New insights in the physicochemical investigation of the vitamin B12 nucleus using statistical physics treatment: interpretation of experiments and surface properties

Abstract

In this research paper, the equilibrium isotherms for the adsorption of cobalt(II)nitrate and cobalt(II)chloride on tetrakis(4-tolylphenyl)porphyrin (H₂TTPP) were obtained at four temperatures for modeling analysis. The experimental data describing the adsorbed quantity of cobalt particles were measured using the quartz crystal microbalance (QCM) strategy. Then, statistical physics formalism was employed to interpret the complexation mechanism by applying the real gas law that contemplates the interaction between the adsorbate particles in the free state. Advanced models treated with the law of van der Waals were applied for the single and L.B.L adsorptions of Co²⁺ at various temperatures (288–318 K). The experimental adsorption data of CoCl₂ on porphyrins were satisfactorily fitted with the monolayer equation, showing that the chlorine particles had no effect on the complexation system, while the nitrate particles were involved in the adsorption of Co(NO₃)₂ and contributed to the layer formation. The physicochemical parameters of statistical physics models were estimated and used to compare the complexation mechanisms of both adsorbates. The study of the cohesion pressure (a) and the co-volume (b) confirmed that cobalt chloride guaranteed more stability during the formation of the vitamin B₁₂ nucleus. Deeper energetic analysis demonstrated that cobalt ions were complexed by ionic or covalent bonds in the case of cobalt chloride (complexation energy (–E_1/2) varies from −48.2 to −50.3), while a physisorption process took place in the case of cobalt nitrate ((–E₁) varies from −33.6 to −36.1), thus indicating that CoCl₂–H₂TTPP was the most stable complex. The statistical physics models were also used to investigate two thermodynamic functions that govern the adsorption mechanisms, namely, the configurational entropy and the Gibbs free enthalpy.