  Fundamental properties of the multicomponent diffuse-interface model (DIM),
such as the maximum entropy principle and conservation laws, are used to
explore the basic interfacial dynamics and phase transitions in fluids. Flat
interfaces with monotonically-changing densities of the components are proved
to be stable. A liquid layer in contact with oversaturated but stable vapour is
shown to either fully evaporate or eternally expand (depending on the initial
perturbation), whereas a liquid in contact with saturated vapour always
evaporates. If vapour is bounded by a solid wall with a sufficiently large
contact angle, spontaneous condensation occurs in the vapour. The external
parameters of the multicomponent DIM -- e.g., the Korteweg matrix describing
the long-range intermolecular forces -- are determined for the water-air
combination. The Soret and Dufour effects are shown to be negligible in this
case, and the interfacial flow, close to isothermal.
