Electrostatic theory concepts

More recently, electrostatic theory has been revived due to the concept of molecular electrostatic potentials. The potential of the solute molecule or ion was used successfully to discuss preferred orientations of solvent molecules or solvation sites 50—54). Electrostatic potentials can be calculated without further difficulty provided the nuclear geometry (Rk) and the electron density function q(R) or the molecular wave function W rxc, [Pg.14]

M. Alcami, O. Mo and M. Yanez, Modeling Intrinsic Basicities The Use of Electrostatic Potentials and the Atoms-in-Molecules Theory, in Molecular Electrostatic Potentials Concepts and Applications (J. S. Murray and K. Sen, Eds.). Elsevier, Amsterdam 1996. Page 407. 

The aluminum oxide surfaces exhibit only a weak Bronsted acidity. Therefore, studies of its surface hydroxyls have usually dealt with their structure. IR spectroscopy appears the most informative and appropriate method for these purposes. The IR spectra cover a wide variety of hydroxyl stretching vibrations, and their assignment is frequently quite a problem. Primarily, it was based on electrostatic concepts and on the electrostatic theory of valency (138). 

The concept of coordination in the second sphere was introduced by Werner. All authors agree that such outer-sphere association exists in solution, hut they disagree about the kind and the extent of this association. Some advocate a second-sphere coordination which is closely analogous to the inner-sphere coordination. The data which support this hypothesis are not very convincing and can be criticized in various ways. The present author finds that the electrostatic theories of N. Bjerrum, Fuoss, and Kraus, according to which the formation of the ion-associates is a result of coulombic attraction, both qualitatively and quantitatively, give the most trustworthy picture of the outer-sphere association. However, this does not exclude the fact that some preferred mutual orientation exists in the ion pairs. 

K5ster, A. M. Leboeuf, M. Salahub, D. R. Molecular Electrostatic Potentials from Density Functional Theory. In Molecular Electrostatic Potentials, Concepts and Applications, Murray, J. S. Sen, K., Eds. Elsevier Amsterdam, 1996 pp 105-142. 

Thus, when investigating the nature and mechanism of adhesion between an adhesive, coating or polymer matrix and the substrate, it is important to consider the possibility of primary bond formation in addition to the interactions that may occur as a result of Dispersion forces and Poiar forces. In addition to the Adsorption theory of adhesion, adhesion interactions can sometimes be described by the Diffusion theory of adhesion, Electrostatic theory of adhesion, or Mechanical theory of adhesion. Recent work has addressed the formation of primary bonding at the interface as a feature that is desirable from a durability point of view and a phenomenon that one should aim to design into an interface. The concept of engineering the interface in such a way is relatively new, but as adhesives become more widely used in evermore demanding applications, and the performance XPS and ToF-SIMS systems continues to increase, it is anticipated that such investigations can only become more popular. 

This article, and related ones giving a more detailed explanation of individnal theories (Adsorption theory of adhesion. Diffusion theory of adhesion. Electrostatic theory of adhesion and Mechanical theory of adhesion), exponnd what could be termed classical theories of adhesion. In cross-referenced articles, more recent ideas are explored. As emphasized above, the concepts of the classical theories overlap and merge seamlessly in providing a model of the empirical observations. The tendency of reducing the interpretation of adhesion phenomena to narrowly conceived theories of adhesion should be avoided, and a broader view should be adopted, using whichever blend of concepts best suits the purpose. 

We extend our imderstanding of the concepts of chemical bonding and reactivity learned in Chapter 3 on metals and Chapter 4 on zeolites to catalysis over metal oxides and metal sulfides in Chapter 5. The featmes that lead to the generation of surface acidity and basicity are described via simple electrostatic bonding theory concepts that were initially introduced by Pauling. The acidity of the material and its application to heterogeneous catalysis are sensitive to the presence of water or other protic solvents. We explicitly examine the effects of the reaction medium in which the reaction is carried out. In addition, we compare and contrast the differences between liquid and solid acids. We subsequently describe the influence of covalent contributions to the bonding in oxides and transition to a discussion on the factors that control selective oxidation. 

