van der Waals Forces
ESS5855 Lecture
Fall 2010
Introduction
• There are 4 principal types of physical
forces acting between atoms and
molecules at the molecular and macroscopic levels
• The first, coulombic interaction involving
at least one formally charged species
• The remaining 3 forces make up what are
commonly termed van der Waals forces
van der Waals Forces
• Intermolecular forces
• The attractive forces between molecules
other than those due to covalent bonds or
to the electrostatic interaction of ions
with one another or with neutral
molecules
• Relatively weak compared to normal
chemical bonds, but play a fundamental
role in diverse fields
van der Waals Forces
• Almost all interfacial phenomena are
influenced to various extents by forces that
have their origin in atomic- and molecularlevel interactions due to the permanent or
induced polarities created in molecules by
the electric fields of neighboring molecules
or due to the instantaneous dipoles caused
by the positions of the electrons around the
nuclei
van der Waals Forces
• Permanent dipole - induced dipole
interaction (Debye equation)
• Permanent dipole - permanent dipole
interaction (Keesom equation)
• Induced dipole - induced dipole interaction
(London equation)
London Force
• Of the three forces mentioned above, the
London force is always present (like the
gravitational force) because it does not
require the existence of permanent polarity or
charge-induced polarity in the molecules
• Even neutral atoms or molecules such as
helium or hydrocarbons give rise to the
London interaction
• Intermolecular forces play very important
parts in determining the strengths of solids,
properties of gases and liquids, heat of
melting and vaporization of solids, and the like
Remarks
Lennard-Jones Potential
Potential energy versus distance of separation for two methane molecules
Dipole–Dipole Interactions
μ1
+Q
-Q
Permanent dipole to Induced dipole
Permanent dipole to Permanent dipole
Induced dipole to Induced dipole
• All energies considered so far can be calculated
using classical physics, which fail to explain the
attraction between non-polar molecules
• To calculate the dispersion force, quantum
mechanical perturbation theory is required
• Consider an atom with its positively charged
nucleus around which electrons circulate with a
high frequency of typically 1015–1016 Hz
• At every instant, the atom is therefore polar and
only the direction of the polarity changes with this
high frequency
• When two such oscillators approach, they start to
influence each other
• Attractive orientations have higher probabilities
than repulsive ones, which leads to an attractive
force
Induced dipole to Induced dipole
van der Waals Forces between Large Particles
and over Large Distances
Homework