London-Van Der Waals

Adsorption can be attributed to the following interactions van der Waals-London interactions, charge transfer/hydrogen bonding, ligand exchanges, ion exchange, direct and induced ion-dipole... 

Van der Waals-London interactions are due to fluctuations in electron distribution as the electrons circulate within their orbits. These instantaneous dipoles are usually weak, but are, regardless, the most common interaction resulting in adsorption.31 Stronger interactions result from charge transfer. 

Although the fact that the cycloamyloses include a variety of substrates is now universally accepted, the definition of the binding forces remains controversial. Van der Waals-London dispersion forces, hydrogen bonding, and hydrophobic interactions have been frequently proposed to explain the inclusion phenomenon. Although no definitive criteria exist to distinguish among these forces, several qualitative observations can be made. 

Gas adsorption (physisorption) is one of the most frequently used characterization methods for micro- and mesoporous materials. It provides information on the pore volume, the specific surface area, the pore size distribution, and heat of adsorption of a given material. The basic principle of the methods is simple interaction of molecules in a gas phase (adsorptive) with the surface of a sohd phase (adsorbent). Owing to van der Waals (London) forces, a film of adsorbed molecules (adsorbate) forms on the surface of the solid upon incremental increase of the partial pressure of the gas. The amount of gas molecules that are adsorbed by the solid is detected. This allows the analysis of surface and pore properties. Knowing the space occupied by one adsorbed molecule, Ag, and the number of gas molecules in the adsorbed layer next to the surface of the solid, (monolayer capacity of a given mass of adsorbent) allows for the calculation of the specific surface area, As, of the solid by simply multiplying the number of the adsorbed molecules per weight unit of solid with the space required by one gas molecule ... 

The recognition is the binding of a carbohydrate by a specific receptor, which is usually part of a protein (enzyme, immunoglobuUn, lectin). The process is due to the sum of weak interactions (van der Waals-London, hydrogen bonds) that may results in high association constants. 

Can the van der Waals forces lead to the formation of chemical compounds The two Cl atoms in a molecule Cl2 will certainly be attracted to one another by a van der Waals-London force but it can be very simply shown that these forces alone can never lead to the formation of a chemical compound. The peculiar property of these forces is that they always strive to bring like atoms together. Suppose we have a dilute gas mixture containing N atoms of A and N atoms of B further let us imagine that the van der Waals-London forces lead to the formation of molecules AB from these atoms the energy of the system is then... 

The energy of the combination AB is thus higher than that of 0-5A2 + 0 5jB2. This result can be proved equally well for other combinations. Van der Waals-London forces, therefore, although very important in the mutual cohesion of atoms and molecules, can never lead to an actual chemical combination. ... 

The forces causing chemical combination must be of a completely different kind and must lead to a definite saturation. When two chlorine atoms combine to form a molecule Cl2, a third atom can not be added whereas the van der Waals-London forces will always tend to add still further particles to an existing combination. These forces can lead to the formation of lattices, but not to separate molecules with a fixed number of particles. 

The van der Waals-London forces play a particularly important part in the formation of molecular lattices even when the molecules still have an ionic structure screening and polarization diminish the effect of electrostatic forces to a great extent, and the van der Waals forces are therefore preponderant. Examples of substances which form molecular lattices are those of the type XY4, such as CC14, CI4, SnCl4, etc. In these a small positive ion is surrounded by four large halogen ions in the form of a tetrahedron, with the result that the external electric field of the positive ion is very weak. Mutual attraction of molecules is often exclusively the result of the van der... 

In fact, the problem is a great deal more complicated because, in addition to the dipole-dipole attractions, the van der Waals-London forces resulting from the polarization of non-polar molecules by polar ones have also to be taken into account. The energy required to remove a molecule from the liquid can in general be split into three separate parts... 

It was seen in Section 46 that the van der Waals forces alone could never lead to the formation of molecules, and yet it appears that they prevent mixing. In fact, the van der Waals-London forces always tend to add like to like. It is due to the change in entropy that mixing still takes place, except where the heat of mixing is positive under the influence of dipole forces. 

It is a known fact that finely divided carbon can adsorb many gases, a property which is utilized in gas masks. Here the adsorption depends on the van der Waals-London forces, from which it follows that all strongly polarizable gases will be adsorbed, and, further, that a carbon mask is not suitable for the adsorption of CO. In practice, the efficacy of carbon for the adsorption of gases can be increased by adding other, usually ionic materials. 

Molecular compounds consist of two or more stable species held together by weak forces. In clathrittes-, a gaseous substance, such as SO . HC1. CO.-, or n rare gas is held in the crystal lattice of a solid, such as beta-quinol. by Van der Waals-London dispersion forces. The gas hydrates, e.g.. CU- 6H 0. contain halogen molecules similarly trapped in ice-like... 

The Influence of Extremely Small Attractive As Well As of Repulsive van der Waals-London Forces on Cell Interactions... 

The theoretical studies referred to above made it evident that the most important forces contributing to the type of interactions that we are considering are the intermoleeular forces known under the general designation of van der Waals—London forces. 

The second problem, namely, the different effects that the presence of rare tautomers and the subsequent miscouplings may introduce into the physicochemical properties of DNA, is, of course, a very broad one. As a particularly interesting aspect we may consider its influence on the stability of the nucleic acid.183 Thus, Table XX indicates the van der Waals-London interaction energies calculated, following the... 

Claverie, P., B. Pullman, and J. Caillet Van der Waals-London interactions between stacked purines and pyrimidines. J. Theoret. Biol. 12, 419 (1966). 

Computations were carried through for values of 0.05 < 0 < 0.95 in increments of 0.05 unit, with C — 2, 3, 4, and 5. It was assumed that lateral interactions were due to attractive van der Waals-London dispersion forces, where the leading term in the energy expansion varies with distance as r-1/6 with R = V2 one finds C = C1/8. Calculations were also carried out in the Fowler-Guggenheim approximation this simply requires the determination of the zero-order inputs Po(a 0), Pj(b °K and P/P°. The results are exhibited in Figures 2 and 3 the broken curves refer to isotherms calculated according to Equations 22 and 23. 

In the study of the texture of catalysts use is made of the physical adsorption of gases. Physical adsorption is an increase of concentration at the gas-solid or gas-liquid interface under the influence of integrated van der Waals-London forces, also known as de Boer-Hamaker forces. 

The van der Waals-London forces between molecules and atoms are well known from the term a/V2 in the van der Waals equation ... 

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