  Isothermal interfacial zones are investigated starting from a local energy
which can be considered as the sum of two terms: one corresponding to a medium
with a uniform composition equal to the local one and a second one associated
with the non-uniformity of the fluid. In an extended van der Waals theory, the
volume internal energy is proposed with a gradient expansion depending not only
on the gradient of density but also on the gradient of entropy. We obtain the
equation of conservative motions for non-homogeneous fluids near its critical
point. For such a medium, it is not possible to obtain shock waves. The waves
are tangential to the interface and the wave celerity is expressed depending on
thermodynamic conditions at the critical point.
