London-van Der Waals Forces Research Articles

Theory for particle collection under london and gravity forces

Theoretical predictions of particle capture from suspensions flowing around spherical, rotating disc and cylindrical collectors are presented. These results are intended to describe collection of non-Brownian particles under negligible double layer forces from water, primarily for application to deep bed filtration. The predictions are obtained from numerical solutions of the trajectory equations of Spielman and Goren for motion of a small particle near a much larger collector with hydrodynamic, gravitational and London-van der Waals forces considered simultaneously. Electrical double layer forces are introduced only to establish a criterion for their neglect. Comparison with experiment is presented in a subsequent paper.

Unretarded London-van der Waals Forces Derived from Classical Electrodynamics with Classical Electromagnetic Zero-Point Radiation

The unretarded London-van der Waals force between a neutral polarizable particle and a conducting wall, and between two neutral polarizable particles are derived from classical electromagnetism under the assumption that the universe contains fluctuating classical electromagnetic radiation with a Lorentz-invariant spectrum, (classical electromagnetic zero-point radiation). The classical derivations correspond in detail to the familiar charge-fluctuation arguments which have been advanced previously as qualitative semiclassical descriptions of quantum calculations of these forces.

Adsorption of n-Alkanes by Wyoming Montmorillonite

MUCH is known about the interlamellar adsorption of both inorganic and organic cations and polar organic molecules by clay minerals, but there is little information about the interlamellar adsorption of non-polar saturated hydrocarbons by expansible clays. Indeed, there seems to be little or no positive evidence for such adsorption. MacEwan1 found no interlamellar adsorption of n-hexane or n-heptane by montmorillonite after boiling the mineral in the organic liquids and he concluded that polar or polarizable groups are necessary to cause penetration and separation of montmorillonite sheets and hence the formation of complexes. Similarly, Barshad2 stated that the lack of expansion in montmorillonite on treatment with saturated aliphatic hydrocarbons was attributable to their non-polar nature. It was thus to be expected that adsorption of non-polar molecules like n-hexane would occur only on external surfaces as a result of “dispersion” or London-Van der Waals forces.

Structuromechanical properties of plastic powder mixtures containing a carboxymethylcellulose plasticizer

1. A study was made of the structuromechanical properties of plastic powder mixtures containing a 30% aqueous solution of CMC. It was established that structuromechanical constants sharply fall with a rise in plasticizer content, from their maximum values at the minimum amount of CMC (lower limiting content) to values corresponding to the constants of the plasticizer (upper limiting content). 2. Structure formation in a plastic powder mixture is the result of adhesion forces acting between the powder particles and the plasticizer and of molecular attraction forces (van der Waals-London forces) acting between the powder particles through a layer of the plasticizer in regions with excess surface energy. 3. Storage alters the structuromechanical properties of a powder mixture containing a 30% aqueous solution of CMC, the most marked increase being shown by the viscosity of the mixture. Remixing restores almost completely the initial properties of the mixture (indicating that the latter exhibits thixotropy).

Phospholipids, fatty acids and fatty aldehydes in rat brain subcellular fractions

Summary Subcellular fractions of rat brain were prepared by differential and sucrose density gradient centrifugation and examined for their lipid composition. Two fractions, retained by 0.9 and 1.0 M sucrose, respectively, had the morphological characteristics of synaptic membranes. The former was characterized by a high concentration of serine phosphoglyceride. The lipids (sphingomyelin excepted) of both synaptic membrane fractions differed from the lipids of myelin and mitochondria in that fatty acids had shorter chain lengths and lesser degrees of unsaturation. Charged phospholipids may play a role in attracting soluble enzymes to membrane fractions. An example is the high relative specific activity of carboxylic ester hydrolases and acetylcholinesterase in synaptic membranes. The data suggest that the short chain length of fatty acids in synaptic membranes militates against strong London-van der Waals forces, while the high concentration of serine phosphoglyceride in membranes retained by 0.9 M sucrose promotes ionic interaction and formation of phospholipid-enzyme complexes.

