Coulombic or Electrostatic Interactions

The interaction between two charged atoms or molecules is potentially the strongest form of physical interaction to be considered at interfaces and in colloidal systems. The basic concepts and equations involved are fundamental to many areas of physics and chemistry and will not be developed in detail here. More will be said about them in Chapter 5 in the context of repulsive interactions between surfaces. A few basic points of review, however, will be useful in order to facihtate reference to them in later discussions of dipolar ininteractions. 

For two point charges Qi and Q2, the free energy of interaction v(r) is given by 

The force of the coulombic interaction, is the differential with respect to r of the free energy 

For two charges of the same sign, both w(r) and F will be positive, which means that the interaction will be repulsive for unlike charges they will be attractive. In terms of magnitude, the force (whether attractive or repulsive) is at a maximum when the distance of separation r is a minimum, that is, when the two ions are in contact and r equals the sum of the two ionic radii. For example, for a sodium and a chloride ion in contact, r will be 0.276 nm, and the binding energy will be 

According to Equation (4.2), the magnitude of the coulombic interaction falls off as the inverse square of the distance between the charges. A quick calculation shows that, in a vacuum, the interaction energy will fall to kT only as the separation distance approaches 60 nm, a large distance in the normal world of atoms, ions, and molecules. 

---

The question is whether the extent of adsorption is determined by charge control (AE, , i.e., coulombic or electrostatic interactions) or by orbital control i.e., attractive interactions between the filled orbitals of the adsorbate and the empty orbitals of the adsorbent). 

The final term in Eq. [11] is the Coulombic or electrostatic potential energy term and can be represented as the interaction of bond dipoles or atomic monopoles. With the latter,... 

The interaction of the molecules in the fluid and the electrode are represented by KJ,(r. This function is in the most general case the sum of two contributions, a Coulomb or electrostatic term plus a non electrostatic, covalent term such as van der Waals, hard core, etc. 

EIectrosta.tlcs. Electrostatic interactions, such as salt bridges, result from the electrostatic attraction that occurs between oppositely charged molecules. These usually involve a single cation, eg, the side chain of Lys or Arg, or the amino terminus, etc, interacting with a single anion, eg, the side chain of Glu or Asp, or the carboxyl terminus, etc. This attractive force is iaversely proportional to the distance between the charges and the dielectric constant of the solvent, as described by Coulomb s law. 

Group (1) Cations and anions which are incapable of donor-acceptor interactions. These are the large univalent ions. Bonding is purely by Coulomb and Madelung electrostatic interactions. From the Lewis point of view these are not acids or bases. They have no cement-forming potential. 

Starting from electrostatic potentials V (R) the electrostatic or coulomb energy of interaction between two particles AE con is obtained by Eq. (4). 

---