London-van Der Waals Attraction Research Articles

London Dispersion Corrections to Density Functional Theory for Transition Metals Based on Fitting to Experimental Temperature-Programmed Desorption of Benzene Monolayers.

Standard implementations of generalized gradient approximation (GGA)-based density functional theory (DFT) describe well strongly bound molecules and solids but fail to describe long-range London dispersion or van der Waals (vdW) attraction interactions that are important in molecular crystals and two-dimensional solids. To provide accurate values for the vdW distance and energies for the metals Cu, Ag, Au, Ni, Pd, and Pt, we determined empirical vdW corrections to Perdew, Burke, and Ernzerhof (PBE) DFT by fitting the experimental adsorption enthalpies measured by temperature-programmed desorption (TPD) from benzene monolayers by Campbell and co-workers ( J. Phys. Chem. C 2016, 120, 25161-25172). Benzene physisorbed to these metals without chemical reaction; therefore, we consider the bonding to be vdW. We use the low gradient form for the vdW corrections, EvdW-LG = -C6LG/[R6 + RvdwLG6] with just two parameters per atom (C6LG and RvdwLG). This LG form leads to negligible changes in bond distances and angles, so adjusting the parameters should not sacrifice accuracy for the bonding interactions. We demonstrate that the parameters fitted to benzene also describe well the physisorption enthalpies for other hydrocarbons (naphthalene, cyclohexane, methane, ethane, and propane) on Pt. We also report low gradient vdW correction parameters for the noble gases that fit the equilibrium lattice parameter and heat of vaporization of the crystals.

Vacuum Friction on a Rotating Pair of Atoms.

Zero-point quantum fluctuations of the electromagnetic vacuum create the widely known London-van der Waals attractive force between two atoms. Recently, there has been a revived interest in the interaction of rotating matter with the quantum vacuum. Here, we consider a rotating pair of atoms maintained by London-van der Waals forces and calculate the frictional torque they experience due to zero-point radiation. Using a semiclassical framework derived from the fluctuation dissipation theorem, we take into account the full electrostatic coupling between induced dipoles. Considering the case of zero temperature only, we find a braking torque proportional to the angular velocity and to the third power of the fine structure constant. Although very small compared to London-van der Waals attraction, the torque is strong enough to induce the formation of dimers in binary collisions. This new friction phenomenon at the atomic level should induce a paradigm change in the explanation of irreversibility.

Metastable states and activated dynamics in thin-film adhesion to patterned surfaces.

We consider adhesion due to London-van der Waals attraction between a thin film and a patterned surface with nanometer asperities. Depending on the surface topography and the stiffness of the film, three regimes of adhesion are identified: complete contact adhesion, partial contact adhesion, and glassy adhesion. For complete contact adhesion, the film conforms to the undulations of the surface, whereas for partial contact and glassy adhesion, the adhesive interface breaks down into microscopic areas of contact. When a film in the glassy regime is peeled off the surface, metastable states develop at which the crack front becomes arrested, analogously to the frustrated motion of the three-phase contact line across a heterogeneous surface. For this glassy regime, we use transition state theory to model the thermally activated progression of the crack front. This theoretical treatment suggests that the rate of the adhesive failure increases exponentially with the applied force.

Zeta-Potential Investigation and Experimental Study of Nanoparticles Deposited on Rock Surface To Reduce Fines Migration

Summary Fines migration is a noticeable problem in petroleum-production engineering. Plugging of throats in porous media occurs because of detachment of fine particles from sand surfaces. Thus, the study of interactions between fines and pore surfaces and the investigation of governing forces are important factors to consider when describing the mechanism of the fines-release process. The main types of these forces are electric double-layer repulsion (DLR) and London–van der Waals attraction (LVA). It may be possible to alter these forces with nanoparticles (NPs) as surface coatings. In comparison with repulsion forces, NPs increase the effect of attraction forces. In this paper, we present new experiments and simple modeling to observe such properties of NPs. For this purpose, the surfaces of pores were coated with different types of NPs: magnesium oxide (MgO), silicon dioxide (SiO2), and aluminum oxide (Al2O3). A zeta-potential test was used to examine changes in the potential of the pore surfaces. Total interaction energy was then mathematically calculated to compare different states. Total interaction energy is a fitting criterion that gives proper information about the effect of different NPs on surface properties. Consequently, total interaction plots are found to be suitable tools for selecting the best coating material. On the basis of experimental results, the magnitude of change in zeta potential for the MgO NP was 45 mV. Our model demonstrated that the magnitude of the electric DLR in comparison with the LVA of the probe and plate surface was considerably diminished when MgO NPs were used to coat the surface of the plate, which agrees completely with our experimental observation.

