# Van der Waals forces

Entry by Robin Kirkpatrick AP 225, 2011

Van der Waals forces refer to the sum of the forces between molecules, exlcuding those that are ionic or covalent in nature. Intermolcular forces are a result of:

1. Repulsion, as a result of the Pauli Exclusion principal.

2. Keesom/electrostatic interactions between charges, dipoles, quadrapoles, and (in general) multipoles.

3. Induction/Debye forces, which result from interaction of a permanent multipole with an induced multipole.

4. Dispersion/London Forces, which result from the interaction of instantaneous multipoles.

While induction and dispersion forces are attractive, electrostatic forces can be either attractive or repulsive depending on orientation. As a result, random thermal motion is usually enough to disrupt the electrostatic component. The Lennard-Jones potential is an approximation to the isotropic component of the Van der Waals force, and is given below by:

V = <math>\epsilon [(rm/r)^{12} - 2(r/rm)^6]</math>

where <math>\epsilon </math> is the depth of the well, rm is the distance at which the potential reached a minimum, and r is the distance between molecules. Pauli respulsion occurs at short range distances due to overlap of electron orbitals and results in the r^-12 term. The r^-6 in the attractive long range term due to dispersion forces.

## References

[1] http://en.wikipedia.org/wiki/Lennard-Jones_potential [2] http://en.wikipedia.org/wiki/Van_der_Waals_force