EoS: Intermolecular Forces

Intermolecular Forces

Molecules are made of positively charged nuclei and negatively charges electrons, therefore, it is useful to think of many of the interactions between molecules as electronic in nature. The electric field intensity is given by:

$\vec E = \frac{\vec F}{Q} = \frac{-\vec \nabla \Gamma}{Q}$

where $\vec \nabla$ denotes a spatial derivative, and $\Gamma$ is the molecular potential energy.

The electrostatic intermolecular interactions that we will consider are:

• Point charges: between ions, or in plasmas
• Permanent Dipoles
• Induced Dipoles
• Dispersion Forces
DEFINITION

Interactions between point charges are the simplest kind of electrostatic interactions. Here we assume that all of the charge of a molecule is concentrated at a point.

The interaction force/potential between point charges is given as:

$F_{12} \propto \frac{Q_1Q_2}{r^2}$

$\Gamma_{12} \propto \frac{Q_1Q_2}{r}$

Note:

If the charges are the same, then the force is positive (repulsive).