Abstract
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.
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