Attractive forces dispersion

Induced dipole/mduced dipole attraction (Section 2 17) Force of attraction resulting from a mutual and complemen tary polanzation of one molecule by another Also referred to as London forces or dispersion forces Inductive effect (Section 1 15) An electronic effect transmit ted by successive polanzation of the cr bonds within a mol ecule or an ion... 

As already mentioned molecules cohere because of the presence of one or more of four types of forces, namely dispersion, dipole, induction and hydrogen bonding forces. In the case of aliphatic hydrocarbons the dispersion forces predominate. Many polymers and solvents, however, are said to be polar because they contain dipoles and these can enhance the total intermolecular attraction. It is generally considered that for solubility in such cases both the solubility parameter and the degree of polarity should match. This latter quality is usually expressed in terms of partial polarity which expresses the fraction of total forces due to the dipole bonds. Some figures for partial polarities of solvents are given in Table 5.5 but there is a serious lack of quantitative data on polymer partial polarities. At the present time a comparison of polarities has to be made on a commonsense rather than a quantitative approach. 

Typically, the ionic liquid is best considered as an ion-pair (or ion, on an individual basis). The main forces of attraction between the ions are the electrostatic forces and dispersion (van der Waals) forces. At intermediate distances, there is a slight... 

Figure 2.8 Attractive dispersion forces in nonpolar molecules are caused by temporary dipoles, as shown in these models of pentane, C5H12-...

The Smoluchowski-Levich approach discounts the effect of the hydrodynamic interactions and the London-van der Waals forces. This was done under the pretense that the increase in hydrodynamic drag when a particle approaches a surface, is exactly balanced by the attractive dispersion forces. Smoluchowski also assumed that particles are irreversibly captured when they approach the collector sufficiently close (the primary minimum distance 5m). This assumption leads to the perfect sink boundary condition at the collector surface i.e. cp 0 at h Sm. In the perfect sink model, the surface immobilizing reaction is assumed infinitely fast, and the primary minimum potential well is infinitely deep. 

In studies of steric stabilizers too little attention is generally paid to the dispersion force attractions between particles and the critical separation distance (H ) needed to keep particles from flocculating. Adsorbed steric stabilizers can provide a certain film thickness on each particle but if the separation distance between colliding particles is less than H the particles will flocculate. The calculation of H is not cr difficult and measurements to prove or disprove such calculations are not difficult either. For equal-sized spheres of substance 1 with radius or in medium 2 the Hamaker equation for the dispersion force attractive energy (Uj2i) at close approach is (7) ... 

Figure 2. Retardation correction factor (f) for dispersion force attractions between spherical particles of radius (a) at separation distance (H), with dispersion force wavelength Xj. (10)...

Combined Electrostatic and Steric Stabilization. The combination of the two mechanisms is illustrated in Figure 4, taken from Shaw s textbook, (13) where the repulsion of the steric barrier during a collision falls off so rapidly as the colliding particles bounce apart that the dispersion force attractions hold the particles together in the "secondary minimum". This is exactly what happens in the system investigated in this paper. 

Figure 4. Potential energy diagrams for a pair of particles with on the left, a steric barrier (V ) and dispersion force attraction (V.) and on the right, with electrostatic repulsion (V ) added. Reproduced with permission from Ref. (13).Copyright 1980, Butterworths.

A. dipole attractions, dispersion forces, hydrogen bonds... 

Simultaneous and correlated excitations of electrons in both molecules lead to correlation of electron motions and to a general net stabilization of the complex. This effect is usually attributed to the so-called attractive dispersion forces and the corresponding energy is therefore called dispersion energy zJ dis-... 

These results point to a significant effect of the configuration of the diol moiety on the intracomplex forces involved in the isomeric [C/j-M/j/j], [C/j-M/js], and [C/j-Mss] adducts. The OH- - -O hydrogen bonding in these complexes is responsible for the bathochromic shifts observed in the corresponding spectra. " Different spectral shifts for diastereomeric complexes are often due to the superimposing effects of attractive dispersive (polarization) and repulsive (steric) interactions. ... 