Gouy and Chapman realised that in an electrolyte solution, the charges are free to move and are subject to thermal motion. They retained the concepts of the electrostatic theory to describe the coulombic metal-counter ion interaction but, in addition, they allowed for the random motion of the ions. The result is a diffuse layer of charge in which the concentration of counter ions is greatest next to the electrode surface and decreases progressively towards a homogeneous distribution of ions within the bulk electrolyte (Fig. 5.1b). 

Color from Transition-Metal Compounds and Impurities. The energy levels of the excited states of the unpaked electrons of transition-metal ions in crystals are controlled by the field of the surrounding cations or cationic groups. Erom a purely ionic point of view, this is explained by the electrostatic interactions of crystal field theory ligand field theory is a more advanced approach also incorporating molecular orbital concepts. 

Viewing things from the perspective of his physical theory of contact electricity, Volta was intrigued by the apparently endless power of the battery to keep the electric fluid in motion without the mechanical actions needed to operate the classical, friction, electrostatic machine, and the electrophorus. He called his batteiy alternately the artificial electric organ, in homage to the torpedo fish that had supplied the idea, and the electromotive apparatus, alluding to the perpetual motion (his words) of the electric fluid achieved by the machine. To explain that motion Volta relied, rather than on the concepts of energy available around 1800, on his own notion of electric tension. He occasionally defined tension as the effort each point of an electrified body makes to get rid of its electricity but above all he confidently and consistently measured it with the electrometer. 

In order to describe the effects of the double layer on the particle motion, the Poisson equation is used. The Poisson equation relates the electrostatic potential field to the charge density in the double layer, and this gives rise to the concepts of zeta-potential and surface of shear. Using extensions of the double-layer theory, Debye and Huckel, Smoluchowski,... 

The mechanism that has been developed for the conversion of methane to methanol by FeO+ is an excellent example of the synergy between experiment and theory. This mechanism includes two key concepts concerted reaction involving the critical [HO—Fe—CH3] insertion intermediate and two-state reactivity. The reaction proceeds as follows electrostatic interaction between FeO+ and methane produces the [OFe- GHJ entrance channel complex. 

As a result of these electrostatic effects aqueous solutions of electrolytes behave in a way that is non-ideal. This non-ideality has been accounted for successfully in dilute solutions by application of the Debye-Huckel theory, which introduces the concept of ionic activity. The Debye-Huckel Umiting law states that the mean ionic activity coefficient y+ can be related to the charges on the ions, and z, by the equation... 

This concept is due to Bjerrum, who in 1926 suggested that in simple electrolytes ions of the opposite charge could associate to form ion-pairs (Szwarc, 1965 Robinson Stokes, 1959). This concept of Bjerrum arose from problems with the Debye-Huckel theory, when the assumption that the electrostatic interaction was small compared with IcTwas not justified. 

An appreciable advance in the theory of electrostatic interaction between ions in solution was made in 1923 by Peter Debye and Erich Hiickel, who introduced the concept of ionic atmosphere to characterize the averaged distribution of the ions. In its initial form the theory was applied to fully dissociated electrolytes hence, it was named the theory of strong electrolytes. 

J. Bjerrum (1926) first developed the theory of ion association. He introduced the concept of a certain critical distance between the cation and the anion at which the electrostatic attractive force is balanced by the mean force corresponding to thermal motion. The energy of the ion is at a minimum at this distance. The method of calculation is analogous to that of Debye and Hiickel in the theory of activity coefficients (see Section 1.3.1). The probability Pt dr has to be found for the ith ion species to be present in a volume element in the shape of a spherical shell with thickness dr at a sufficiently small distance r from the central ion (index k). 

