In physical chemistry, the Polanyi potential theory, also called Polanyi's potential theory of adsorption or Eucken–Polanyi potential theory, is a model of adsorption proposed independently by Michael Polanyi and Arnold Eucken. Under this model, adsorption can be measured through the equilibrium between the chemical potential of a gas near the surface and the chemical potential of the gas from a large distance away. In this model, the attraction largely due to Van der Waals forces of the gas to the surface is determined by the position of the gas particle from the surface, and that the gas behaves as an ideal gas until condensation where the gas exceeds its equilibrium vapor pressure. While the adsorption theory of William Henry is more applicable in low pressure and the adsorption isotherm equation from Brunauer–Emmett–Teller (BET) theory is more useful at from 0.05 to 0.35 P/P0, the Polanyi potential theory has much more application at higher P/P0 (≈ 0.1–0.8).
History In 1914, Arnold Eucken introduced his theory of adsorption and coined the term adsorption potential (German: Adsorptionspotential). A few months later, Michael Polanyi wrote his first paper on adsorption where he proposed a model for the adsorption of gas onto a solid surface. Afterwards, he published a fully developed paper in 1916, which included experimental verification by his students and other authors. During his research in the University of Budapest, his mentor Georg Bredig, sent his research findings to Albert Einstein. Einstein wrote back to Bredig stating:
The papers of your M. Polanyi please me a lot. I have checked over the essentials in them and found them fundamentally correct. Polanyi later described this event by saying:
Bang! I was a scientist. Polanyi and Einstein continued to write to each other on and off for the next 20 years.
Criticism Polanyi's model of adsorption was met with much criticism for several decades after publication years. His simplistic model for determining adsorption was formed during the time of the discovery of Peter Debye's fixed dipoles, Niels Bohr's atomic model, and well as the developing theory of intermolecular forces and electrostatic forces by key figures in the chemistry world including W.H. Bragg, W.L. Bragg, and Willem Hendrik Keesom. Opponents of his model claimed that Polanyi's theory did not take into account these emerging theories. Criticism included that the model did not take into account the electrical interactions of the gas and the surface, and that the presence of other molecules would screen off the attraction of the gas to the surface. Polanyi's model was furthermore put under scrutiny following the adsorption model of Irving Langmuir from 1916 to 1918 through whose research would eventually win the Nobel Prize in Chemistry in 1932. However, Polanyi was not able to participate in many of these discussions because he served as a medical officer for the Austro-Hungarian army in the Serbian front during World War I. Polanyi wrote about this experience saying:
I myself was protected for a while against any knowledge of these developments by serving as a medical officer in the Austro-Hungarian Army from August 1914 to October 1918, and by the subsequent revolutions and counter revolutions that lasted until the end of 1919. Members of less-well-informed circles elsewhere continued to be impressed for some time by the simplicity of my theory and its wide experimental verifications.
Defense Polanyi described that the “turning point” of the acceptance of his model of adsorption occurred when Fritz Haber asked him to defend his theory in full in the Kaiser Wilhelm Institute for Physical Chemistry in Berlin, Germany. Many key players in the scientific world were present in this meeting including Einstein. After hearing Polanyi's full explanation of his model, Haber and Einstein claimed that Polanyi “had displayed a total disregard for the scientifically established structure of the matter”. Years later, Polanyi described his ordeal by concluding,
Professionally, I survived the occasion only by the skin of my teeth. Polanyi continued to provide supporting evidence in proving the validity of his model years after this meeting.
Refutation Polanyi considered Eucken theory erroneous and claimed that Eucken modified his own theory in 1922 to fit with his. Polanyi's 'deliverance' (as he described it) from these rejections and criticism of his model occurred in 1930, when Fritz London proposed a new theory of cohesive forces founded on the theories of quantum mechanics on the polarization of electronic systems. Polanyi wrote to London asking,
“Are these forces subject to screening by intervening molecules? Would a solid acting by these forces possess a spatially fixed adsorption potential?” After computational analysis, a joint publication was made between Polanyi and London claiming that the adsorptive forces behaved similarly to the model that Polanyi had proposed.
Further research Polanyi's theory has historical significance whose work has been used a foundation for other models, such as the theory of volume filling micropores (TVFM) and the Dubinin–Radushkevich theory. Other research have been performed loosely involving the potential theory of Polanyi such as the capillary condensation phenomenon discovered by Richard Adolf Zsigmondy. Unlike Poylani's theory which involves a flat surface, Zsigmondy's research involves a porous structure like silica materials. His research proved that condensation of vapors can occur in narrow pores below the standard saturated vapour pressure.
Description
The Polanyi potential adsorption theory is based on the assumption that the molecules near a surface move according to a potential, similar to that of gravity or electric fields. This model is applicable in the case of gases at a surface at constant temperature. Gas molecules move closer to that surface when the pressure is higher than the equilibrium vapor pressure. The change in potential relative to the distance from the surface can be calculated using the formula for difference of the chemical potential,
… excerpt ends here. Continue reading the full article.
