Thermodynamic properties of M− C 60 dimers immersed in an inert gas

Abstract

Abstract When fullerenes C 60 capture an alkali metal M − C 60 , they acquire a powerful permanent dipole. This suggests that if the system is in an inert atmosphere, like Argon, they tend to form dimers ( M − C 60 ) 2 , producing a gas whose thermodynamical properties can be studied. On this work, we focus on the degrees of freedom that appear when the dimer has formed. The fraction of ( M − C 60 ) 2 that can be created is calculated by using the standard physical statistics and can be graphed against the temperature on the 273.15 K and 373.15 K interval. The usual procedure to obtain the Keesom energy is revised to show that it can only be valid for weakly interacting dipoles or when the separation distance between them is considerable. To solve the problem we use the correction to the Keesom energy obtained by Battezzati and Magnasco in order to introduce it into the partition function for the new degree of freedom. The energy, the Helmholtz energy and the entropy is calculated through both means. The results were represented on graphs against the distance of separation between the M − C 60 . These results show that using the Keesom term alone is unsuitable when the separation distances are lower than 0.7 nm. Lastly, the calculations of the Helmholtz, Gibbs, and enthalpy as functions of temperature show that the dimer formation is both spontaneous and exothermic, as well as stable and an important fraction of the total amount of gas molecules in the system.