  We analyze the localized phase of a general model of a directed polymer in
the proximity of an interface that separates two solvents. Each monomer unit
carries a charge, $\omega_n$, that determines the type (attractive or
repulsive) and the strength of its interaction with the solvents. In addition,
there is a polymer--interface interaction and we want to model the case in
which there are impurities $\tilde\omega_n$, that we call again charges, at the
interface. The charges are distributed in an in--homogeneous fashion along the
chain and at the interface: more precisely the model we consider is of quenched
disordered type.
  It is well known that such a model undergoes a localization/delocalization
transition. We focus on the localized phase, where the polymer sticks to the
interface. Our new results include estimates on the exponential decay of
averaged correlations and the proof that the free energy is infinitely
differentiable away from the transition. Other results we prove, instead,
generalize earlier works that typically deal either with the case of copolymers
near an homogeneous interface ($\tilde\omega\equiv 0$) or with the case of
disordered pinning, where the only polymer--environment interaction is at the
interface ($\omega\equiv 0$). Moreover, with respect to most of the previous
literature, we work with rather general distributions of charges (we will
assume only a suitable concentration inequality).
