We present a simple approximate analytical estimate for self-energy of a charge in the middle of cylindrical channel of a high permittivity epsilon_1 in a media of a low permittivity epsilon_2 (for the cases of infinitely long and comparatively short channels) and show that this estimate is in a good quantitative agreement with exact solution of Poisson equation. Further, using these estimates, we explain the observed a lower conductivity, caused by an increased the self-free-energy for ions, whose diameter is by ~1 angstrom less than that of the channel (as compared to ions, whose diameter is equal to that of the channel).
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