  Isoscalar and isovector particle densities are derived analytically by using
the approximation of a sharp edged nucleus within the local energy density
approach with the proton-neutron asymmetry and spin-orbit effects. Equations
for the effective nuclear-surface shapes as collective variables are derived up
to the higher order corrections in the form of the macroscopic boundary
conditions. The analytical expressions for the isoscalar and isovector tension
coefficients of the nuclear surface binding energy and the finite-size
corrections to the $\beta $ stability line are obtained.
