We consider the plasmapause as a stationary boundary layer (a sheath) separating two types of plasmas characterized by different temperatures and densities: on one side, the hot trapped particles imbedded in the cold exospheric plasma of ionospheric origin, and on the other side the plasmatrough including the hot ring current particles. The structure of this layer is described by a kinetic model using the Vlasov-Maxwell's equations for the charged particles and fields. In absence of any collisional effects or wave-particle interactions a minimum value for the thickness of the plasmapause is deduced which is of the order of 5 times the Larmor radius of the cold ions.