Физико-химические аспекты изучения кластеров, наноструктур и наноматериалов (Dec 2024)
Equation of interphase adsorption equilibrium
Abstract
Despite intensive development of adsorption science, there are still many unresolved issues. In this regard, at least one of them can be highlighted in the present paper. As early as 1927 [1], it was established that solids change their dimensions during adsorption of gases and vapors. However, the development of research in this area was hampered for a long time by the lack of theoretical concepts and great experimental and methodological difficulties. Meanwhile, the study of the deformation of solids during adsorption is of great importance both for the development of adsorption thermodynamics and for practical purposes. In the theory of adsorption, a method for describing adsorption equilibrium is widely used, in which the role of a solid is reduced only to the creation of an adsorption force field in which the adsorbate is located. The adsorbent itself is considered thermodynamically inert. It is obvious that the analysis of a two-component system is replaced by consideration of only one component – the adsorbate. However, the very fact of the existence of deformation of a solid during adsorption directly indicates the insufficient rigor of the description of adsorption equilibrium in the language of concepts of the one-component system, as was accepted in deriving, for example, the classical Henry, Langmuir or BET (Brunauer-Emmett-Teller) equations. Therefore, the greatest value is represented by those works in which attempts are made to construct a general theory of adsorption (and absorption), including phenomena on deformable adsorbents (absorbents). From such a general equation, as special cases, at least the known classical adsorption equations should follow. In work [2], such an equation was proposed and it was shown that from it, as special cases, follow the classical Henry, Langmuir and BET equations with constants that have a clear physical meaning. Thus, the constant in the Henry equation is determined by temperature, the specific surface area adsorbent, size of adsorbate molecules, molar mass of adsorbate, and the isosteric heat of adsorption (the energy of interaction of the adsorbate molecules with the adsorbent surface). In the derived particular BET equation, in contrast to the classical version, a clear dependence of the equation constant on specific physical characteristics of the adsorption system is indicated for the first time. It is determined by the concentration of adsorbate molecules in the liquid phase at temperature under consideration, concentration of adsorbate molecules during the formation of a dense monolayer on the adsorbent surface, the energy of interaction of adsorbate molecules with the adsorbent surface, and the heat of condensation. The approach presented in this work can serve as a basis for modeling a wide variety of adsorption and absorption phenomena, including adsorption on microporous adsorbents and deformable bodies. The present work continues the studies conducted. It is shown that particular cases of the equation presented in [2] are the well-known classical Fowler-Guggenheim, Temkin equations.
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