This structure gives stability to the layers. The fourth, non-bonding pair of electrons is delocalized and free to move within the layers. This gives graphite the ability to conduct an electric current. There are no intramolecular forces between the layers. Dispersion forces attract one layer to another, enabling layers to slide by one another. Graphite feels slippery as a result of this characteristic. In fact, many industrial processes use graphite as a lubricant. 

Distinguish between dipole-dipole attraction forces and dispersion (London) forces. Is one type of these intermolecular forces stronger than the other Explain. 

On the basis of these comparable mechanisms, the observed regioselectivity with various 3-substituents summarized in Table I might be best interpreted in terms of the balance of three effects, namely attractive dispersion force, steric hindrance, and electrostatic repulsion which would all be operative between the 3-substituent and the ferricyanide ion in the rate-determining step. 

This result suggests that we can look at the partitioning of various compounds (i.e., vary i) from the gas phase and expect that their relative tendencies to go into or onto differing media (i.e., vary the chemical nature of medium 1) will depend, at least in part, on predictable dispersive force attractions. Partitioning that is in excess of what... 

The distance dependence of 1/r6 for the attractive potential is characteristic of the interaction between dipoles. This is because the attractive dispersion forces result from the mutual induction of electrostatic dipoles. Although a nonpolar molecule has no net dipole averaged over a period of time, at any one instant there will be dipoles due to the local fluctuations of electron density. Because the energies depend on the induction of a dipole, polarizability is an important factor in the strength of the interaction between any two atoms. 

All adsorption processes result from the attraction between like and unlike molecules. For the ethanol-water example given above, the attraction between water molecules is greater than between molecules of water and ethanol As a consequence, there is a tendency for the ethanol molecules to be expelled from the bulk of the solution and to concentrate at die surface. This tendency increases with the hydrocarhon chain-length of the alcohol. Gas molecules adsorb on a solid surface because of die attraction between unlike molecules. The attraction between like and unlike molecules arises from a variety of intermolecular forces. London dispersion forces exist in all types of matter and always act as an attractive force between adjacent atoms and molecules, no matter how dissimilar they are. Many oilier attractive forces depend upon die specific chemical nature of the neighboring molecules. These include dipole interactions, the hydrogen bond and the metallic bond. 

Weak attractive forces between nonbonded atoms are called van der Waals attractive forces, London forces, or dispersion forces, and are of great importance in determining the properties of liquids. They also can be expected to play a role in determining conformational equilibria whenever the distances between the atoms in the conformations correspond to the so-called van der Waals minima. 

Polarizability is a measure of the ease with which the electrons of a molecule are distorted. It is the basis for evaluating the nonspecific attraction forces (London dispersion forces) that arise when two molecules approach each other. Each molecule distorts the electron cloud of the other and thereby induces an instantaneous dipole. The induced dipoles then attract each other. Dispersion forces are weak and are most important for the nonpolar solvents where other solvation forces are absent. They do, nevertheless, become stronger the larger the electron cloud, and they may also become important for some of the higher-molecular-weight polar solvents. Large solute particles such as iodide ion interact by this mechanism more strongly than do small ones such as fluoride ion. Furthermore, solvent polarizability may influence rates of certain types of reactions because transition states may be of different polarizability from reactants and so be differently solvated. 

Roughly 60 years ago Derjaguin, Landau, Verwey, and Overbeek developed a theory to explain the aggregation of aqueous dispersions quantitatively [66,157,158], This theory is called DLVO theory. In DLVO theory, coagulation of dispersed particles is explained by the interplay between two forces the attractive van der Waals force and the repulsive electrostatic double-layer force. These forces are sometimes referred to as DLVO forces. Van der Waals forces promote coagulation while the double layer-force stabilizes dispersions. Taking into account both components we can approximate the energy per unit area between two infinitely extended solids which are separated by a gap x ... 

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