Enthalpy of the Acid-Base Interaction

Polymers containing polar groupings are often one of many components of a coating composition. Other components can be plasticisers, additives, solvents, fillers and pigments. Non-dispersive interactions among these components may affect numerous properties of the system, including the rheology, the mechanical properties and the adhesion characteristics. Almost all materials mentioned previously, have some electron donor or electron acceptor character and the non-dispersive enthalpy of such interactions can be estimated as the enthalpy of the acid/base interactions. 

Enthalpy of the acid-base interaction Enthalpy of adsorption Planck constant /i/2tt Drop height Fracture toughness... 

This interaction actually involves both covalent (homopolar) and ionic (heteropolar) factors. Thus, Drago et al. introduced four parameters for the prediction of the enthalpies of the acid-base interactions. For an A-B pair, the enthalpy or molar energy of the adduct formation can be expressed by the following empirical relation ... 

Basic Theory An important method of estimating bonding energies in adhesion related calculations utilizes acid-base interactions, where polar organic molecules are considered to be either electron donors or acceptors. As will be explored below, the estimation of the strength of the interaction utilizes infrared spectroscopy. For example, the shift in the -OH stretching frequency can be used to calculate the enthalpy of such acid-base interactions, AH. 

Upon combining Eqs. 88 and 85, one finds that the total work of adhesion is related to the surface energies of the contacting materials as well as the enthalpy associated with acid-base interactions by... 

However, the strength of Lewis acid-base interaction can be expressed in energy terms, such as the exothermic molar heat, —for the equilibrium (III) of adduct formation. The enthalpy term is preferred because entropy effects accompanying the formation of coordinative bonds are difficult to determine. Various models have been proposed for the theoretical estimation of the enthalpy term based on molecular properties of reactants and are reviewed in Ref 5. The most significant developments have been the hard and soft acid-base principle of Pearson [6], the E C equation of Drago and Wayland [7], the donor and acceptor numbers of Gutmann [8], and the perturbation theory of Hudson and Klopman [9]. 

The influence of a solvent complicates any understanding of acid-base interactions in solutions. The calculation of aqueous acidity constants would require the calculation of the free enthalpy of solvation of the acidic site, conjugate base, and proton, in addition to the gas-phase term for the change of free enthalpy during deprotonation and, of course, all entropy terms. While the entropy o/deprotona-tion is nearly independent of the acid-base couple, the free enthalpy of solvation can be obtained only with difficulty from molecular dynamics calculations [13]. 

Fowkes was one of the first people to emphasize the importance of acid-base interactions in the context of adhesion. The volume of papers collected to honour his 75 birthday provides an important source of detailed information. Fowkes has argued that the acid-base component of the work of adhesion can be related to the molar enthalpy, for the acid-base reaction by... 

Calorimetric enthalpy data have also been reported for the acid-base interaction of 2,2,2-trifluoroethanol with a variety of Lewis bases the correlations observed have been compared with similar data for phenol and 1,1,1,-3,3,3-hexafluoropropan-2-ol (see above) to reveal the relative abilities of phenyl, trifluoroethyl, and hexafiuoroisopropyl substituents to withdraw electron density from OH and to characterize their rdle in hydrogen bond formation. Interest is growing in the use of 2,2,2-trifluoroethanol as a solvent in solvolysis studies. - Values of the standard enthalpies of formation of the fluorinated propan-l-ols CFgCHgCHgOH, CHFj-CFaCHj-OH, and CFg-CFa CHa-OH are now available. Trifluoroethanol and heptafluorobutan-l-ol exert a narcotic action on the central nervous system, and the noaximum permissible concentrations of the vapours of these alcohols in air are 10 mg m and 20 mg m , respectively. - ... 

The chemistry of Lewis acid-base adducts (electron-pair donor-acceptor complexes) has stimulated the development of measures of the Lewis basicity of solvents. Jensen and Persson have reviewed these. Gutmann defined the donor number (DN) as the negative of the enthalpy change (in kcal moL ) for the interaction of an electron-pair donor with SbCls in a dilute solution in dichloroethane. DN has been widely used to correlate complexing data, but side reactions can lead to inaccurate DN values for some solvents. Maria and Gal measured the enthalpy change of this reaction... 

In the case of the four-parameter equation, the enthalpies of interaction of a large number of acids and bases were determined calorimetrically in an inert solvent. With these values being known, a value of 1.00 was assigned for EA and CA for the Lewis acid iodine. The experimental enthalpies for the interaction of iodine with several molecular Lewis bases were fitted to the data to determine EB and CB values for the bases. Values were thus established for the four parameters for many acids and bases so that they can be used in Eq. (9.112) to calculate the enthalpies of the interactions. The agreement of the experimental and calculated enthalpies is excellent in most cases. However, the four-parameter approach is used primarily in conjunction with interactions between molecular species, although extensions of the approach to include interactions between charged species have been made. Table 9.7 gives the parameters for several acids and bases. 

It is the interaction of the Lewis acid and base in the gas phase that gives the enthalpy of the A B bond, AHAB, but what is more often (and conveniently) measured is the enthalpy change when the reaction is carried out in solution, AH AB. 

Steric interactions between bulky substituents such as t-Bu, leading to larger C-E-C bond angles, obviously affect the Lewis basicity caused by the increased -character of the electron lone pair. However, the strength of the Lewis acid-base interaction within an adduct as expressed by its dissociation enthalpy does not necessarily reflect the Lewis acidity and basicity of the pure fragments, because steric (repulsive) interactions between the substituents bound to both central elements may play a contradictory role. In particular, adducts containing small group 13/15 elements are very sensitive to such interactions as was shown for amine-borane and -alane adducts... 

The major disadvantage of the HSAB principle is its qualitative nature. Several models of acid-base reactions have been developed on a quantitative basis and have application to solvent extraction. Once such model uses donor numbers [8], which were proposed to correlate the effect of an adduct on an acidic solute with the basicity of the adduct (i.e., its ability to donate an electron pair to the acidic solute). The reference scale of donor numbers of the adduct bases is based on the enthalpy of reaction. A//, of the donor (designated as B) with SbCb when they are dissolved in 1,2-dichloroethane solvent. The donor numbers, designated DN, are a measure of the strength of the B—SbCb bond. It is further assumed that the order of DN values for the SbCb interaction remains constant for the interaction of the donor bases with all other solute acids. Thus, for any donor base B and any acceptor acid A, the enthalpy of reaction to form B A is ... 

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