  Electrostatic charging changes the critical temperature of superconducting
thin layers. To understand the basic mechanism, it is possible to use the
Ginzburg-Landau theory with the boundary condition derived by de Gennes from
the BCS theory. Here we show that a similar boundary condition can be obtained
from the principle of minimum free energy. We compare the two boundary
conditions and use the Budd-Vannimenus theorem as a test of approximations.
