CONFINED SYSTEMS WITHIN THERMODYNAMIC FRAMEWORK:TIME DEPENDENT VS. TIME INDEPENDENT TREATMENT

Porous material in contact with vapor tends to adsorb fluid in the pores. The amount of adsorbed fluiddepends on the vapor pressure, but depends in some parameter ranges as well on the history of the system.When raising the vapor pressure the adsorption may be smaller than while lowering the pressure. Thus, themeasured adsorption isotherm consists of two branches that appear stable on the time scale of experiments.The phenomenon is known as adsorption hysteresis and has been extensively discussed in the literature asit is in distinct contrast to the expectations of thermodynamics:the system has more than one answer to oneset of boundary conditions (here: grand canonical boundary conditions). The common explanation offeredin the literature introduces the concept of metastable states, conceiving either or both branches of theisothermas being metastable. Even though the concept of metastability cannot be separated from theconcept of a lifetime against decay into the corresponding ground state, this aspect is usually not discussedin the literature. Within the adsorption community it is agreed upon that the concept of metastable statesbrings the experimental findings in harmony with the theory of thermodynamics while the lifetime of theconceived metastable states is disregarded. In the present paper we challenge this notion. We argue thatthe characteristic lifetime sys τ of a system against decay into its ground state must be compared with theduration exp τ of the experimental technique employed to investigate the behavior of the system. Based onexperimental evidence and based on previous theoretical results we find that the relation exp τ >>τ sysholds for typical adsorption systems. As thermodynamics is founded on the assumption exp τ