Electrostatic Interactions Involving Dipoles

There are three types of electrostatic interactions involving dipoles ... 

Interactions between the adsorbent surface and the solute can vary from nonspecific ones (such as dispersion or van der Waals forces) to specific ones (electrostatic interactions involving permanent dipoles or electron-donor-acceptor interactions, such as hydrogen bonding). Retention on silica gel or alumina is governed mainly by... 

Usually adsorption, i.e. binding of foreign particles to the surface of a solid body, is distinguished as physical and chemical the difference lying in the type of adsorbate - adsorbent interaction. Physical adsorption is assumed to be a surface binding caused by polarization dipole-dipole Van-der-Vaals interaction whereas chemical adsorption, as any chemical interaction, stems from covalent forces with plausible involvement of electrostatic interaction. In contrast to chemisorption in which, as it has been already mentioned, an absorbed particle and adsorbent itself become a unified quantum mechanical system, the physical absorption only leads to a weak perturbation of the lattice of a solid body. 

All of these interactions involve a host and a guest as well as their surroundings like solvation, crystal lattice, and gas phase. Electrostatic interactions are the driving force behind the ion pairing (ion-ion, ion-dipole, dipole-dipole, etc.) interactions, which are undeniably important in natural and supramolecular systems. The electrostatic interaction energy E is given by... 

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). 

One of the simplest orientational-dependent potentials that has been used for polar molecules is the Stockmayer potential.48 It consists of a spherically symmetric Lennard-Jones potential plus a term representing the interaction between two point dipoles. This latter term contains the orientational dependence. Carbon monoxide and nitrogen both have permanent quadrupole moments. Therefore, an obvious generalization of Stockmayer potential is a Lennard-Jones potential plus terms involving quadrupole-quadrupole, dipole-dipole interactions. That is, the orientational part of the potential is derived from a multipole expansion of the electrostatic interaction between the charge distributions on two different molecules and only permanent (not induced) multipoles are considered. Further, the expansion is truncated at the quadrupole-quadrupole term. In all of the simulations discussed here, we have used potentials of this type. The components of the intermolecular potentials we considered are given by ... 

Having introduced the major principles of complexation, association, and organization, it is important to review the physicochemical forces that lead to supramolecular ensemble formation. As mentioned above, supramolecular interactions are by definition noncovalent. In the order of the polarity of the partners involved, they comprise ionic or electrostatic interactions,17 18 ion-dipole interactions,19 dipole-dipole interactions, (ionic) hydrogen bonding, cation-tt and anion-tt interactions,... 

---