Structuromechanical properties of plastic powder mixtures. Part I

1. A study was made of the structuromechanical properties of plastic powder mixtures containing 20% starch paste. It is demonstrated that the powder particles are joined to the plasticizer by adsorption bonds, while bonding between the particles themselves is effected by the van der Waals-London forces through plasticizer layers in areas with a disturbed structure, which possess free surface energy. 2. The effects of the amount of plasticizer and the specific surface area, particle shape, and material of the powder on the structuromechanical properties of plastic powder mixtures have been examined.

The stability of amorphous colloidal silica

The stability of amorphous silica hydrosols has been studied in the presence of alkali metal cations at various pH's. Results show that silica is very stable within appropriate pH ranges, notwithstanding the presence of very high electrolyte concentrations (sometimes reaching saturation). Moreover, coagulation is often reversible. Although all the alkaline cations are able to coagulate silica between pH's about 7 and 11, they can be classified into two groups according to their behavior in more alkaline media: above pH 11, Li + and Na + cations bring coagulation whereas K +, Rb +, and Cs + remain ineffective. It is suggested that the attraction forces responsible for coagulation are related to acid-base interparticle bonding of two types (salt bonding by acidic cations and silanol-dissociated silanol bonds) rather than to London-van der Waals forces.

Collision efficiency of equal spherical particles in a shear flow. The influence of London-van der Waals forces

A theoretical and experimental approach to the investigation of the influence of London-Van der Waals forces on orthokinetic flocculation is described. The theoretical treatment evaluates the effect of these forces on the interaction of two spherical particles in a laminar shear flow, to give a value of the collision efficiency (stability ratio). The experimental measurements on the flocculation of monodisperse polystyrene beads in a laminar shear flow reported provide a test of the theory. By making use of the theoretical approach described in this paper the London-Hamaker constant for polystyrene in aqueous suspension can be calculated from measurements of orthokinetic flocculation : the values obtained are in good agreement with those derived by other methods. The method additionally provides a means of measuring the “characteristic wavelength” of these forces.

Composition and energy relationships for some thin lipid films, and the chain conformation in monolayers at liquid-liquid interfaces

Optically black films have been formed in aqueous media from solutions of glyceryl mono-oleate in aliphatic hydrocarbons. The thicknesses of the hydrocarbon cores of the films were estimated from electrical capacitance measurements and the compositions from interfacial tension data. The thicknesses and the compositions were found to be interrelated in a simple way and were markedly dependent on the chain length of the hydrocarbon solvent.An electrical potential applied across a liquid film subjects it to a large compressive force under which most types of film became significantly thinner. From thickness measurements in applied fields the strengths of the steric interactions which stabilize the films were calculated. From a knowledge of the steric interaction, together with an estimate of the London-van der Waals forces from contact angle measurements, the curve of potential energy against film thickness has been calculated for one system. The magnitude of the steric interaction at a given film thickness varies considerably for films of different solvent content. As a consequence, a general picture of the time-average conformations of the hydrocarbon chains of glyceryl mono-oleate in the black films and at different hydrocarbon/water interfaces may be deduced.

NMR Chemical Shifts and Coupling Constants as Probes into London–van der Waals Forces

NMR Chemical Shifts and Coupling Constants as Probes into London–van der Waals Forces

NMR coupling constants as probes into London-van der Waals interactions

Dispersion forces make small but significant contributions to nuclear magnetic resonance coupling constants. The heteronuclear 117SnCH, 119SnCH, and 207PbCH coupling constants in tetramethyllead and tetramethyltin are very well correlated with the change in the dispersion interaction. Measurements are performed in n-heptane-carbon disulfide mixtures differing markedly in their refractive indices, but not in their dielectric constants. It is thus possible to isolate a contribution to the coupling parameter from London-van der Waals forces by using nonpolar solutes in nonpolar solvents. This contribution appears to be of a similar magnitude to the reaction field and/or Stark terms proposed for polar solutes in polar media. This contribution is to be considered in addition to these terms when the solvent dependencies of coupling constants for polar solutes are being analyzed.

