Characterization of thermodynamic properties on Al3+/dopamine system

Abstract

The interactions of dopamine (Dop−) with aluminum were studied in NaCl(aq) at different ionic strengths (0.15 ≤ I/mol dm−3 ≤ 1.0) and temperatures (288.15 ≤ T/K ≤ 310.15). The investigations were carried out by using ISE-[H+] potentiometry, UV–Vis spectrophotometry and 1H NMR. The results obtained from the different techniques were in good agreement, both in terms of speciation model and stability constants of the complexes. The speciation model is featured by mononuclear complexes, namely: AlDop; AlDopOH; AlDop(OH)2 and AlDop(OH)3 (charges omitted). From 1H NMR investigations, it resulted that both the amino and the phenolic groups in meta position of aromatic ring are involved in the protonation equilibria, while the Al3+ complexation occurs from the catechol side of the ligand. The dependence on the ionic strength and temperature of the complex formation constants was modelled by a modified extended Debye-Hückel equation, containing a term for the dependence on T/K. The SIT approach allowed the calculation of the specific ion interaction coefficients of the aluminum:dopamine complexes. From the enthalpy change values obtained, it resulted that the formation of the species is exothermic in nature and that the entropy is the driving force of the processes. The sequestering ability of dopamine towards Al3+ was evaluated by the calculation of pL0.5 parameter at different ionic strengths, pHs and temperatures; the dependence of the pL0.5 on these variables was modelled by using an empirical predictive equation that allows the calculation of the effective sequestering ability of dopamine at desired conditions. The precipitates collected at the end of the potentiometric titrations were studied by means of thermogravimetric (TGA) analysis.