General concept of acid-base interactions

Acid-base interactions have found numerous applieations in research dealing with adsorption of molecules of liquids on the surfaees of solids. The main focus of this research is to estimate the thermodynamic work of adhesion, determine mechanism of interactions, analyze the morphology of interfaces and various surfaee coatings, develop surface modifiers, study the aggregation of macromolecular materials, explain the kinetics of swelling and drying, understand the absorption of low molecular weight compounds in polymeric matrices, and determine the properties of solid surfaces. In addition to these, there are many other applieations. 

Several techniques are used to determine and interpret acid-base interaetions. These include contact angle, inverse gas ehromatography, IGC, Fourier transform infiared, FTIR, and X-ray photoeleelron speetroseopy, XPS. These methods, as they are applied to solvents are diseussed below. 

Contaet angle measurements have long been used beeause of eommon availability of instruments. In reeent years, fliey have been developed from simple optical devices to the present day preeise, sophistieated, computer-controlled instruments with sufficient precision. Van Oss and Good - developed the basic theory for this method. Their expression for surfaee free energy is used in die following form  

7 Lewis acid parameter of surface free energy 

Y Lewis base parameter of surface fiee energy. 

---

Lewis has defined acids and bases in a general way as electron acceptors and donors, respectively (21). Accordingly, a compound or element capable of accepting electrons (electron seeking) is termed a Lewis acid. Conversely a compound or element capable of giving (or sharing electrons) is a Lewis base. In other words, those elements which are deficient in electrons --that is, have unfilled electron shells -- will seek out those elements carrying extra electrons (lone pairs). This is the basis for the Lewis concept of "acid -base interaction. Thus ... 

Therefore, the generalized solvosystem concept proposed above is applicable to the description of protic and aprotic solvents, although just for this case this does not allow us to obtain basically new results. As for the case of ionic melts, it is more important, since the theoretical basis of acid-base interactions in this kind of solvents is not developed so extensively. Nevertheless, there are many results on oxoacidity obtained in ionic solvents of the first and the second kinds, which have been treated in a proper way. 

The Lewis definitions of acid-base interactions are now over a half a century old. Nevertheless they are always useful and have broadened their meaning and applications, covering concepts such as bond-formation, central atom-ligand interactions, electrophilic-nucleophilic reagents, cationic-anionic reagents, charge transfer complex formation, donor-acceptor reactions, etc. In 1923 Lewis reviewed and extensively elaborated the theory of the electron-pair bond, which he had first proposed in 1916. In this small volume which had since become a classic, Lewis independently proposed both the proton and generalized solvent-system definitions of acids and bases. He wrote ... 

I. Definition of Basicity A. General Comments on the Acid-base Concept The current general definition of the acid-base concept is based on the definitions by Bronsted and Lewis. According to Bronsted s theory (1923) an acid-base interaction can be described by the general relation (1) ... 

The expansion of the concept to encompass cyclic electron delocalization or homoaromaticity occurred in the late 1950s. In 1956 Applequist and Roberts pointed out that the cyclobutenyl cation resembles the cyclopropenium cation . Doering and colleagues suggested that the cycloheptatriene carboxylic acids could be regarded as planar pseudoaromatic type structures with a homoconjugative interaction between C(l) and C(6) . Based on the results of solvolytic studies on the bicyclo[3.1. OJhexyl system, Winstein set out the general concept of homoaromaticity in 1959 ... 

Hence, on the basis of the generalized solvosystem concept it is possible to formulate the main distinctive features of different kinds of high-temperature ionic melts as media for acid-base interactions by the Lux-Flood method. 

More recently, it has been shown, in particular by Fowkes and co-workers [2,6,7], that electron acceptor and donor interactions, according to the generalized Lewis acid-base concept, could be a major type of interfacial forces between two materials. This approach is able to take into account hydrogen bonds which are often involved in adhesive joints. Inverse gas chromatography at infinite dilution for example is a well adapted technique [8-10] for determining the acid-base characteristics of fibres and matrices. Retention data of probes of known properties, in particular their electron acceptor (AN) and donor (DN) numbers according to Gutmann s semi-empirical scale [11], allow the determination of acid-base parameters, and Kj), of fibre and matrix surfaces. It becomes then possible to define a "specific interactions parameter" A at the fibre-matrix interface, as the cross-product of the coefficients and Kq of both materials [10,11] ... 

The Concept of Hard and Soft Acids and Bases, General Lessons for Lewis Acid-Base Interactions, and Relative Nucleophilicity and Electrophilicity... 

Application of the concepts of acceptor-donor interaction is an active area of re- search in adhesion science. In the last few years the foundations have been laid that will allow XPS, and to a lesser extent SSIMS. to play an important role in such investigations. Although information cotKeming the acid-base properties of polymer and inorganic surfaces is generally difficult to obtain, it is clear that the goal of achieving a predictive approach to adhesion and to the hydrolytic stability of adhesive joints and coated substrates is now much closer than a decade ago. 

This review clearly illustrates the recent advances in the use of ILs as solvent in heterocyclic synthesis along with the advantages, such as commercial availability, increased reaction rates, high product yield, easy workup procedures, their recyclability, and reuse, etc. In this review more than 80% of the articles involved the use of neutral reactants and neutral reaction conditions. The ILs used in most of cyclocondensation reactions appreciably improved the yield and shortened the reaction time. Despite, in few cases IL Bronsfed acid cafalysis (e.g., NH or CH of imid-azolium-based ILs) or Lewis acidic/basic catalysis (interaction of IL cation or anion with substrates) have also been reported, we believe the most important benefit of ILs in organic reactions can be explained by the general concepts of solvent effects, where the ability of stabilizing the charged activated complex via solvent-solute-type interactions, enhances the reaction rates and improves yield in comparison to same reaction when performed in molecular solvents. 

A further variant of Fowkes early work is due to van Oss et al. (1988). It is based on the concept that weak acid-base interactions (of the Lewis type) may contribute to interfacial tensions. Couched in a formalism inspired by Fowkes earlier work, it is proposed that a (general) surface tension may be written as ... 

Resonance such as (5.28a)-(5.28c) is inherently a quantal phenomenon, with no classical counterpart. In NBO language, each of the resonance interactions (5.28a)-(5.28c) corresponds to a donor-acceptor interaction between a nominally filled (donor Lewis-type) and unfilled (acceptor non-Lewis-type) orbital, the orbital counterpart of G. N. Lewis s general acid-base concept. As mentioned above, Lewis and Werner (among others) had well recognized the presence of such valence-like forces in the dative or coordinative binding of free molecular species. Thus, the advent of quantum mechanics and Pauling s resonance theory served to secure and justify chemical concepts that had previously been established on the basis of compelling chemical evidence. 

Drago and co-workers Introduced an empirical correlation to calculate the enthalpy of adduct formation of Lewis acids and bases ( 5). In 1971, he and his co-workers expanded the concept to a computer-fitted set of parameters that accurately correlated over 200 enthalpies of adduct formation ( ). These parameters were then used to predict over 1200 enthalpies of interaction. The parameters E and C are loosely Interpreted to relate to the degree of electrostatic and covalent nature of the Interaction between the acids and bases. This model was used to generalize the observations involved in the Pearson hard-soft acid-base model and render it more quantitatively accurate. 

---