Induced dipole cohesion

Quantum mechanical calculations were also applied to evaluate the nature of the weak interactions between atoms. These weak interactions are responsible for the induced dipole cohesion of neutral atoms. It has been shown that the dispersion interaction energy of two atoms or ions with closed electronic configurations varied according to their polarizability and ionization energies. London used perturbation theory to obtain a simplified expression for the dispersion energy, Ejj, of two interacting species i and /, at separation r. 

The intermolecular forces of adhesion and cohesion can be loosely classified into three categories quantum mechanical forces, pure electrostatic forces, and polarization forces. Quantum mechanical forces give rise both to covalent bonding and to the exchange interactions that balance tile attractive forces when matter is compressed to the point where outer electron orbits interpenetrate. Pure electrostatic interactions include Coulomb forces between charged ions, permanent dipoles, and quadrupoles. Polarization forces arise from the dipole moments induced in atoms and molecules by the electric fields of nearby charges and other permanent and induced dipoles. 

The interactions between molecules which produce the cohesive energy characteristic of the liquid phase are described in the section entitled Secondary Forces Between Solvent and Solute Molecules. These involve the dispersion forces, dipole-dipole and dipole-induced dipole interactions, and specific interactions, especially hydrogen bonding. If it is assumed that the intermolecular forces are the same in the vapor and liquid states, then -E is the energy of a liquid relative to its ideal vapor at the same temperature. It can be described as the energy required to vaporize 1 mole of liquid to the saturated vapor phase (Af U) plus the energy required for the isothermal expansion of the saturated vapor to infinite volume. Detailed discussion of the theory and derivations is given in the publications by Hildebrand and associates cited above. 

Dispersive Forces. In the absence of permanent or induced dipoles, London dispersive forces (17) become important. Random fluctuations in the electron cloud produce a time-varying, temperature-independent intermolecular force of attraction termed the dispersive force. The magnitude of these dispersive forces (typically 0.1 to 2 kcal/mole) can be represented by a variety of cohesive parameters Including the dispersive component of the Hansen solubility parameter (16). 

If a nonpolar component (aliphatic hydrocarbon or tetrachloromethane) is added, almost nothing happens (Figure 3.8a). The solubility of octane in either the micellar solution or the liquid crystal is very limited. This is true of any molecule exhibiting only dispersion cohesive forces (induced dipole-induced dipole van der Waals forces). 

From the Solubility Parameters The cohesive properties of a material are related to the solubility parameter, 5., that originates from different types of interactions dispersive or atomic, molecular of the type polar and hydrogen bonding, induced dipoles, metallic, etc. The first three types are most important, thus respectively ... 

Until the advent of quantum mechanics the reasons for the stability of molecules were unknown. The cohesive energy of ionic crystals could be adequately interpreted on the purely classical basis of the electrical attraction of the oppositely charged ions. Some attempts were made to interpret the interaction of all atoms on the basis of the electrical interaction of positive and negative charges, electrical dipoles, induced dipoles, and so on. These classical calculations indicated that the bonding between two like atoms, such as two hydrogen atoms, should be very much weaker than it is. This is another problem that classical physics failed to solve. 

Intermolecular forces in saturated hydrocarbons are practically entirely due to the London dispersion forces. These forces are the result of the interaction of fluctuating electric dipoles with the induced dipoles they contribute to the cohesion in all substances, but their magnitude depends on the type of material and its density. Many substances have other intermolecular forces in addition to the dispersion forces. In the case of mercury, the interatomic forces involve the dispersion forces and the metallic bond in the case of water, they involve dispersion forces and dipole interactions (mainly hydrogen bonds). Since the dispersion forces are not appreciably influenced by other intermolecular forces, one can assume dispersion forces and other intermolecular forces generally to be additive. Thus, in the case of the surface tension of water, the surface tension can be considered the sum of a contribution resulting from dispersion forces, y and a contribution resulting from the dipole interactions, mainly hydrogen bonds,... 

Assume that the cohesive energy, — U, of a substance consists of contributions due to interactions through dispersion or nonpolar forces, — 2 tf, through dipole-dipole and dipole-induced dipole forces, — i/p, and hydrogen bonds, — ... 

The remaining interactions in molecules that have permanent dipole moments, that is, the dipole-dipole and dipole-induced-dipole interactions, have the same dependence on intermolecular separation as the London potential, varying as and are of lesser magnitude at ordinary temperatures (Atkins, 1998). These two interactions and the previously mentioned dispersion interaction are collectively known as van der Waals interactions. They are related to such measurable properties as surface tension and energy of vaporization, and to concepts such as the internal pressure, the cohesive energy density (energy of vaporization per unit volume, A UIV), and the solubility parameter, 6, which is the square root of the cohesive energy density (Hildebrand and Scott, 1962). 

We consider first the effect of aqueous solvation on all intermolecular stabilizations that derive from the interaction of charged or dipolar species. Because of its small size and significant dipole and quadrupole, water interacts strongly with all ionic and dipolar species. A binding event of charged or dipolar compounds thus proceeds with a significant loss of favorable cohesive interactions between solutes and water. The effect is most profound for ionic interactions similar ameliorations of solute-solute interaction apply to multipole-multipole and dipole-induced dipole interactions. 

Van der Waals Interactions, Involving either transiently induced or permanent electric dipoles, occur In all types of molecules, both polar and nonpolar. In particular, van der Waals interactions are responsible for the cohesion between molecules of nonpolar liquids and solids, such as heptane, CH3—(CFl2)5—CFI3, that cannot form hydrogen bonds or ionic interactions with other molecules. The strength of van der Waals Interactions decreases rapidly with Increasing distance thus these noncovalent bonds can form only when... 

In the crystals of inherent gases the cohesion between atoms is given by the van der Waals interaction. The origin of this interaction consists in the fact that the electrons are in movement around the nucleus, even in the lowest energetic state, so even for OK. The electrons movement produces a dipole moment instantaneous non-null, which will induce a dipole moment instantaneous in the neighboring atom and so one. The van der Waals interaction is un-oriented, thereby, in these structures will appear the tendency that an atom will be siuroimded with a maximum number from other atoms. 

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