Eos: Induced Dipoles and Dispersion Forces

Induced Dipoles and Dispersion Forces

Symmetric molecules have no net charge and a symmetrical distribution of charges, so there is no dipole interaction between them (Ex: $CO_2$, $CH_4$).

Nevertheless, proximity to permanent dipoles can induce a temporary dipole in these symmetric molecules.

DEFINITION

An induced dipole is caused when a molecule that would not by itself have a dipole moment is brought close to a molecule with a dipole moment. (That is, the dipole "pushes" the electron cloud of the symmetric molecule.)

DEFINITION

The polarizability, $\alpha$, of a molecule characterizes the ease to which that molecule's electrons are "pushed". What do you think influences this value?

$F_{12} \propto -\frac{\mu_1^2\alpha_2}{r^7}$

$\Gamma_{12} \propto -\frac{\mu_1^2\alpha_2}{r^6}$

Even if two symmetric molecules (with no net charge) come into proximity, fluctuations in the electron cloud density can induce dipole-like behavior.

DEFINITION

Dispersion (London) Forces arise when two non-polar (i.e., do not have a dipole) molecules are brought close together so that their time-varying electron clouds "push" each other.

$F_{12} \propto -\frac{\alpha_1\alpha_2}{r^7}$

$\Gamma_{12} \propto -\frac{\alpha_1\alpha_2}{r^6}$

Definition

Dipole-dipole, dipole-induced-dipole, and dispersion forces are all attractive and all decay as $r^7$ (potential decays as $r^6$). All are lumped together as van der Waals forces.

Outcome

Define van der Waals forces and relate it to the dipole moment and polarizability of a molecule.