Van der Waals attractive force

A second factor of importance in this connection is found in the emphasis which was placed in the 1890 s and early 1900 s on secondary association of molecules. Coordination complexes, the concept of partial valences, and van der Waals forces attracted wide attention... 

Surface Forces. Surface forces can be measured with thin film balances where freely suspended horizontal films are submitted to controlled pressures. If the film drainage has been smooth enough, so that no early rupture has occurred, a regime where interactions between the two sides of the film become significant can be attained this is in a thickness region of about 100 nm and less. Typical interactions are van der Waals forces (attractive) and electrostatic forces (repulsive). Short range forces (steric, hydration) are also present. 

Satoh, T., Higuchi, S., and Shimoiizaka, J., Dispersibility and van der Waals force attractive energy of magnetite colloid in cyclohexane. Abstracts of the 19th Annual meeting of The Chemical Society of Japan I, p. 293 (1966). 

There are three basic regions of interaction between the probe and surface (i) free space, (ii) attractive region, and (iii) repulsive region. At short distances, the cantilever mainly senses interatomic forces the very short range ( 0.1 nm) repulsive forces, and the longer-range (up to 10 nm) van der Waals forces. Attractive forces near the surface ean arise from a layer of contamination present on all surfaces in ambient air. The eontamination is typically an aerosol composed of water vapor and hydrocarbons. On the other hand, the repulsive force occurs between any two atoms or molecules that approach so closely that... 

According to the above arguments, from a consideration of van der Waals forces, attractive interactions between bacteria and electrodes are expected irrespective of the nature of charges on them. The observations made above with a number of bacterial species suggest the dominant nature of the bactericidal action of the generated species on the surface. 

When both An and A22 are either greater or less than A33, the interparticle interaction Ai32 is predicted to be positive and, therefore, the van der Waals force attractive. If, however, A33 has an intermediate value between An and A22, then A132 is negative and the van... 

In the rubber industry the distribution of particle size is considered to be important as it affects the mechanical properties and performance. Aggregate size also varies with particle size. Aggregates can have any shape or morphology. The fundamental property of the filler used in a filled elastomer is the particle size. This affects the reinforcement of elastomer most strongly. One of the sources of reinforcement between the carbon black surface and the rubber matrix is the van der Waals force attraction. Also, rubber chains are grafted onto the carbon black surface by covalent bonds. The interaction is caused by a reaction between the functional group at the carbon black particle surface and free radicals on polymer chains. Hence, filler-rubber interface is made up of complex physical-chemical interaction. The adhesion at the rubber-filler interface also affects the reinforcement of rubber. When the polymer composites are filled with spherical filler (aspect ratio of the particle is equal to unity), the modulus of the composite depends on the modulus, density, size, shape, volume ratio, and number of the incorporated particles. 

The important feature of this work was the ability to manipulate the particle size and density by proper choice of surface hydrophobicity. These results suggested more efficient nucleation on the hydrophobic substrate. This is in direct contradiction to the model of biomineralization discussed above which suggest that highly charged, cation adsorbing surfaces are needed to promote nucleation. The SAM surface most certainly does not bind cadmium but may, through van der Waals forces, attract neutral ion pair species or subnanometer colloidal particles. 

Much of chemistry is concerned with the short-range wave-mechanical force responsible for the chemical bond. Our emphasis here is on the less chemically specific attractions, often called van der Waals forces, that cause condensation of a vapor to a liquid. An important component of such forces is the dispersion force, another wave-mechanical force acting between both polar and nonpolar materials. Recent developments in this area include the ability to measure... 

In 1930, London [1,2] showed the existence of an additional type of electromagnetic force between atoms having the required characteristics. This is known as the dispersion or London-van der Waals force. It is always attractive and arises from the fluctuating electron clouds in all atoms that appear as oscillating dipoles created by the positive nucleus and negative electrons. The derivation is described in detail in several books [1,3] and we will outline it briefly here. 

