Relevance of the Hard-Soft Acid-Base (HSAB) Principle to Solid Adhesion

In this paper, the acid-base interaction is shown to consist of at least two major components: the electrostatic (or ionic) and charge transfer (or covalent). 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. Our findings indicate that the absolute hardness is directly linked to the average energy gap. 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).