Composition at the interface between multicomponent nonequilibrium fluid phases

The computation of mass transfer between two nonequilibrium phases requires interfacial composition. For a system with a total of M components, the assumption of local thermodynamic equilibrium provides only M equations while the number of unknown interfacial compositions is 2(M-1). In binary systems (M=2), the number of equations matches the number of unknowns and the interfacial composition is readily computed at any temperature and pressure. In multicomponent systems (M>or=3), the number of unknowns exceeds the number of equations and additional constraints are required. To the best of our knowledge, a general solution to the problem of computing interfacial composition in multicomponent systems has not been presented in the past, despite its fundamental importance. In this work we present a general and consistent method of computing interfacial composition where the additional constraints are obtained from mass balance across the interface. The derivations reveal that in multicomponent systems interfacial composition depends on the diffusion coefficients in the two bulk phases. This is a fundamental difference from binary systems. The differences are demonstrated by comparing examples of mixing of nonequilibrium phases in binary and ternary (M=3) systems.