London-van Der Waals Interaction Research Articles

Interaction of Nanoparticles in Electrolyte Solutions.

The interaction between nanoparticles includes several components; however, the most frequently used are electrostatic, caused by overlapping double electrical layers, and London-van der Waals interactions, caused by quantum and thermodynamic fluctuations of electromagnetic fields. Only these two kinds of interaction are considered below. The electrostatic interaction is calculated based on the linearized Poisson-Boltzmann equation for particles with constant electrical potential of the surfaces (constant ζ potentials). An exact solution of the problem is obtained for both identical particles and particles of different sizes. For the London-van der Waals interaction, the screening of static fluctuations and the retardation of electromagnetic fields for the dispersive part of the interaction are taken into account. The total interaction energy for two particles was calculated for a range of possible nanoparticle sizes from 1 to 103 nm and electrolyte concentration from 10-2 to 10-6 mol/L. The predominance of the London-van der Waals force over the shielded electrostatic repulsion force was found at high electrolyte concentrations in the range from 10-2 to 10-3 mol/L at large interparticle distances.

Fractional ionization and size of cetyltrialkyl ammonium bromide and hydroxide micelles as a function of head-group lipophilicity and temperature

We have measured the diffusion coefficients, D, of aqueous micelles formed by cetyltriethyl-, cetyltripropyl- and cetyltributyl-ammonium bromides (CTEABr, CTPABr and CTBABr, respectively) and cetyltriethyl- and cetyltripropyl-ammonium hydroxides (CTEAOH and CTPAOH, respectively) by dynamic light scattering (DLS) at several temperatures from 15 to 55 °C and a range of surfactant (0.01–0.05 M) and salt (0.02–0.06 M NaBr; 0.05–0.3 M NaOH) concentrations. From values of D, we derived the respective fractional ionization values of micellar surfaces. For surfactants with bromide counterion we obtained fits of the diffusivity data using the linear interaction/DLVO approach, thus yielding estimates of the micellar hydrodynamic radius, Rh, and the micellar fractional ionization, α, which ranged from 0.26 to 0.35. For CTEAOH and CTPAOH, the fits appeared to be poorly sensitive to changes in the London-Van der Waals interactions, as expressed by the Hamaker constant, and only a large fractional ionization could account for the observed diffusivities.

Orientation-Dependent London-van der Waals Interaction Energy between Macroscopic Bodies.

The aim of this work is to derive formulas for numerical calculations of the orientation-dependent London-van der Waals (vdW) interaction energy (V(A)) between two rectangular bodies with arbitrary dimensions, arranged at arbitrary relative angles (θ) and separations in twisted and coplanar rotational modes. The formulation is made using a simple volume-element-integration method in the framework of the microscopic approach, in which V(A) is the sum of the local vdW energy (Vp) between body 1 and each thin plate constituting body 2. Examples of the calculation results are the following: (1) The θ values that give maximal and minimal values of V(A) depend on their shapes and relative positions. (2) As the bodies come close to each other, the variations of V(A) with θ and thus vdW dispersion torques generated are drastically intensified. (3) Upon increasing the length of crossing rods in twisted configurations, the V(A) values become constant beyond a critical length (depending on θ and separation), where the length effect on V(A) disappears. (4) The distribution curves of Vp show that the region in body 2 which interacts effectively with body 1 (i.e., the effective interaction region) is more sharply localized in the vicinity of the surface (closest to body 1) as the separation is decreased.

Nonperturbative calculation of the London–van der Waals interaction potential

The Helmholtz free energy of an oscillator coupled to a heat bath is used to derive the London-van der Waals interaction potential for all separations and temperatures, extending the well-known results for the interaction energy beyond the weak-coupling limit.

