  In the present work, we study an electrolyte solution confined between planar
surfaces with nonopatterned charged domains, which has been connected to a bulk
ionic reservoir. The system is investigated through an improved Monte Carlo
(MC) simulation method, suitable for simulation of electrolytes in the presence
of modulated surface charge distributions. We also employ a linear approach in
the spirit of the classical Debye-H\"uckel approximation, which allows one to
obtain explicit expressions for the averaged potentials, ionic profiles,
effective surface interactions and the net ionic charge confined between the
walls. Emphasis is placed in the limit of strongly confined electrolytes, in
which case local electroneutrality in the inter-surface space might not be
fulfilled. In order to access the effects of such lack of local charge
neutrality on the ionic-induced interactions between surfaces with modulated
charge domains, we consider two distinct model systems for the confined
electrolyte: one in which a salt reservoir is explicitly taken into account
{\it via} the osmotic equilibrium with an electrolyte of fixed bulk
concentration, and a second one in which the equilibrium with a charge neutral
ionic reservoir is implicitly considered. While in the former case the osmotic
ionic exchange might lead to non-vanishing net charges, in the latter model
charge neutrality is enforced through the appearance of an implicit Donnan
potential across the charged interfaces. A strong dependence of the
ionic-induced surface interactions in the employed model system is observed at
all particle separations. These findings strongly suggest that due care is to
be taken while choosing among different scenarios to describe the ionic
exchanging in electrolytes confined between charged surfaces, even in cases
when the monopole (non zero net charge) surface contributions are absent.
