Relevance of the density-functional theory to acid-base interactions and adhesion in solids

In this paper, the acid-base interaction in terms of the molecular interaction is discussed from a broader background. Thus, the overall nature of the acid-base interaction consists of electrostatic, charge (or electron) transfer, exchange, polarization, and dispersion components. Among them, the electrostatic (or ionic) and charge transfer (or covalent) are the two major components. It is pointed out that one of the important criteria determining whether there is any molecular interaction is the interatomic distance that affects each of these components in different ways. The optimum distance appears to be around 2 angstrom. To complement existing acid-base theories, we further demonstrate the relevance of the density-functional theory to surface interactions and solid adhesion. On the basis of the density-functional theory, two chemical parameters, i.e. chemical potential μ and absolute hardness η, will be shown to govern an acid-base interaction. From these, the number of transferred electrons ΔN can be estimated. By extending the density-functional theory for atoms and molecules to solids, we have found two important physical properties of solids, i.e. the work function Φ and the average energy gap EgAv, which are equivalent to the above two chemical parameters. Thus, we propose to estimate the number of transferred electrons for solid interactions involving metals or polymers by the derived equation: ΔN ΔΦ/ΣEgAv. Basically, this equation somewhat resembles Ohm's law. In practice, as ranked by the average energy gaps, all metals are soft for having narrow gaps; for the same reason all semimetals and semiconductors are generally soft, and all insulators including polymers are comparatively hard for having wide gaps. Furthermore, to achieve a reasonable rate for an interfacial acid-base interaction or solid adhesion, it is advantageous to consider the HSAB principle: hard bases (or donors) prefer to interact with hard acids (or acceptors), and soft bases (or donors) with soft acids (acceptors).