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    Porous material in contact with vapor tends to adsorb fluid in the pores. The amount of adsorbed fluid depends 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, the measured 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 as it is in distinct contrast to the expectations of thermodynamics:the system has more than one answer to one set of boundary conditions (here: grand canonical boundary conditions). The common explanation offered in the literature introduces the concept of metastable states, conceiving either or both branches of the isothermas being metastable. Even though the concept of metastability cannot be separated from the concept of a lifetime against decay into the corresponding ground state, this aspect is usually not discussed in the literature. Within the adsorption community it is agreed upon that the concept of metastable states brings the experimental findings in harmony with the theory of thermodynamics while the lifetime of the conceived metastable states is disregarded. In the present paper we challenge this notion. We argue that the characteristic lifetime sys τ of a system against decay into its ground state must be compared with the duration exp τ of the experimental technique employed to investigate the behavior of the system. Based on experimental evidence and based on previous theoretical results we find that the relation exp τ >>τ sys holds for typical adsorption systems. As thermodynamics is founded on the assumption exp τ << τ sys it cannot be the appropriate theory for describing adsorption systems.Several schemes found in the literature seem to provide such a time dependent approach. Our analysis, however, shows that neither of these attempts describes the time dependence in a realistic way. Thus, we have to conclude that no valid theory for the propagation of adsorption systems in time has emerged yet. We propose to develop a new generally valid time dependent theory for confined systems whose time independent limit for τ exp τ sys → 0 would be suited to handle adsorption systems
    5 years ago by @ijac
     
      10.5121/ijac.2015.1101DOI:
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