Role of long- and short-range hydrophobic, hydrophilic and charged residues contact network in protein’s structural organization

BackgroundThe three-dimensional structure of a protein can be described as a graph where nodes represent residues and the strength of non-covalent interactions between them are edges. These protein contact networks can be separated into long and short-range interactions networks depending on the positions of amino acids in primary structure. Long-range interactions play a distinct role in determining the tertiary structure of a protein while short-range interactions could largely contribute to the secondary structure formations. In addition, physico chemical properties and the linear arrangement of amino acids of the primary structure of a protein determines its three dimensional structure. Here, we present an extensive analysis of protein contact subnetworks based on the London van der Waals interactions of amino acids at different length scales. We further subdivided those networks in hydrophobic, hydrophilic and charged residues networks and have tried to correlate their influence in the overall topology and organization of a protein.ResultsThe largest connected component (LCC) of long (LRN)-, short (SRN)- and all-range (ARN) networks within proteins exhibit a transition behaviour when plotted against different interaction strengths of edges among amino acid nodes. While short-range networks having chain like structures exhibit highly cooperative transition; long- and all-range networks, which are more similar to each other, have non-chain like structures and show less cooperativity. Further, the hydrophobic residues subnetworks in long- and all-range networks have similar transition behaviours with all residues all-range networks, but the hydrophilic and charged residues networks don’t. While the nature of transitions of LCC’s sizes is same in SRNs for thermophiles and mesophiles, there exists a clear difference in LRNs. The presence of larger size of interconnected long-range interactions in thermophiles than mesophiles, even at higher interaction strength between amino acids, give extra stability to the tertiary structure of the thermophiles. All the subnetworks at different length scales (ARNs, LRNs and SRNs) show assortativity mixing property of their participating amino acids. While there exists a significant higher percentage of hydrophobic subclusters over others in ARNs and LRNs; we do not find the assortative mixing behaviour of any the subclusters in SRNs. The clustering coefficient of hydrophobic subclusters in long-range network is the highest among types of subnetworks. There exist highly cliquish hydrophobic nodes followed by charged nodes in LRNs and ARNs; on the other hand, we observe the highest dominance of charged residues cliques in short-range networks. Studies on the perimeter of the cliques also show higher occurrences of hydrophobic and charged residues’ cliques.ConclusionsThe simple framework of protein contact networks and their subnetworks based on London van der Waals force is able to capture several known properties of protein structure as well as can unravel several new features. The thermophiles do not only have the higher number of long-range interactions; they also have larger cluster of connected residues at higher interaction strengths among amino acids, than their mesophilic counterparts. It can reestablish the significant role of long-range hydrophobic clusters in protein folding and stabilization; at the same time, it shed light on the higher communication ability of hydrophobic subnetworks over the others. The results give an indication of the controlling role of hydrophobic subclusters in determining protein’s folding rate. The occurrences of higher perimeters of hydrophobic and charged cliques imply the role of charged residues as well as hydrophobic residues in stabilizing the distant part of primary structure of a protein through London van der Waals interaction.

Cohesive energy curves for noble gas solids calculated by adiabatic connection fluctuation-dissipation theory

We present first-principles calculations for the fcc noble gas solids Ne, Ar, and Kr applying the adiabatic connection fluctuation-dissipation theorem (ACFDT) to evaluate the correlation energy. The ACFDT allows us to describe long-range correlation effects including London dispersion or van der Waals interaction on top of conventional density functional theory calculations. Even within the random phase approximation, the typical $1∕{V}^{2}$ volume dependence for the cohesive energy of the noble gas solids is reproduced, and equilibrium cohesive energies and lattice constants are improved compared to density functional theory calculations. Furthermore, we present atomization energies for ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, and ${\mathrm{O}}_{2}$ within the same post-density-functional-theory framework, finding an excellent agreement with previously published data.

Thermophoresis of dissolved molecules and polymers: Consideration of the temperature-induced macroscopic pressure gradient.

The movement of molecules and homopolymer chains dissolved in a nonelectrolyte solvent in response to a temperature gradient is considered a consequence of temperature-induced pressure gradients in the solvent layer surrounding the solute molecules. Local pressure gradients are produced by nonuniform London-van der Waals interactions, established by gradients in the concentration (density) of solvent molecules. The density gradient is produced by variations in solvent thermal expansion within the nonuniform temperature field. The resulting expression for the velocity of the solute contains the Hamaker constants for solute-solvent and solute-solute interactions, the radius of the solute molecule, and the viscosity and cubic coefficient of thermal expansion of the solvent. In this paper we consider an additional force that arises from directional asymmetry in the interaction between solvent molecules. In a closed cell, the resulting macroscopic pressure gradient gives rise to a volume force that affects the motion of dissolved solutes. An expression for this macroscopic pressure gradient is derived and the resulting force is incorporated into the expression for the solute velocity. The expression is used to calculate thermodiffusion coefficients for polystyrene in several organic solvents. When these values are compared to those measured in the laboratory, the consistency is better than that found in previous reports, which did not consider the macroscopic pressure gradient that arises in a closed thermodiffusion cell. The model also allows for the movement of solute in either direction, depending on the relative values of the solvent and solute Hamaker constants.

