Specific Association Term

The energy of solvation can be further broken down into terms that are a function of the bulk solvent and terms that are specifically associated with the first solvation shell. The bulk solvent contribution is primarily the result of dielectric shielding of electrostatic charge interactions. In the simplest form, this can be included in electrostatic interactions by including a dielectric constant k, as in the following Coulombic interaction equation ... 

The statistical thermodynamic approach of Pitzer (14), involving specific interaction terms on the basis of the kinetic core effect, has provided coefficients which are a function of the ionic strength. The coefficients, as the stoichiometric association constants in our ion-pairing model, are obtained empirically in simple solutions and are then used to predict the activity coefficients in complex solutions. The Pitzer approach uses, however, a first term akin to the Debye-Huckel one to represent nonspecific effects at all concentrations. This weakens somewhat its theoretical foundation. 

Several hundred individual hydrocarbon chemicals defined as petroleum-based have been identified. Furthermore, each individual crude oil and each individual petroleum product has a specific mixture of the various constituents because of the variation in petroleum composition (Chapter 2), and this variation is reflected in the composition of the finished petroleum product. At this point it is worthy of note that the term petroleum hydrocarbons (PHC) is widely used to refer to the hydrogen- and carbon-containing compounds originating from crude oil, but petroleum hydrocarbons should be distinguished from total petroleum hydrocarbons because the term total petroleum hydrocarbons is specifically associated with environmental sampling and analytical results (Weisman, 1998 CFR, 2004). 

The four electrostatic interactions which involve the dipole moments and the polarizabilities of the solute and solvent molecules are termed non-specific because they do not imply a fixed geometry or stoichiometry of the molecules. In a specific association the molecules form a loose complex, of which hydrogen bonding is the most important example. Hydrogen bonding takes place between a protic (proton donor or acid) molecule such as water and a proton acceptor (base) such as an amine (Figure 3.48). 

If the reaction mixture is very dilute in the reactants and the products, the activity coefficients can all be approximated by unity. Then the last term on the right hand side of Eq. (2.20) vanishes, and the left hand side can be written as AG° = -RT n ATsolution, the equilibrium quotient becoming the equilibrium constant. Under ordinary conditions, however, the activity coefficient term must be taken into account, since there are solvent effects on all the terms on the right hand side except -RTIn K". The fact that different numbers of solvent molecules may specifically associate with the reactants and the products and that solvent molecules may be released or consumed in the reaction should not be included explicitly, since this effect is already covered by the terms in AG s of solvation of the reactants and products according to our definition of this concept. 

The wavenumber displacement of a solute vibration is a complex function of both solute and solvent properties and can be explained in terms of weak nonspecific electrostatic interactions (dipole-dipole, dipole-induced dipole, etc.) and of strong specific association of solute with solvent molecules, usually of the hydrogen-bond type [140], It should be realized that the duration of vibrational transitions is very short with respect to motion of the solvent molecules e.g. for an O—H stretching vibration, the frequency is ca. 10 s ). Thus, it is possible to observe such transitions even for short-lived entities such as may arise after a collision in the liquid phase (collision complexes) [140],... 

Another illustrative example is that of phenylacetylene. Table 6-7 summarizes the H NMR chemical shifts of its alkyne H-atom in a variety of solvents [273], Most solvents (except aromatic solvents) decrease the shielding of the acetylenic hydrogen nuclei. The corresponding low-field shifts have been interpreted in terms of weak specific association between the alkyne as hydrogen-bond-donor and electron pair-donor groupings of the solvent [273], The high-field shifts in aromatic solvents arise from the magnetic anisotropy of the solvent molecules (see below). The order of effectiveness of the solvent... 

Aromaticity is a term which requires careful definition in terms of thermodynamic, kinetic, and mechanistic criteria. For the purpose of this review it is defined as an additional stabilization of a molecule specifically associated with delocalization of tt electrons contained in a closed molecular orbital shell. The determination of this specific stabilization or resonance energy may be carried out experimentally or theoretically. Table XVIII contains a number of experimentally accessible parameters and their theoretical counterparts which have all been used to arrive at its value. 

In this chapter we describe ceramic dielectrics. A dielectric is by definition an electrical insulator (p is high and is large). That means that dielectric behavior is a property associated with certain ceramics and polymers but not a property associated with metals. We begin with a background section. Some of this material may have been covered before but perhaps not specifically in terms of ceramics. 

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