Clay Swelling and Colloid Stability. Martin Smalley. CRC Press, Taylor and Francis Group, (2006), 272 pp. [ISBN 0-8493-8079-0] Price $139.95.

The DLVO (Derjaguin-Landau-Verwey-Overbeek) theory, formulated by Derjaguin and Landau (1941) and later independently by Vervey and Overbeek (1948), laid the foundations for our understanding of particle interactions in colloidal lyophobic dispersions and their kinetic stabilities. The configuration of the electrical double layers had been described previously by the Gouy-Chapman model with refinements later by many other investigators. In this model the counterions were reduced to point charges, liquid media were represented by a uniform dielectric constant, and colloidal particles by flat and uniformly charged large surfaces. The particle interactions in DLVO theory were assumed to be essentially controlled by two opposing forces, the screened Coulombic repulsion and by the London-van der Waals attraction. These oversimplifications have caused several problems in the application of the DLVO theory since its inception. For the last 50 years, various refinements and corrections were proposed to explain these discrepancies or to reconcile the theory with the observed data. This monograph calls an end to these corrections for the DLVO theory both on experimental and theoretical grounds. It is, therefore, a very important publication. It will be beneficial to the reader if I present this review in two parts: Part 1 presents the author’s own experimental data collected over the years that led him down the path for the formulation of a new theory for the stability of hydrophobic colloids. Chapters 1–7 are devoted to this section which is the essence of the monograph. Part 2 discusses the configurations of counterions and water molecules in the interfaces between macro-ions i.e. charged platy colloidal particles in Chapters 8–10. The effects of a special polymer, PEO, on the stability of n -C4H9NH+3-vermiculite dispersion, on its interlamellar separation, and the stacking order and crystallinity of the vermiculite layers are examined in Chapters …

Hydration forces between silica surfaces: Experimental data and predictions from different theories

Silica is a very interesting system that has been thoroughly studied in the last decades. One of the most outstanding characteristics of silica suspensions is their stability in solutions at high salt concentrations. In addition to that, measurements of direct-interaction forces between silica surfaces, obtained by different authors by means of surface force apparatus or atomic force microscope (AFM), reveal the existence of a strong repulsive interaction at short distances (below 2 nm) that decays exponentially. These results cannot be explained in terms of the classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, which only considers two types of forces: the electrical double-layer repulsion and the London-van der Waals attraction. Although there is a controversy about the origin of the short-range repulsive force, the existence of a structured layer of water molecules at the silica surface is the most accepted explanation for it. The overlap of structured water layers of different surfaces leads to repulsive forces, which are known as hydration forces. This assumption is based on the very hydrophilic nature of silica. Different theories have been developed in order to reproduce the exponentially decaying behavior (as a function of the separation distance) of the hydration forces. Different mechanisms for the formation of the structured water layer around the silica surfaces are considered by each theory. By the aid of an AFM and the colloid probe technique, the interaction forces between silica surfaces have been measured directly at different pH values and salt concentrations. The results confirm the presence of the short-range repulsion at any experimental condition (even at high salt concentration). A comparison between the experimental data and theoretical fits obtained from different theories has been performed in order to elucidate the nature of this non-DLVO repulsive force.

Mass transfer of volatile organic carbons through aqueous foams

A model is developed to study diffusive mass transfer of hydrocarbon vapor through a flexible foam blanket. The model accounts for the diffusion of hydrocarbon vapor through gas-phase and liquid lamellae, the combined gravity and capillary drainage from the plateau border, the thinning of foam lamellae caused by the forces of capillary suction, London–van der Waals attraction, and electrostatic double-layer repulsion, and foam collapse. Uniform bubble size is assumed, and hence, interbubble gas diffusion arising out of variation in bubble sizes alone is not incorporated into the model. A high-stability aqueous foam formulation that remains stable in the presence of oil (hexane) at foam–oil contact was developed using surfactants, stabilizers, and viscosifiers. Emission of hexane vapor through the foam was measured. The model predicts that the initially taller foam columns collapse faster. Their mass-transfer resistance is higher before the onset of collapse but not very different from that of the shorter foam columns at long times. If the solubility and diffusivity of the hexane gas in the foam liquid are unaffected, the foams with higher viscosities persist longer and provide greater diffusive mass-transfer resistance. Foam bubble size does not significantly impact the mass-transfer resistance of the foam column before the onset of foam collapse. However, the foams with smaller bubbles collapse earlier, and their ability to act as a mass-transfer barrier to the diffusing hydrocarbon vapor diminishes rapidly. The experimental results compared reasonably with the model for varying initial foam heights and bubble sizes.

