  In continuum mechanics, the equations of motion for mixtures are derived
through the use of Hamilton's extended principle which regards the mixture as a
collection of distinct continua. The internal energy is assumed to be a
function of densities, entropies and successive spatial gradients of each
constituent. We first write the equations of motion for each constituent of an
inviscid miscible mixture of fluids without chemical reactions or diffusion.
Our work leads to the equations of motion in an universal thermodynamic form in
which interaction terms subject to constitutive laws, difficult to interpret
physically, do not occur. For an internal energy function of densities,
entropies and spatial gradients, an equation describing the barycentric motion
of the constituents is obtained. The result is extended for dissipative
mixtures and an equation of energy is obtained. A form of Clausius-Duhem's
inequality which represents the second law of thermodynamics is deduced. In the
particular case of compressible mixtures, the equations reproduce the classical
results. Far from critical conditions, the interfaces between different phases
in a mixture of fluids are layers with strong gradients of density and entropy.
The surface tension of such interfaces is interpreted.