Polymer thermophoresis in solvents and solvent mixtures

The thermophoresis of homopolymer chains dissolved in a pure non-electrolyte solvent or solvent mixture is theoretically examined. Thermophoresis is related to the temperature-dependent pressure gradient in the solvent layer surrounding the monomer units (mers). The gradient is produced by small changes in the solvent or solvent mixture density due to the mer-solvent interaction. The London-van der Waals interaction was considered as the main reason of the excess pressure around mers. The resulting expression for the thermophoretic mobility (TM) contains the Hamaker constant for mer-solvent interaction, as well as solvent thermodynamic parameters, including the cubic thermal expansion coefficients of the solvents and the temperature coefficient of the solvent partition factor (for the solvent mixture). This expression is used to calculate the interaction constants for polystyrene and poly(methyl methacrylate) in several organic solvents and binary solvent mixtures using thermophoretic data obtained from thermal field-flow fractionation. The calculated constants are compared with values in the literature and found to follow the same order among the different solvents and to be of the same order of value although several times larger. Furthermore, the model explains weak polymer thermophoresis in water compared with less polar solvents, which correlates also with monomer size. The concentration dependence of polystyrene TM in solvent mixtures also provides a satisfactory explanation by the proposed theory using a concept of secondary diffusiophoresis due to secondary temperature-induced solvent concentration gradient. The method for the evaluation of the diffusiophoresis contribution is proposed.

Adsorption of Comb Macromolecule on Fused Silica Particles and the Calculation of Interparticle Potential Energy

We have discussed the effect of comb macromolecule adsorbed on fused silica particles to the dispersion of the particle. The effect of the amount and thickness of the polymers adsorbed on the fused silica particles on the total interparticle potential energy was investigated. The evaluation of dispersed property of the particles in solvent was considered by magnitude of interparticle potential energy. The total interparticle potential energy was calculated by summing up the steric hindrance effect and London-van der Waals interaction. The amount of polymers adsorbed were not influenced by the minimum of the total potential energy. The maximum radius of the particles that can be dispersed in polymer solution was in proportion to the thickness of the adsorbed layer.

The influence of interparticle surface forces on the coagulation of weakly magnetic mineral ultrafines in a magnetic field

In this paper, it is shown that the coagulation of dispersions of weakly magnetic mineral ultrafines (such as hematite and chromite) in an external magnetic field can be described theoretically by invoking interparticle forces. Essentially, coagulation occurs when the short-range London—van der Waals interactions and the long-range magnetic forces outweigh the stabilizing electric double layer repulsion. From classical colloid chemistry theory, we have calculated the various components of the potential energy for different-sized particles at a series of ionic strengths and magnetic field intensifies. Principles governing the stability of the suspensions were derived and the computations lead to the establishment of criteria which can be used to predict the stability of the suspensions of weakly magnetic oxide mineral ultrafines in a “wet magnetic separation process”. Experimentally, the magnetic-field induced coagulation of ultrafines of natural hematite and chromite in aqueous suspensions at moderate ionic strength was investigated using a laboratory-scale electromagnetic solenoid. The experimental results relate the coagulation process (as determined by magnetosedimentation analysis) to particle size, slurry pH and the external magnetic field. In the magnetic fields, maximum coagulation occurred near the pH of the point of zero charge (pH PZC) of the minerals (where the electrostatic double layer repulsion was reduced to a minimum) enabling the particles to enter the “primary minimum” energy sink. In contrast, in cases where the electrostatic repulsion was not suppressed, the long-range magnetic forces enabled coagulation to occur in the “secondary minimum”. This caused the formation of chains which appeared to be relatively stable at enhanced rates of settling. The experimental results could be interpreted from a theoretical analysis of the interparticle forces controlling the process.

