  The model under consideration is a semi-infinite two-dimensional
two-component plasma (Coulomb gas), stable against bulk collapse for the
dimensionless coupling constant $\beta<2$, in contact with a dielectric wall of
dielectric constant $=0$. The model is mapped onto an integrable sine-Gordon
theory with a ``free'' Neumann boundary condition. Using recent results on a
reflection relationship between the boundary Liouville and sine-Gordon
theories, an explicit expression is derived for the surface tension at a
rectilinear dielectric -- Coulomb gas interface. This expression reproduces the
Debye-H\"uckel $\beta \to 0$ limit and the exact result at the bulk collapse
border, the free-fermion point $\beta =2$, where the surface tension keeps a
finite value. The surface collapse, identified with the divergence of the
surface tension, occurs at $\beta =3$.
