The appearance of the first adsorption theories in the 20th century

Abstract

      In the 19th century, the adsorption of gases by coal was studied in detail, but there were difficulties in comparing the experimental data, because the studies have not been standardized. At the beginning of the 20th century, much attention was paid to the reproducibility of the experiment, which made it possible to compare the experimental data among themselves, to find certain dependencies, and to create theories of adsorption. In 1914-1918, independently of each other, two theories of adsorption were proposed - the potential theory of A. Eucken and M. Polanyi and the monomolecular theory of I. Langmuir. The historical context of the indicated theories and their main provisions are shown.
       A. Eucken, using the works of W.H. Nernst in the field of thermodynamics and electrochemistry, proposed a potential theory of adsorption. He suggested the existence of the «atmosphere» of adsorbed gas in a compressed state that adsorbed molecules, which do not interact with each other and that the interaction between the adsorbent and molecules is expressed by the adsorption force of attraction and depends on the
distance between them.
         M. Polanyi, also based on the works of W.H. Nernst, suggested that the van-der-Waals forces act between the adsorbent and the adsorbate, which form layers on the surface of a solid, and the gas obeys the laws of thermodynamics. If the force of attraction between the adsorbent and the gas compresses the gas to a certain extent, then it condenses on the surface of the adsorbent.  In this case, the adsorbed gas can have a
thickness of several molecules on the surface of the adsorbent, and the adsorbate molecules are located on the surface in a certain order. In this case, neither Eucken nor Polanyi proposed an equation for the adsorption isotherm. This was done by Langmuir when he created a new theory of adsorption.
        I. Langmuir in 1916-1918 introduced the concept of monolayer adsorption on energetically homogeneous solid surfaces and derived an equation for the adsorption isotherm obtained from kinetic studies. Langmuir suggested the occurrence of electrostatic forces during the adsorption process and used in his works the G.N. Lewis’s electron-pair theory of chemical valence (1916). In this respect, his approach was
innovative for that time.
       Langmuir's theory received support of many scientists, as it was in good agreement with thermodynamics and with the theory of intermolecular forces of Debye-Hückel. Supporters of his theory was F. Haber, A. Einstein. The latter were at the same time opposed to the theory of Polanyi, believing that his theory was not sufficiently well founded. Supported the theory of Polanyi H.M. Freundlich and several other scientists. However, Freundlich believed that by that time an enough experimental facts had not been accumulated that would unequivocally testify in favor of one of them (all the theories are correct).
         At the beginning of the 20th century, after a long period of accumulation of experimental material on adsorption in the 19th century, a period of active creation of adsorption theories came. Their appearance was facilitated by advances in the development of chemistry and physics in the early 20th century, related to the development of chemical thermodynamics and theories of chemical bonding.