Dipole–Induced Dipole Forces

When one molecule is polar and the other isn't

A third kind of attractive force operates between a molecule that has a permanent dipole and a neighbouring molecule that has no permanent dipole of its own. The permanent dipole's electric field distorts (polarises) the electron cloud of the neutral neighbour, and in doing so it induces a temporary dipole on that otherwise non-polar molecule (Fig. 5.3). The two — one permanent, one induced — then attract each other.

Just as with dispersion forces, the interaction energy here is proportional to $\dfrac{1}{r^{6}}$, where $r$ is the separation between the two molecules.

What controls the strength of the induced dipole

Two factors decide how big the induced dipole (and hence the attraction) turns out to be:

• the dipole moment of the permanent-dipole molecule — a stronger permanent dipole distorts the neighbour's cloud more, and
• the polarisability of the non-polar molecule — larger molecules, whose electron clouds are held less tightly, are polarised more easily (recall this from Unit 4), so higher polarisability means a stronger induced dipole and a stronger attraction. …