  Near-critical binary mixtures containing ions and confined between two
charged and selective surfaces are studied within a Landau-Ginzburg theory
extended to include electrostatic interactions. Charge density profiles and the
effective interactions between the confining surfaces are calculated in the
case of chemical preference of ions for one of the solvent components. Close to
the consolute point of the binary solvent, the preferential solubility of ions
leads to the modification of the charge density profiles in respect to the ones
obtained from the Debye-H\"uckel theory. As a result, the electrostatic
contribution to the effective potential between the charged surface can exhibit
an attractive well. Our calculations are based on the approximation scheme
valid if the bulk correlation length of a solvent is much larger than the Debye
screening length; in this critical regime the effect of charge on the
concentration profiles of the solvent is subdominant. Such conditions are met
in the recent measurements of the effective forces acting between a substrate
and a spherical colloidal particle immersed in the near-critical water-lutidine
mixture [Nature V. 451, 172 (2008)]. Our analytical results are in a
quantitative agreement with the experimental ones.