The influence of magnetic and surface forces on the coagulation of hematite and chromite

The coagulation of dispersed ultrafine weakly-magnetic oxide mineral particles (e.g., natural hematite and chromite) in an external magnetic field can be described by interparticle forces. Essentially, coagulation occurs when the short-range London-van der Waals interactions and the long-range magnetic forces outweigh the stabilizing electrical double-layer repulsion. Using the classical colloid chemistry theory, the various components of the potential energy for different-sized particles at a series of ionic strengths and magnetic field intensities were calculated. The results of this theoretical analysis were then related to the results obtained from experimental studies in which the magnetic field-induced coagulation of the ultrafines of natural hematite and chromite in aqueous suspensions were investigated using a laboratory-scale electromagnetic solenoid. The experimental results relate the coagulation process (as determined by magneto-sedimentation analysis) to the particle size, the slurry pH and the external magnetic field. In the magnetic fields, the maximum coagulation occurred near the pH of zero-point-of-charge (pHzpc) of the minerals (where the electrostatic double layer repulsion was reduced to a minimum) enabling the particles to enter the “primary minimum” energy sink. On the other hand, in cases where the electrostatic repulsion was not suppressed, the long-range magnetic forces enabled coagulation to occur in the “secondary minimum.” This caused the formation of chains that appeared to be relatively stable during sedimentation.

Preparation of monolayer particle coated powder by the dry impact blending process utilizing mechanochemical treatment

Spherical silica particles 0.3, 0.6 and 0.9 μm in diameter, and spherical polyethylene particles 5.2 and 11.9 μm in diameter were used as coating and core materials respectively. Scanning electron microscopy revealed that the silica particles were fixed on the surface of the polyethylene particles, and a monolayer particle coated powder was formed by a dry impact blending preparation method when the ratios of the coating and core particle sizes were 0.3/11.9, 0.6/11.9, 0.9/11.9 and 0.3/5.2. In the dry impact blending of a binary powder mixture which contains two different sizes, it was shown that the ratio of the core and coating particle sizes, the particle sizes themselves and the action of impulsive forces during the operation were important factors in the effective preparation of the monolayer particle coated powder. The binding energies were calculated on the assumption that the Coulomb and London—van der Waals interactions were the basis of adhesion for these monolayer particle coated polyethylene powders. Consequently, it was found that the Coulomb interaction between particles with different electric charges was not always advantageous for the formation of the monolayer particle coated powder.

Uniaxial nematic phase in fluids of biaxial particles

Abstract We construct a mean field theory for fluids of biaxial particles interacting via London-van der Waals interactions. This model is shown to belong to a class of systems previously studied by Straley. In the special case when the psueodpotential can be constructed from the invariants of the dielectric susceptibility tensor, the model is shown to be equivalent to that of Freiser and others. A discussion is given of the difference between molecular properties, as obtained in N.M.R. experiments, and bulk properties.

Van der Waals interaction between linear molecules

First a standard expansion in spherical harmonics for the interaction of elongated axial-symmetric molecules obeying the law r−2n is derived, where an integral representation for the coefficients is given. In the second step we will show how the integrals can be evaluated and present the coefficients as analytical functions of the distance between two molecules and their dimensions as well. This is carried out explicitly in the case of the London–van der Waals interaction (r−6) for linear centrosymmetric molecules up to the fourth order term.

On the stability of micelles in apolar media

An attempt has been made for the first time to calculate explicitly the stability of micelles built up by ionic surfactants in apolar media. The quantitative treatment based on a geometrical model closely corresponding to the shape and structure of these micelles refers to coulombic and London-Van der Waals interactions. The latter is applied under the suitable assumption that the dispersion forces are additive (“microscopic theory”). The experimentally observed properties of the micelles are well confirmed by this treatment with respect to size, stability, and solvent dependence of the mean aggregation number of the constituent monomers.

