The image force is considered to be one of the non-specific sources of surface charge and origin of the double charge layer in the interface between a non-polar liquid dielectric and a polar solid. It is assumed that there is no interaction between phases, neither chemical interaction nor preferential adsorption, due to their specific chemical nature. Two possible physical mechanisms have been investigated. The first takes into account the existence of a potential barrier to the charge movement as it leaves the surface. Therefore, the charges that are in liquid are attracted to the surface. The second is the reduction of dissociation energy of ion pairs close to the interface in comparison with bulk dissociation. Both mechanisms result in a decrease of recombination rate and a sharp increase of dissociation rate. It has been shown that both these mechanisms could lead to the creation and accumulation of an abnormal non-specific surface charge carrier.

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It is expected that in liquid dielectrics a special layer with increasing conductivity can be formed close to the boundary with the solid. This behaviour has both chemical and physical origins. According to our computations one of the reasons could be enhanced dissociation of dissolved electrolytes near the solids with higher permittivity. We have performed experiments with transformer oil and ceramics. Instead of using the usual method for surface conductivity (SC) measurements, a new method based on foil electrodes of different thicknesses between two plane ceramic samples is developed. Experimental data show that the SC σs is proportional to the bulk conductivity raised to the power two-thirds . This corresponds neither to our theoretical treatment nor to the usual treatment taking into consideration the ζ-potential of the double electric layer.

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