The authors have constructed functional expansions for single-particle distribution functions in the surface layer of a multicomponent system for different sets of thermodynamic variables. The authors indicate variables which significantly simplify the use of the constructed expansions for describing the state of a binary system close to the critical point. They have obtained the asymptotics for the local component concentration at the liquid-gas boundary close to the critical stratification point of a binary system. The authors have found two independent combinations of single-particle distribution functions, of which only one has the specific long-range behavior in the critical region. They consider the gravitational effect in a binary system.