Spontaneous rupture of thin liquid films

The rupture of a liquid film on a solid surface and of a free liquid film have been studied using hydrodynamic stability theory. The films are not thicker than several hundred Angstrom. A small perturbation applied to the free interface generates motions in the film, and the assumption is made that the Navier–Stokes equations can be used to describe them. The difference in forces acting upon an element of liquid in a thin film and in a bulk fluid is accounted for by introducing a body force in the Navier–Stokes equations. This force is calculated from the potential energy per unit volume in the liquid caused by the London–van der Waals interactions with the surrounding molecules of the liquid and with those of the solid. If the perturbation grows, it leads to the rupture of the film. The range of wavelengths of the perturbation for which instability occurs is established and the time of rupture is evaluated. The effect of insoluble and soluble surface active agents is analyzed. Available experimental data concerning condensation on a solid surface and coalescence of bubbles are explained on the basis of the obtained results.

Solvent Effects on Electronic Spectral Intensities

A theory is developed for determining the effect of the London–van der Waals interaction on the electronic spectral intensities of an absorbing substance dissolved in a transparent solvent. The formulation is based on quantum-statistical double perturbation theory: time varying in the applied field and time independent in the intermolecular force field. A virial expansion is developed for the extinction coefficient, integrated over a band, in terms of molecular parameters and molecular distribution functions, and the first and second virial coefficients are worked out in detail. It is shown that the leading term produces the oscillator strength of the free absorbing molecule in a rarefied gas. The results are analyzed in terms of local field contributions and distortions of electron distributions by the surroundings. The relevance of the theory to allowed transitions and forbidden transitions made allowed is discussed.

Enthalpies of Mixing in Binary Liquid Alkali Iodide Mixtures

The enthalpies of mixing of the binary liquid alkali iodide mixtures have been measured at 740°C. The results are compared with similar data for the binary alkali bromide, chloride and fluoride mixtures. They indicate that it is not possible to represent the enthalpies of mixing for a wider range of binary fused salt systems by any simple general formula. However, the results for all the considered binary iodides alone are well represented by a single semiempirical expression ΔHM = N1N2(U0++ − 540δ122kcal/mole. In this expression N1 and N2 are the mole fractions of the two components; δ12 = (d1 − d2) / d1d2, where d1 and d2 are characteristic interionic distances in the two salts, while U0++ represents a numerical estimate of the positive contribution to the enthalpy of mixing which arises from the London–van der Waals interaction between second nearest neighbor cations.

The effect of dispersion interaction on nuclear magnetic shielding

A theory is developed for calculating the effect of the London-van der Waals interaction on the nuclear magnetic shielding of a closed shell atom. The effect of a single perturber at fixed internuclear distance is first obtained by the use of triple perturbation theory. In the closure approximation, this pair shift is proportional to the van der Waals energy and the unperturbed shielding constant. The solvent shift due to the fluid medium is then obtained statistically. The results are compared with existing heuristic expressions based on field simulation, and the parameters used in the field techniques are analyzed and interpreted. Numerical values are given for the van der Waals shifts of the noble gas liquids. Difficulties associated with the extension of the theory to polyatomic molecules are discussed, and appropriately modified parameters are suggested. Numerical estimates so obtained for CH 4 and CF 4 are in good agreement with experimental data.

Interaction des Ions Métalliques avec le DNA. II. Adsorption préférentielle de l'Ion césium par les Acides nucléiques

AbstractPreferential adsorption of cations occurs in the solvated shell surrounding polyanions, especially polynucleotides and nucleic acids. The solvation parameter Γ is determined from sedimentation coefficients which are considered as pienometric determinations of the specific volume of the sample. By comparing two identical concentrations (0.1M) of monovalent salts, NaCl and CsCl, and using the model of DNA described in the preceding paper, one can relate the ratio SCs/SNa to solvation parameters ΓCs, and ΓNa.On the other hand, Γ can be related to the difference of energy W, between cation and water, to be extracted from the solution and placed in the vicinity of the polyanion. One finds generally a preferential solvation of the polarizable caesium ion by means of London–van der Waals interactions with bases. With reasonable assumptions, an estimate of W can be made, and the predicted value of Γ can be compared to the experimental one. An average of six sites is found around each base in a single‐stranded polymer, but‐only one in the double‐stranded structure. The amount of secondary structure can thus be estimated from the simple measurement of the ratio of two sediment constants.