) (m = 2-5) again with 2(n) (n = 2-5) the cyclic tetraselenadiynes 5(m.n) resulted, with methylene chains of length m and n between the SeC triple bond CSe units. The structures of seven ring systems could be investigated in the solid state. These investigations reveal that the molecular structures are determined by the rigid SeC triple bond CSe units, which try to adopt torsion angles of the CH(2)-Se sigma-bonds between 60 degrees and 90 degrees. In the solid state, the systems 5(3.3) and 5(5.5) show columnar structures that can be traced back to close contacts between Se atoms of neighboring rings.     

Effect of ionic van der Waals forces on the diffuse potential of model colloids

In this work, the role of ionic dispersion forces in specific ion effects is evaluated through Monte Carlo simulations in the primitive model. More specifically, we assess the effect of such forces on the diffuse potential, since this property is essential to understand the electrokinetic behavior of colloids. In this way, ion specificity arises naturally since ion dispersion forces depend on ionic polarizability, which differs for ions with the same valence. This property is included in the primitive model by means of the Lifshitz theory. The results for different ions are summarized with the help of a nondimensional parameter characterizing the relative weight of ionic van der Waals interactions. Our data reveal that for small ions with high polarizability the ionic dispersion forces can considerably contribute to the specificity of the diffuse potential. In any case, the specific ion effects due to ion polarizability are strongly influenced by ion size.     

Improved evaluation of liquid densities using van der Waals molecular models

A new approach is presented to estimate molar volumes and densities of liquids in ambient conditions from van der Waals models, taking advantage of the correlation between the intermolecular volume and the atomic contributions to the molecular surface area. Using this approach, the role of hydrogen bonds can be quantified. The densities obtained prove remarkably close to the values derived from the ACD group contribution method. However, the present approach requires much less empirical parameters and may be applied to arbitrary H-C-N-O-F-S-Cl-Br compounds. Moreover, it is more reliable than earlier empirical procedures, including quantitative structure property relationships (QSPR). While it does not correct the deficiencies of group contribution methods, it provides a natural approach to introduce temperature effects.     

Analytical solution for Newtonian flow inside carbon nanotube taking into consideration van der Waals forces

In this paper, we study the flow of water inside a carbon nanotube of radius 20 Å taking into account the molecular interactions between water and the nanotube, and the slip boundary condition. Incompressible and Newtonian fluid is employed, and the radial, axial and slip velocities are derived analytically for an impermeable wall. Both radial and axial velocities are found to obey the prescribed boundary conditions, and the axial velocity and the flux turn out to be approximately sevenfold larger when the pressure generated by the tube entry is considered.     

Upper and lower bounds to polarizabilities and van der Waals forces I. General theory

Upper and lower bounds to polarizabilities, and long-range interactions of atoms and molecules are derived using operator inequalities. The formulae for the dispersion and the three-body non-additive forces are given in terms of the S(k), related by sum rules to properties of the separated systems. Some known approximations are obtained but with additional information about their nature, as well as several new inequalities. Their interest is that they give explicit and rigorous bounds involving quantities available from experimental data.     

Capillary and van der Waals forces between uncharged colloidal particles linked by a liquid bridge

This work presents a theoretical study of the forces established between colloidal particles connected by means of a concave liquid bridge, where the solid particles are partially wetted by a certain amount of liquid also possessing a dry portion of their surfaces. In our analysis, we adopt a two-particle model assuming that the solids are spherical and with the same sizes and properties and that the liquid meniscus features an arc-of-circumference contour. The forces considered are the typical capillary ones, namely, wetting and Laplace forces, as well as the van der Waals force, assuming the particles uncharged. We analyze different parameters which govern the liquid bridge: interparticle separation, wetting angle, and liquid volume, which later determine the value of the forces. Due to the dual characteristic of the particles' surfaces, wet and dry, the forces are to be determined numerically in each case. The results indicate that the capillary forces are dominant in most of the situations meanwhile the van der Waals force is noticeable at very short distances between the particles.     

van der Waals interactions across stratified media

Working at the Lifshitz level, we investigate the van der Waals interactions across a series of layers with a periodic motif. We derive the complete form of the van der Waals interaction as an explicit function of the number of periodic layers. We then compare our result with an approximation based on an anisotropic-continuum representation of the stratified medium. Satisfactory agreement between discrete-layer and continuum models is reached only for thicknesses of ten or more layers.     

Capillary pressure of van der Waals liquid nanodrops

The dependence of the surface tension on nanodrop radius is important for the new-phase formation process. It is demonstrated that the famous Tolman formula is not unique and the size-dependence of the surface tension can distinct for different systems. The analysis is based on a relationship between the surface tension and disjoining pressure in nanodrops. It is shown that the van der Waals interactions do not affect the new-phase formation thermodynamics since the effects of the disjoining pressure and size-dependent component of the surface tension cancel each other.     

The perturbation calculation of van der Waals potentials

Summary The unsymmetrized perturbation theory for interaction potentials is reformulated in such a way that the overlap and exchange effects can be taken into account in a satisfactory and conceptually simple way. This formulation, known as the generalized Heitler-London theory, its shown to be valid regardless of the ultimate limit to which the polarization approximation converges. Within the framework of this theory, the van der Waals potential of the triplet H₂(³ _u ) state is calculated and shown to be in excellent agreement with the exactab initio results. Both the exchange energy and the polarization energy are obtained from a perturbation calculation.     

Nonadditivity in van der Waals interactions within multilayers

Working at the macroscopic continuum level, we investigate effective van der Waals interactions between two layers within a multilayer assembly. By comparing the pair interactions between two layers with effective pair interactions within an assembly we assess the significant consequences of nonadditivity of van der Waals interactions. This allows us to evaluate the best numerical estimate to date for the Hamaker coefficient of van der Waals interactions in lipid-water multilamellar systems.     

Phototransformation of stilbene in van der Waals nanocapsules

We have utilized para-hexanoylcalix[4]arene nanocapsules as hosts to carry out phototransformations of cis- and trans-stilbene. Single-crystal X-ray diffraction studies were performed to define precisely the location of encapsulated stilbenes inside the capsule and to analyze possible pathways of phototransformation. cis-Stilbene stacks as a pi-pi dimer located at the center of the capsule, whereas trans-stilbene does not form such a dimer. Irradiation of the crystalline inclusion complexes of each isomer of stilbene in the solid state leads to the appearance of the second isomer, and after prolonged photolysis, photodimerization also occurs. syn-Tetraphenylcyclobutane is formed as the major product of dimerization and its yield depends on the time and intensity of irradiation. In most cases, the single crystals of the complexes remain intact during irradiation; hence, the nanocapsules have the potential to serve as robust nanoreactors in the solid state. The confinement in the nanocapsules is sufficient to keep the reacting molecules together, although this is less restrictive than for trans-stilbene crystals, in which the molecules cannot achieve a favorable orientation for dimerization.