Question

what is interfacial polarisation

Answer 1

At the interface between two materials in contact, there is a polarization that takes place in the nearby atoms caused by electromagnetic interaction between them. Assuming the two materials were made from "ordinary" matter where the nucleus is composed of neutrons and protons surround by the electrons (simplified model of the atom, I now, but suites the concept), the the electrons on the surface facing each other will repel and cause a polarization on that surface.

Answer 2

As a note, if you are talking about interfacial polarization in the context of electrostatics (like when studying a capacitor!) this could also refer to polarization at the interface between the terminals and the dielectric material. This "interfacial polarization" is induced by application of an electric field. You must be careful about how you view this, the entire dielectric material is polarized, but internal to the interfaces the net charge is canceled.

Answer 3

Also, there can be space charge layers induced in the capacitors by a number of effects. For example, under electric fields, vacancy defects within a dielectric can migrate to the electrode dielectric interface, in effect creating a second layer or third layer to the capacitor structure. Also the electric field within the metal of the interface will be imperfectly screened, meaning that it takes a finite distance for this screening to take effect (known as Thomas-Fermi screening. this leads to a small space charge layer. You may also want to look into Maxwell Wagner effects which are space charge regions at the interfaces between two dielectric materials.

All of teh above can cause interfacial polarisation effects in capacitors. Interfacial polarisation is a complex field of study.