Dipoles, cause

A fourth possibility is electrodynamic bonding. This arises because atoms and molecules are not static, but are dynamically polarizable into dipoles. Each dipole oscillates, sending out an electromagnetic field which interacts with other nearby dipoles causing them to oscillate. As the dipoles exchange electro-magnetic energy (photons), they attract one another (London, 1937). 

The author believes that dipoles cause deformation hardening because this is consistent with direct observations of the behavior of dislocations in LiF crystals (Gilman and Johnston, 1960). However, most authors associate deformation hardening with checkerboard arrays of dislocations originally proposed by G. I. Taylor (1934), and which leads the flow stress being proportional to the square root of the dislocation density instead of the linear proportionality expected for the dipole theory and observed for LiF crystals. The experimental discrepancy may well derive from the relative instability of a deformed metal crystal compared with LiF. For example, the structure in Cu is not stable at room temperature. Since the measurements of dislocation densities for copper are not in situ measurements, they may not be representative of the state of a metal during deformation (Livingston, 1962). 

Dispersion forces result from temporary dipoles caused by polarization of electron clouds... 

Almost all interfacial phenomena are influenced to various extents by forces that have their origin in atomic- and molecular-level interactions due to the induced or permanent polarities created in molecules by the electric fields of neighboring molecules or due to the instantaneous dipoles caused by the positions of the electrons around the nuclei. These forces consist of three major categories known as Keesom interactions (permanent dipole/permanent dipole interactions), Debye interactions (permanent dipole/induced dipole interactions), and London interactions (induced dipole/induced dipole interactions). The three are known collectively as the van der Waals interactions and play a major role in determining material properties and behavior important in colloid and surface chemistry. The purpose of the present chapter is to outline the basic ideas and equations behind these forces and to illustrate how they affect some of the material properties of interest to us. 

Doubt can arise at this point as to whether molecules with unusual shapes are really ionic compounds. It must be remembered, however, that dipoles arise because the particles still retain some charge, otherwise the moment would be zero. The problem still remains as to how the particles in HC1, NO and CO can have charges which are only fractions of that of an indivisible electron. The difficulty is partly resolved by bringing the polarization of the ions into consideration, for if both ions in NaCl are polarized, then in both ions dipoles are created, which are opposite to the dipole caused by the charges see Figure 31). What, in fact, is measured is the resultant of these three dipoles. The observed dipole is therefore always smaller than the dipole which is caused by the ionic charges. This... 

Alefeld and co-workers (24, 25) have discussed the hydrogen-hydrogen attractive interaction by using the elasticity theory developed for defects in solids by Eshelby and others (46). The strength of the elastic dipole moment is related to the volume expansion resulting from the interstitial hydrogen (25) (Equation 10), where P is the strength of the elastic dipole caused by the interstitial species... 

Induction forces. These arise when a molecule with a permanent dipole caused by a polar group (C-Cl, C=0, C-NO2), induces a dipole in a neighboring molecule. This effect is particularly strong with aromatics because of the high polarizability of the easily displaced -electrons-e.g., low molecular weight esters and polystyrene, or benzene and poly (vinyl acetate). 

The induced dipole moment of the HD-X systems, with X = He, Ar, H2, HD, is well known from the fundamental theory, for the purely rotational bands and also for most fundamental bands [59]. To the induced dipole, the permanent dipole moment of HD has to be added vectorially, accounting for the linear variation with density which differs from the density variation of the induced dipole components [391]. According to the theory of intracollisional interference (as the process was called, to be distinguised from the intercollisional interference considered elsewhere in this monograph), interference occurs for those induced components that are of the same symmetry as the allowed dipole, namely A1A2AL = 0110 and 1010 [178, 179, 321, 389]. These induced components are always parallel or antiparallel to the allowed dipole, causing constructive or destructive interference. 

The forces involved in the interaction al a good release interface must be as weak as possible. They cannot be the strong primary bonds associated with ionic, covalent, and metallic bonding neither arc they the stronger of the electrostatic and polarization forces that contribute to secondary van der Waals interactions. Rather, they are the weakest of these types of forces, the so-called London or dispersion forces that arise from interactions of temporary dipoles caused by fluctuations in electron density. They are common to all matter. The surfaces that are solid at room temperature and have the lowest dispersion-force interactions are those comprised of aliphatic hydrocarbons and fluorocarbons. 

