  For binary solvent mixtures composed of ions and two kinds of polar solvents,
electric double layer near a charged object is strongly affected by not only
the binary solvent composition but also nature of solvents such as volume and
dipole moment of a solvent molecule. Accounting for difference in sizes of
solvents and orientational ordering of solvent dipoles, we theoretically obtain
general expressions for the spatial distribution functions of solvents and
ions, in planar geometry and within the mean-field approach. Although focusing
on long-range electrostatic interaction, we neglect short-range interactions
such as preferential solvation, our approach predicts an asymmetric depletion
of the two solvents from the charged surface and a behavior of decreased
permittivity of the binary solvent mixture. Furthermore, we suggest that the
key factor for the depletion is the ratio of the solvent dipole moment to the
solvent volume. The influence of binary solvent composition, volume of solvent
and dipole moment of solvent on the number density of solvents, permittivity
and differential capacitance are presented and discussed, respectively. We
conclude that accounting for difference in the volume and dipole moment between
polar solvents is necessary for new approach to represent more realistic
situations such as preferential solvation.
