  The three-phase contact line of a droplet on a smooth surface can be
characterized by the Young-Dupr\'e equation. It relates the interfacial
energies with the macroscopic contact angle $\theta_e$. On the mesoscale,
wettability is modeled by a film-height-dependent wetting energy $f(h)$. Macro-
and mesoscale description are consistent if $\gamma \cos \theta_\mathrm{e}
=\gamma+f(h_\mathrm{a})$ where $\gamma$ and $h_\mathrm{a}$ are the liquid-gas
interface energy and the thickness of the equilibrium liquid adsorption layer,
respectively.
  Here, we derive a similar consistency condition for the case of a liquid
covered by an insoluble surfactant. At equilibrium, the surfactant is spatially
inhomogeneously distributed implying a non-trivial dependence of
$\theta_\mathrm{e}$ on surfactant concentration. We derive macroscopic and
mesoscopic descriptions of a contact line at equilibrium and show that they are
only consistent if a particular dependence of the wetting energy on the
surfactant concentration is imposed.This is illustrated by a simple example of
dilute surfactants, for which we show excellent agreement between theory and
time-dependent numerical simulations.
