The role of quasi-free dipoles in polar liquid behavior

At the molecular level, the structure of liquids is in a state of constant turmoil. If the liquid is a polar dielectric, its response to an applied field will critically depend on the freedom of dipoles to respond to it. In general, each dipolar molecule will be subjected to a constant bombardment from its neighbours due to their thermal energy, which creates a so-called inner friction. However, at any particular instant some of the dipolar molecules will be more free to move than others. Indeed a small proportion would be expected to be able to move with very little interference from others, and such molecules can be said to be quasi-free. They will only be in that state for a short time, but when they revert to the more usual state of movement hindered by bombardment, other molecules will in their turn become quasi-free. Because of the high number density of molecules, statistically there will be a steady spectrum of molecular freedom stretching from the normal state of hindered movement to the state of quasi-freedom. Although only a very small proportion of molecules can be expected to be in the quasi-free state, because of their relative freedom of motion they can be expected to exercise a disproportionate influence on the dielectric properties of the liquid. This variation in the hindrance to motion of the molecules can be visualized as being caused by a distribution of barrier heights. The skewed arc behavior observed for some dielectric liquids can be described in terms of a distribution of activation energies caused by the distribution of barrier heights. Thus the variation in the hindrance to movement experienced by the molecules is linked directly to the distribution of relaxation times derivable from experimental observations