London-van der Waals Force between Glass Plate and Its Film Fragments

London-van der Waals Force between Glass Plate and Its Film Fragments

The role of the plasma membrane in the secretion of milk fat

On the basis of electron microscopic evidence, Bargmann & Knoop (1959) proposed that milk fat is secreted from the lactating cell by progressive envelopment of the fat droplet with plasma membrane at the apex of the cell. Ultimately the fully wrapped globule is pinched off from the cell. Support for this hypothesis is presented here in the form of biochemical data together with a rationale for the biophysics of the process. The biochemical evidence indicates that fat droplets within the cell are relatively devoid of certain membrane constituents, particularly phosphatidyl ethanolamine and carotenoids characteristically present in the surface coat of secreted milk fat globules. From a combination of compositional analyses and turnover studies with 14C-fatty acid, the logical origin of these components is the plasma membrane of the cell. The envelopment of the droplet appears adequately explained on the basis of London-van der Waals forces which are estimated to achieve an attraction force of one atmosphere when the separating distance between membrane and droplet is reduced to 20 Å. Milk proteins appear to be secreted by emptying of vacuoles through the plasma membrane. Membranes around such vacuoles may replenish the plasma membrane used in fat secretion, thus relating the two processes.

THE MECHANISM OF ADHESION OF CELLS TO GLASS. A STUDY BY INTERFERENCE REFLECTION MICROSCOPY.

An optical technique for measuring the thickness of thin films has been adapted and evaluated for studying the structure of the adhesion of cells to glass in tissue culture. This technique, which is termed interference reflection microscopy, has been used to study embryonic chick heart fibroblasts. These findings have been observed: in normal culture medium the closest approach of the cell surface to substrate in its adhesions is ca. 100 A, much of the cell surface lying farther away; chemical treatments which bring the cell surface to near its charge reversal point reduce the closest approach of adhesions to <50 A, probably to <30 A; chemical treatments which increase surface charge increase the nearest approach of cell and substrate in adhesions from ca. 100 A; high osmotic concentration of a non-polar substance, i.e. sucrose, does not affect the distance between cell and substrate in the adhesions. In addition, optical evidence indicates that there is no extracellular material between cell and glass in the adhesions. When cells de-adhere from glass, they appear not to leave fragments behind. The adhesive sites in these fibroblasts appear to be confined to the edge of the side of the cell facing the substrate and to the pseudopods. The significance of this is discussed in relation to the phenomenon of contact inhibition. Evidence is presented that the mechanism of cell adhesion does not involve calcium atoms binding cells to substrate by combining with carboxyl groups on cell surface, substrate, and with a cement substance. Osmium tetroxide fixation results in a final separation of 100 to 200 A between cell and substrate: there are reasons for thinking that this fairly close approach to the condition in life is produced as an artefact. The results can be accounted for only in terms of the action of electrostatic repulsive forces and an attractive force, probably the van der Waals-London forces. Biological arguments suggest that these results are equally applicable for cell-to-cell adhesions.

Physical aspects of protein and DNA synthesis

The idea that the synthetic processes of a bacterial cell take place by random collision and specific selection has been examined in terms of Brownian movement theory. The observed rates of protein, deoxyribonucleic acid and ribosome synthesis are used to calculate the necessary concentrations of the metabolites used in the synthetic process. If the collision process must involve the presentation of the active part of the colliding molecule at first impact, then the observed rates require excessive concentrations for soluble ribonucleic acid. The concentration required for nucleotide triphosphates in the synthesis of deoxyribonucleic acid is such that many points of synthesis are called for. If, instead, a very temporary complex can form, of the nature expected from the operation of London-Van der Waals forces, the free rotation of the colliding molecule provides a variety of aspect changes, and the collision process is much more efficient. If this process takes place, the transfer of energy from rotation to vibration becomes important. The precision of assembly of deoxyribonucleic acid is examined in terms of the idea that precision in timing is important. It is shown that the cell could not function in that way if any large molecule had to move during the assembly. It is suggested that enzymes having high activation energies must be present in abnormally high concentrations.

