Charge-induced dipole forces types

Induction of electric charge occurs when a charge on one object causes a change in the distribution of charge on a nearby object. Rubbing a balloon to make it "stick" to a wall is an example of charging by induction. There are two types of charge-induced dipole forces. 

Ion-induced dipole and dipole-induced dipole forces are the two types of charge-induced dipole forces they are most important in solution, so we ll focus on them in Chapter 13. Nevertheless, polarizability affects all intermolecular forces. 

Ion-induced dipole forces are one of two types of charge-induced dipole forces, which rely on the polarizability of the components. They result when an ion s charge distorts the electron cloud of a nearby nonpolar molecule. This type of force plays an essential biological role that initiates the binding of the Fe " " ion in hemoglobin and an O2 molecule in the bloodstream. Because an ion increases the magnitude of any nearby dipole, ion-induced dipole forces also contribute to the solubility of salts in less polar solvents, such as LiCl in ethanol. 

The van der Waals radius determines the shortest distance over which intermolecu-iar forces operate it is aiways larger than the covalent radius. Intermolecular forces are much weaker than bonding (intramolecular) forces. Ion-dipole forces occur between ions and poiar molecules. Dipole-dipole forces occur between oppositely charged poles on polar molecules. Hydrogen bonding, a special type of dipole-dipole force, occurs when H bonded to N, O, or F is attracted to the lone pair of N, O, or F in another molecule. Electron clouds can be distorted (polarized) in an electric field. Dispersion (London) forces are instantaneous dipole-induced dipole forces that occur among all particles and increase with number of electrons (molar mass). Molecular shape determines the extent of contact between molecules and can be a factor in the strength of dispersion forces. 

These types of attractions occur when the charge on an ion or a dipole distorts the electron cloud of a nonpolar molecule. This induces a temporary dipole in the nonpolar molecule. These are fairly weak interactions. Like an ion-dipole force, this type of force requires the presence of two different substances. 

A second type of force between water molecules and the metal consists of the dispersion forces. Dispersion forces (or London forces) can be seen classically as follows A time-averaged picture of any atom shows spherical symmetry because the charge due to the electrons orbiting around the nucleus is smoothed out in time. An instantaneous picture of, say, a hydrogen atom, would, however, show a proton here and an electron there—two charges separated by a distance. Hence, every atom has an instantaneous dipole moment of course, the time average of all these dipole moments is zero. This instantaneous dipole will induce an instantaneous dipole in a contiguous atom, and an instantaneous dipole-dipole force will arise. When these... 

If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. If the solvent is nonpolar, then the rough description of the interaction is dipole-induced dipole type. In polar solvents, dipole-dipole interactions also become important. The London forces are always present. For the calculation of dipole-dipole interaction energy, point dipole approximations are made which are poor description for large extended molecules. 

Van der Waals Forces. Van der Waals interactions are of two types one attractive and one repulsive. Attractive van der Waals forces involve interactions among induced dipoles that arise from fluctuations in the electron charge densities of neighboring nonbonded atoms. Such interactions amount to 0.1-0.2 kcal/mol despite their small size, the large number of such interactions that occur when molecules come close together makes such interactions quite significant. Van der Waals forces favor close packing in folded protein structures. 

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

There are different types of intermolecular forces. Between nonpolar molecules, the force is weak and is called a dispersion force, or induced dipole. The force between oppositely charged ends of two polar molecules is called a dipole-dipole force. The more polar the molecule, the stronger the dipole-dipole force. The third force, a hydrogen bond, is especially strong. It forms between the hydrogen end of one dipole and a fluorine, oxygen, or nitrogen atom on another dipole. 

A phenomenon called dielectrophoresis has been used to pattern a variety of cell types on 2D substratesand more recently in 3D culture constructs. Unlike electrophoresis where charged species move in an applied electric field due to Coulombic forces (F = qE), dielectrophoresis capitalizes on the ability of a cell to become polarized when placed in an electric field. Dielectrophoresis is most often used in conjunction with dtemating current (AC) electric fields since AC fields eliminate electrophoretic movement, and have less physiological impact on cells than direct current (DC) fields. When a cell is placed in an AC field, the magnitude and polarity of the induced dipole depend on the frequency of the applied field and the conductivities of the cell and the surrounding medium, described by the equation... 

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