Hydrodynamic and surface interaction forces on a particle in a pore

The interaction between a spherical particle in a pore near a constriction and the wall is calculated in order to understand the behavior of dispersions on their transport through a porous medium. A simplified geometry of the pore is assumed which permits, for a spherical particle: 1. (a) Analytical calculation of the London-van der Waals attraction; 2. (b) Calculation of electrostatic repulsion by numerical solution of the Poisson-Boltzmann equation and application of the Langmuir equation for the repulsive pressure; and 3. (c) Calculation of hydrodynamic interactions by numerical solution of the Navier-Stokes equation by means of a finite element method. The method permits solution of the Navier-Stokes equation, in the low Reynolds number range (e.g., Re = 5 × 10 −4), as long as the closest distance between the particle and the wall is at least 0.3 μm. The results indicate that under the conditions covered by the present calculations, for a stationary particle near a pore constriction, the hydrodynamic interactions by far predominate over colloid chemical interactions.

Spectral analysis of light scattered by liquid films. II. Light scattering in the long wavelength limit

This paper describes what information can be obtained when the spectral distribution of light scattered by thin liquid films, like free soap films, is analyzed. To this end we have worked out the so-called long wavelength limit (i.e., when the wavelengths of the surface waves are much greater than the film thickness) of the thermal fluctuation power spectrum describing the dynamics of the interfacial displacements occurring on a film. The analysis reveals that the squeezing mode spectrum as well as the bending mode spectrum exhibit a three peak structure: two peaks shifted symmetrically around zero frequency (Brillouin doublet) and one peak centered around zero frequency (pure diffusive component). For the squeezing mode the diffusive component, which is typical for a film, is strong, whereas for the bending mode the energy contained in the Brillouin doublet is large when compared with the central peak. These observations and also the large difference in characteristic time scales, in which the two modes occur, enables one to study their features separately by means of dynamic light scattering. The width of the central peak of the squeezing mode depends on: the film thickness, the viscosity of the film liquid, the interfacial tension, and the second derivative with respect to film thickness of the free energy of film formation. The free energy used in this paper consists of a sum of electrical double layer repulsion and London–van der Waals attraction. The effect of the so-called surface elasticity on the central peak width of the squeezing mode is calculated. We also derived the thermal fluctuation power spectrum for interfacial waves when the film is suspended in a second medium. It turns out that for the bending mode the effect of the film bounding medium on the frequency shift of the Brillouin doublet is important, even if this is a low density vapor phase. The shift frequency for the bending depends on the film tension and the total mass that participates in this kind of fluctuation. The width of these shifted peaks depends mainly on the properties of the film bounding medium.

Spectral analysis of light scattered by liquid films. I. General considerations

This paper deals with dynamic light scattering from liquid films. We have obtained the thermal fluctuation power spectrum of surface waves on films by solving the electrohydrodynamic equations of motion where the interaction forces (electrical double layers repulsion and London–van der Waals attraction) are included in the momentum equation. The resulting power spectrum for the mixed transversal-longitudinal surface waves can be split up into two contributions: a squeezing mode part and a bending mode part. In this paper the general expressions for both modes are discussed for arbitrary film thicknesses and arbitary frequencies in the hydrodynamic regime. When the film is thick the spectrum exhibits the well known two peak structure of a single fluid interface. When the film becomes thin the spectrum consists of three peaks for each mode, i.e., the time evolution of both modes comprises a purely diffusive and a propagating component. It is found that the squeezing mode shows an intense diffusive peak, whereas for the bending mode the propagating peak is strong compared with the diffusive peak.

Model calculations on vertical common black equilibrium soap films: The relation of contact angle to Fresnel diffraction patterns from the film-border transition

Profiles of vertical Common Black soap films were calculated numerically, taking into account double-layer repulsion and London-van der Waals attraction. The definition of a contact angle is discussed and attention is payed to the case that no contact angle can be defined. From the profiles Fresnel diffraction patterns were calculated. It turned out that (1) they are not very sensitive for the details of the transition region, between a film and its meniscus, (2) for contact angles larger than 16′ the results of H. M. Princen and S. Frankel ( J. Colloid Interface Sci. 35, 368 (1971)) coincide with ours within 5%, (3) this optical method will not result in reliable contact angles when they are smaller than about 10′.

