  We define the dielectric constant (susceptibility) that should enter the
Maxwell boundary value problem when applied to microscopic dielectric
interfaces. The dielectric constant (susceptibility) of the interface is
defined by exact linear-response equations involving correlations of
statistically fluctuating interface polarization and the Coulomb interaction
energy of external charges with the dielectric. The theory is applied to the
interface between water and spherical solutes of altering size studied by
molecular dynamics (MD) simulations. The effective dielectric constant of
interfacial water is found to be significantly lower than its bulk value, and
it also depends on the solute size. For TIP3P water used in MD simulations the
interface dielectric constant changes from 9 to 4 when the effective solute
radius is increased from ~5 to 18 A.
