A statistical physics study of the interaction of [7]-helicene with alkali cations (K+ and Cs+): new insights on microscopic adsorption behavior

Abstract

The adsorption of a metal ion to a polycyclic aromatic molecule such as helicene is the object of our study. The latter could function as a chiral molecular clamp for the cationic alkali metal. The main contribution of this work is to attribute new microscopic interpretations for the adsorption of potassium and cesium ions onto a thin layer of helicene achieved by the QCM technique. Throughout the grand canonical ensemble and some theoretical considerations, statistical physics processing has been used for modeling of experimental adsorption isotherms. A new model has been developed and chosen as an appropriate one to present a good correlation with experimental isotherms. Six physico-chemical parameters are obtained from the fitting of the experimental adsorption isotherms. Thanks to the steric parameters, we have found that an increase in temperature promotes a rise in the numbers of ions per site n₁ and n₂ and raises the adsorption capacities N_M1 and N_M2. The two energetic parameters c₁ and c₂ allow deduction of the adsorption energies at four temperatures. It is found from the calculated energies that physical adsorption takes place; the helicene can function as potassium and cesium captor and the K⁺–helicene complex is more stable than the Cs⁺–helicene complex.