It is assumed that the quantity Cc is not a function of the electrolyte concentration c, and changes only with the charge cr, while Cd depends both on o and on c, according to the diffuse layer theory (see below). The validity of this relationship is a necessary condition for the case where the adsorption of ions in the double layer is purely electrostatic in nature. Experiments have demonstrated that the concept of the electrical double layer without specific adsorption is applicable to a very limited number of systems. Specific adsorption apparently does not occur in LiF, NaF and KF solutions (except at high concentrations, where anomalous phenomena occur). At potentials that are appropriately more negative than Epzc, where adsorption of anions is absent, no specific adsorption occurs for the salts of... 

To exploit the concept of PMF to represent solvent in free energy calculations, practical approximations must be constructed. A common approach is to treat the two components Z H/"P(X) and Z lYelec(X) separately. Approximations for the nonpolar term are usually derived from geometric considerations, as in scaled particle theory, for example [62], The electrostatic contribution is usually derived from continuum electrostatics. We consider these two contributions in turn. 

The two major theories of flocculation, the bridging model (1) and the electrostatic patch model (2, 3 ), provide the conceptual framework for the understanding of polymer-aided flocculation, but they do not directly address the kinetics of the process. Smellie and La Mer (4) incorporated the bridging concept into a kinetic model of flocculation. They proposed that the collision efficiency in the flocculation process should be a function of the fractional surface coverage, 0. Using a modified Smoluchowski equation, they wrote for the initial flocculation rate... 

The final part is devoted to a survey of molecular properties of special interest to the medicinal chemist. The Theory of Atoms in Molecules by R. F.W. Bader et al., presented in Chapter 7, enables the quantitative use of chemical concepts, for example those of the functional group in organic chemistry or molecular similarity in medicinal chemistry, for prediction and understanding of chemical processes. This contribution also discusses possible applications of the theory to QSAR. Another important property that can be derived by use of QC calculations is the molecular electrostatic potential. J.S. Murray and P. Politzer describe the use of this property for description of noncovalent interactions between ligand and receptor, and the design of new compounds with specific features (Chapter 8). In Chapter 9, H.D. and M. Holtje describe the use of QC methods to parameterize force-field parameters, and applications to a pharmacophore search of enzyme inhibitors. The authors also show the use of QC methods for investigation of charge-transfer complexes. 

Compared to all other intermolecular interactions, the Coulomb interaction is described by a simple law, i.e.. Equation 15.2. A theory for Coulombic interaction, therefore, uses the concepts and laws that have been developed in classical electrostatics. However, it is worth pointing out that the dielectric constant is a macroscopic property and it is therefore, in principle, not correct to describe the solvent as a dielectric continuum on the molecular level. Nevertheless, experience has shown that it is in fact a useful approximation. 

The concept of bond valence, which, as will be shown below, is the same as the bond flux derived in Chapter 2, grew out of attempts to refine Pauling s principles determining the structures of complex ionic crystals (Section 1.7). In this empirical evolution of Pauling s model, both the electrostatic and short-range components were developed simultaneously. Only later did it become apparent that it was also possible to derive the properties of the electrostatic component independently using the ionic theory. 

The conception of the electrostatic bond has been found to be most valuable in the field of inorganic chemistry and has helped to clarify a great many phenomena. It has frequently made it possible to predict properties both of unknown compounds and of those which have been little investigated. Nevertheless, a number of difficulties have been encountered. For example, in Section 43, it was shown that the theory did not really provide a satisfactory explanation of the low symmetry of molecules. Again, the theory failed to explain the volatility and small dipole moment of the compounds CO, NO, C120, etc., and lastly, it threw no light whatever on the reason for the existence of molecules of elements such as H2, Cl2, Oa, N2, etc., especially as dipole measurements show that the formulae H+H, Ci+ci are excluded. 

Relation of Electrostatic and Statistical-Mechanical Approaches to Interionic Theory. We believe that the ionic cloud concept is appropriate for the... 

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