A comprehensive equation for the rate of adsorption of colloidal particles and for stability ratios

A general correlating equation which can be used for predicting adsorption rates of colloidal particles on substrates of various geometry under a wide range of double layer interactions is presented. This equation is applicable as long as the depth of the secondary minimum in the surface interaction energy profile is low (⪷0.5 k B T) and can be used in the case of constant-potential or constant-charge-density double layer interactions or when a suitable combination is appropriate. The following parameters are needed for using the equation: the height of the potential barrier, V max; the magnitude of the London-van der Waals attraction, A/6 k B T (where A is the Hamaker constant); the Peclet number, Pe (which represents the influence of convection); and the ratio or radii of the two interacting surfaces. Four special cases, for adsorption on spherical and cylindrical substrates and on rotating discs, and for mutual coagulation, are derived from the general equation as illustrative examples. The extensions of the equation for adsorption in porous media and fibrous mats (useful in the theory of physical separation processes) are also presented. Since the correlating equation is a closed-form expression, it offers a convenient method for determining the effective potential barrier (the activation energy for adsorption) from experimentally determined adsorption rates.

On enzymatic clotting processes. Part 3.—Flocculation rate constants of paracasein and fibrin

Flocculation rate constants of micellar paracasein and fibrin have been determined from the lag phase in the enzymatic clotting. Without exception these rate constants turn out to be very small. Different arguments are put forward to demonstrate that the retardation in the clotting of paracasein is chiefly of a steric nature. The activation energy of the clotting process becomes negative beyond 310 K, which is ascribed to the growing influence of the London–van der Waals attraction as the micelles become discharged by the enzyme.

The London-Van der Waals attraction constant of secretory granules and its significance

The DLVO theory of colloidal particle interaction has been applied to electrokinetic data from isolated chromaffin granules to obtain an estimate of the London-Van der Waals attraction constant. Interaction energy curves were derived for chromaffin granules and their significance discussed in relation to exocytosis.

An experiment concerning the dispersion forces between very small metal spheres

The London-Van der Waals attraction between very small spheres is determined in an experiment with colloidal particles. The measured value of the attraction constant is a function of the particle size. It is ten times smaller than values calculated with the macroscopic theory of dispersion forces on the basis of data for a bulk material.

Effect of Concentration and Intermolecular Forces on the Sedimentation of Polystyrene Spheres

At low concentration, the sedimentation coefficient s can be related to the sedimentation coefficient at zero concentration s0 through a power series expansion in the volume fraction Φ, i.e., s = s0(1 + KΦ···). We discuss the calculation of the constant K for sedimenting charged spheres. The spheres interact hydrodynamically as well as through a potential consisting of a hard-core repulsion, a screened Coulomb repulsion, and a London–van der Waals attraction. To calculate K one must know the value of the electrostatic potential in the vicinity of the surface of the sedimenting sphere ψδ, and the Hamaker constant, the constant which measures the strength of the London–van der Waals attraction. It is shown that knowledge of any two of the three quantities K, ψδ, and the Hamaker constant allows one to find the third. It is also shown that if both ψδ and the Hamaker constant are unknown the experimental determination of K can be used to obtain upper and lower bounds on the Hamaker constant. Using Cheng and Schachman's experimental value of K for polystyrene latex spheres, the following bounds for the Hamaker constant for polystyrene in aqueous solution are found: 3–8 × 10−14 erg. These bounds are in good agreement with previous experimental determinations.

Force-balances and stability in hexagonally-packed polyelectrolyte systems

The London-van der Waals attraction is calculated for long cylinders of finite diameter, and is compared to the electrical double-layer repulsion for the cylinders, taken to have a uniform surface charge, in the absence of salt and in an excess of salt. It is shown that in principle a stable balance can be achieved between the attractive force and the repulsive force for hexagonally-packed, charged cylinders in the ionic solution. The position of this balance depends critically on the value of (a) the van der Waals constant, (b) the surface potential of the polyelectrolyte, and (c) the Debye-Hückel parameter, but the distance between surfaces at the stable balance point can be of the same order as the diameter of the cylinders themselves. It is suggested that there are a number of biological systems where this model is relevant; in particular muscle fibres and microtubule systems.

Classical coagulation. London-van der Waals attraction between macroscopic objects

Classical coagulation. London-van der Waals attraction between macroscopic objects

The London—van der Waals attraction between spherical particles

Summary Frequently we experience the existance of adhesive forces between small particles. It seems natural to ascribe this adhesion for a large part to London-v.d. Waals forces. To obtain general information concerning their order of magnitude the London-v. d. Waals interaction between two spherical particles is computed as a function of the diameters and the distance separating them. A table is calculated which enables numerical application of the formulae derived. Besides approximations are added, which may be used when the distance between the particles is small. In a separate section it is investigated how the results must be modified, when both particles are immersed in a liquid. Here we are led to the important conclusion that even in that case London-v. d. Waals forces generally cause an attraction.