The second general cause of a variable heat of adsorption is that of adsorbate-adsorbate interaction. In physical adsorption, the effect usually appears as a lateral attraction, ascribable to van der Waals forces acting between adsorbate molecules. A simple treatment led to Eq. XVII-53. 

Adsorbates can physisorb onto a surface into a shallow potential well, typically 0.25 eV or less [25]. In physisorption, or physical adsorption, the electronic structure of the system is barely perturbed by the interaction, and the physisorbed species are held onto a surface by weak van der Waals forces. This attractive force is due to charge fiuctuations in the surface and adsorbed molecules, such as mutually induced dipole moments. Because of the weak nature of this interaction, the equilibrium distance at which physisorbed molecules reside above a surface is relatively large, of the order of 3 A or so. Physisorbed species can be induced to remain adsorbed for a long period of time if the sample temperature is held sufficiently low. Thus, most studies of physisorption are carried out with the sample cooled by liquid nitrogen or helium. 

As the tip is brought towards the surface, there are several forces acting on it. Firstly, there is the spring force due to die cantilever, F, which is given by = -Icz. Secondly, there are the sample forces, which, in the case of AFM, may comprise any number of interactions including (generally attractive) van der Waals forces, chemical bonding interactions, meniscus forces or Bom ( hard-sphere ) repulsion forces. The total force... 

In accordance with equation (Bl.20.1). one can plot the so-called surface force parameter, P = F(D) / 2 i R, versus D. This allows comparison of different direct force measurements in temis of intemiolecular potentials fV(D), i.e. independent of a particular contact geometry. Figure B 1.20.2 shows an example of the attractive van der Waals force measured between two curved mica surfaces of radius i 10 nun. 

A substance exists as a liquid rather than a gas because attractive forces between molecules (mtermolecular attractive forces) are greater m the liquid than m the gas phase Attractive forces between neutral species (atoms or molecules but not ions) are referred to as van der Waals forces and may be of three types... 

These forces are electrical m nature and m order to vaporize a substance enough energy must be added to overcome them Most alkanes have no measurable dipole moment and therefore the only van der Waals force to be considered is the induced dipole/mduced dipole attractive force... 

Nonbonded interactions are the forces be tween atoms that aren t bonded to one another they may be either attractive or repulsive It often happens that the shape of a molecule may cause two atoms to be close in space even though they are sep arated from each other by many bonds Induced dipole/induced dipole interactions make van der Waals forces in alkanes weakly attractive at most distances but when two atoms are closer to each other than the sum of their van der Waals radii nuclear-nuclear and electron-electron repulsive forces between them dominate the fvan derwaais term The resulting destabilization is called van der Waals strain... 

Polypropylene chains associate with one another because of attractive van der Waals forces The extent of this association is relatively large for isotactic and... 

In addition to hydrogen bonding between the two polynucleotide chains the double helical arrangement is stabilized by having its negatively charged phosphate groups on the outside where they are m contact with water and various cations Na" Mg and ammonium ions for example Attractive van der Waals forces between the... 

The well-known DLVO theory of coUoid stabiUty (10) attributes the state of flocculation to the balance between the van der Waals attractive forces and the repulsive electric double-layer forces at the Hquid—soHd interface. The potential at the double layer, called the zeta potential, is measured indirectly by electrophoretic mobiUty or streaming potential. The bridging flocculation by which polymer molecules are adsorbed on more than one particle results from charge effects, van der Waals forces, or hydrogen bonding (see Colloids). 

Sorption of nonionic, nonpolar hydrophobic compounds occurs by weak attractive interactions such as van der Waals forces. Net attraction is the result of dispersion forces the strength of these weak forces is about 4 to 8 kj/mol ( 1 2 kcal/mol). Electrostatic interactions can also be important, especially when a molecule is polar in nature. Attraction potential can develop between polar molecules and the heterogeneous sod surface that has ionic and polar sites, resulting in stronger sorption. 

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