Classical Electrostatic Description of the Work Function and Ionization Energy of Insulators

A simple classical model utilizing image forces has been developed which shows that the energy required to extract an electron from an insulating particle depends inversely on: 1) the size of the particle, 2) the dielectric constant, and 3) the maximum natural excursion distance of the electron from the surface of the insulating particle. As anticipated, data confirm that the ionization energy of an insulating polymer molecule is inversely related to the number of monomer units in the polymer molecule. Considering the solid insulator as the limiting case of a macroscropic particle, the predicted work function of the solid is the minimum value of the steadily decreasing ionization energy with size. This is in agreement with the limited data which show that the work function of an insulating polymer is approximately 1/2 of the ionization energy of the monomer in the gas phase. The model also predicts that the work function decreases with increasing dielectric constant in agreement with some limited available data.