The opposite effect to electrophoresis is the generation of a sedimentation potential. If a charged particle moves in the gravitational field or in a centrifuge, an electric potential arises — the sedimentation potential. While the particle moves, the ions in the electric double layer lag somewhat behind due to the liquid flow. A dipole moment is generated. The sum of all dipoles causes the sedimentation potential. 

London (dispersion) forces Attractive forces between transient dipoles caused by random changes in the electron distribution of a molecule (one part of a molecule temporarily becomes slightly positively charged while another part becomes slightly negatively charged). 

In contrast to the LS3 pore, the water molecules were frozen in the tetrameric LS2 pore, with diffusion coefficients of zero. They were found to be aligned antiparallel to the helix dipole caused by the orientation of the hydroxyl groups of the serine residues. This enabled formation of a water wire network important for the transport of protons by the proton wire or Grotthiis mechanism. 

When infrared light interacts with the fluctuating electric dipole caused by the vibration of the constituent atoms in molecules or crystals, absorption may occur when the energy of the radiation matches that of the vibration. This fluctuating electric dipole can be considered to arise if two centers (atoms) of equal and opposite charge ( Q) are separated by a distance r, when the dipole moment ( x) will be ... 

For SFe, the measurements of Rosenberg and Bimbaum give a very small value of B, comparable to those obtained from atomic gas mixtures. By neglecting possible contributions from dipoles caused by overlap forces, and attributing all of to the effects of the hexadecapole moment, Rosenberg and Bimbaum calculate an upper limit of + 17 x 10 Cm for [Pg.52]

The excitation spectrum of di-8-ANEPPS is altered when it lines up (symmetrically or asymmetrically) with the membrane dipoles causing electronic redistributions within the probe molecule (see e.g. Fig. 5a). This promotes red or blue shifts in the excitation spectrum depending on the magnitude and direction of the dipole moment of the ambient environment that the probe finds itself in as shown in Fig. 5b. Preparation of membranes with sterols etc (ie that possess quite different dipole-moments to PC) promote changes in the membrane dipole potential, and significant variations of the intensity and position of the excitation maximum are observed. The excitation spectrum of di-8-ANEPPS in phosphatidylcholine (PC) membranes for example is significantly altered when 15mol% of either 6-ketocholestanol (KC) or phloretin are added to such membranes. In the case of phloretin the difference spectmm has a minimum at 450 nm and a maximum at 520 nm (Fig. 5b). In the case of KC, however, the difference spectrum has a maximum at 450 nm and a minimum at 520 nm, which is the opposite effect to that of phloretin. 

An electric field applied to a molecule with a permanent dipole causes a shift in charge center. If field-induced displacement is x and the total charge is then the induced dipole moment is ... 

Sensors based on adsorption of species onto or into lattice structures have been reported for molecules besides water. For example, devices based on the detection of carbon dioxide adsorption onto semiconductor materials have been developed [10]. In other cases, dielectric materials that have some degree of chemical specificity have been used for making chemically-sensitive layers. One such application is the use of the highly porous zeolite lattice to detect adsorbed hydrocarbons [11]. The specific dimensions and shape of the zeolite pores allows for size and chemical selectivity in the lattice. As in the case of the humidity devices, the adsorbed molecules dipoles cause a local change in the electric fields that can be detected through a capacitive effect. 

Ferroelectric means the spontaneous alignment of electric dipoles caused by interactions between them domains form in an analogous manner to the domains of magnetic dipoles in a ferromagnetic material (see Figure 20.25 and related discussion). 

Fig. 5.2. A chart showing the work function of several metals (a) [81] and the HOMO/LUMO levels of several organic semicondnctors relative to the vacuum level (b) [82] [83] in eV. Theoretically, in n-type devices we want to inject electrons and therefore wonld expect to see the best performance using electrodes whose work function lines up with the LUMO, and for p-type contacts (injecting holes) hned up with the the HOMO. In practice, noble metals appear to work best for most material systems, (c) Contact dipoles caused by chemical interactions with the metal surface, dopants, and contaminants can vary the energy level/contact offsets-those seen by [82] [83] are shown. On this diagram, these offsets appear as a shift in the vacuum level. Note that the effective energy levels for organic semiconductors wiU differ on different surfaces these are both measured deposited in a thin layer on gold.

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