  We theoretically investigate the threshold for the director reorientation
from the homeotropic state to the hybrid homeotropic-planar state and vice
versa in a cell filled with a flexoelectric nematic liquid crystal (NLC)
subjected to an electric field. The liquid crystal is doped by a CTAB-like
substance, a part of molecules of which dissociates into positive and negative
ions. The anchoring on one of the cell surfaces is assumed to be strong and
homeotropic, while the other surface can adsorb positive ions which play the
role of an orienting surfactant for NLC molecules on this surface. At certain
voltages, the orientational transitions in the bulk of the NLC are possible due
to the changing conditions for the director on the adsorbing surface. We
calculate respective threshold voltages as functions of anchoring parameters.
The existence of the critical values of these parameters, beyond which the
orientational transitions do not take place, is established.
