Separation chemistry commonly uses methods, such as liquid-liquid extraction, that involve phenomena that occur at the mesoscopic scale, around a few nanometres. At this intermediate scale, long-range interactions, geometrical effects or the role of entropy are particularly important. This makes it necessary to use a special modelling approach, intermediate between molecular models, where atoms and molecules are described precisely, and macroscopic models, representing the phenomenon as a whole. We present here how, in a general way, such descriptions can be elaborated by a multi-scale analysis. We show that it is a question of creating a thermodynamic description whose variables describing the system are more precise than those generally considered, since they can, for example, model the movements of solutes, the effects of curvatures of interfaces, or the presence of temporary species of all sizes. The validity of such an approach is guaranteed, at least in principle, for equilibrium properties, but it remains dependent on an assumption of time scale decomposition if it is to be used to determine the kinetics of physical and chemical processes.