Mechanical properties of plastic-disperse systems at very small deformations

A system containing flocculated solid particles in a liquid shows viscoelastic behavior at very small deformations. A model is described by means of which the elastic modulus of the system can be correlated with the forces acting between the particles. It is assumed that these forces are due to van der Waals-London attraction, and their contribution to the modulus is calculated. A method is indicated which allows the contribution of the van der Waals-London forces to the modulus of the material to be measured under specified conditions. Experiments on materials containing fat crystals in oil show agreement between predicted and experimental values for this contribution, if it is assumed that the solid particles in the aggregate approach each other to a distance of about 10 A. The energy content of the van der Waals-London bonds in these materials has been estimated from measurements of their rate of breaking in creep experiments, i.e., under the influence of a constant, low shearing stress. The energy content of about 40 kT-units is in satisfactory agreement with the results of stiffness measurements.

Aspects of electrical breakdown of liquid insulating material I

The electrical breakdown strength of insulating oil depends on the size of foreign particles which may form bridges in a place of maximum electric stress. This theoretical relation was verified with colloid suspensions of particles of known radius. Mineral oils may deteriorate if the particles unite by the process of flocculation, the occurrence of which depends on the relative magnitudes of the attractive London-Van der Waals forces acting between the particles and the repulsive forces between their ion atmospheres. The possibility of using a mineral oil as an insulator depends on the existence of an upper limit of the size of particle complexes due to the rapid fall-off of the L.-v.d.W. forces at diameters exceeding 500 A. The latter value of 2r corresponds to a breakdown strength of 1 kV/mm. If acids are being formed, the upper limit of 500 A will shift towards larger values, and correspondingly the breakdown strength may drop below 1 kV/mm.

On the additivity of London-Van der Waals forces: An extension of London's oscillator model

Calculations are given concerning the additivity of London-Van der Waals forces between two groups of atoms, the atoms being represented as isotropic harmonic oscillators. The results indicate that deviations of 1030% from additivity can be obtained if only dipole-dipole interaction between oscillators of one group is assumed. The effect can be expected to be relatively large if the symmetry of the arrangements of the oscillators in the group is low and it is dependent upon the relative spatial position of the groups.

Mooney Viscosity of Carbon Black-Rubber Mixtures

Abstract The principal properties of carbon black which must be considered in the study of rubber mixtures containing carbon black are (1) particle size and the related property, (2) surface area, (3) hydrogen content, (4) oxygen content, and finally a property which is more difficult to evaluate but which is related to (5) the tendency to agglomerate. This latter term is referred to in the trade as a “structure” factor, and it is usually determined from oil absorption measurements. Unvulcanized rubber-carbon black mixtures are considered to be analogous to mixtures of carbon black and with other liquid media, for example, mineral oil. It has been demonstrated by Weltmann and Green that the “plastic viscosities” of concentrated suspensions obey a modified “Arrhenius equation”. In this paper it is shown that unvulcanized rubber-carbon black mixtures also obey a similar equation. The rheological properties of concentrated suspensions are profoundly influenced by long-range London-van der Waals forces. Some of the laboratory tests by which these forces are studied by colloid chemists are specific volume of a powdered solid under pressure, sedimentation volume, oil absorption, and plasticity. A correlation is demonstrated between data obtained by these tests and Mooney viscosity. A general equation is presented relating the Mooney viscosity with the concentration of carbon black and oil absorption. This equation applies to all of the data which have been examined in natural rubber. In GR-S another term, the oxygen content of the carbon black, must be included.

On the interaction of colloidal particles V. nature of repulsion between particles in dilute sols

AbstractAn extension is proposed to the theory of Verwey and Overbeek on the force between two colloidal particles in a dilute sol. The effect of the Coulomb interaction of the ions in the dispersion medium is considered. It is shown that this contributes an additional repulsion term to the mutual free energy of the two particles. This correction term behaves as an inverse power and is the predominant effect at large separations, although it is a second-order effect at small separations. For parallel plates, this power is the fourth, and for two spheres, the sixth. A tentative calculation shows that, at least for thin plates and low electrolyte concentrations, our new repulsion will outweigh the attractive London-van der Waals force considered by Verwey and Overbeek, and more recently by Casimir and Polder, when the plates are far apart.