The Effect of Metal Cations on the Aqueous Behavior of Dopamine. Thermodynamic Investigation of the Binary and Ternary Interactions with Cd(2+), Cu(2+) and UO(2)(2+) in NaCl at Different Ionic Strengths and Temperatures

The interactions of dopamine [2-(3,4-Dihydroxyphenyl)ethylamine, (Dop(−))] with cadmium(II), copper(II) and uranyl(VI) were studied in NaCl(aq) at different ionic strengths (0 ≤ I/mol dm(−3) ≤ 1.0) and temperatures (288.15 ≤ T/K ≤ 318.15). From the elaboration of the experimental data, it was found that the speciation models are featured by species of different stoichiometry and stability. In particular for cadmium, the formation of only MLH, ML and ML(2) (M = Cd(2+); L = dopamine) species was obtained. For uranyl(VI) (UO(2)(2+)), the speciation scheme is influenced by the use of UO(2)(acetate)(2) salt as a chemical; in this case, the formation of ML(2), MLOH and the ternary MLAc (Ac = acetate) species in a wide pH range was observed. The most complex speciation model was obtained for the interaction of Cu(2+) with dopamine; in this case we observed the formation of the following species: ML(2), M(2)L, M(2)L(2), M(2)L(2)(OH)(2), M(2)LOH and ML(2)OH. These speciation models were determined at each ionic strength and temperature investigated. As a further contribution to this kind of investigation, the ternary interactions of dopamine with UO(2)(2+)/Cd(2+) and UO(2)(2+)/Cu(2+) were investigated at I = 0.15 mol dm(−3) and T = 298.15K. These systems have different speciation models, with the MM’L and M(2)M’L(2)OH [M = UO(2)(2+); M’ = Cd(2+) or Cu(2+), L = dopamine] common species; the species of the mixed Cd(2+) containing system have a higher stability with respect the Cu(2+) containing one. The dependence on the ionic strength of complex formation constants was modelled by using both an extended Debye–Hückel equation that included the Van’t Hoff term for the calculation of the formation enthalpy change values and the Specific Ion Interaction Theory (SIT). The results highlighted that, in general, the entropy is the driving force of the process. The quantification of the effective sequestering ability of dopamine towards the studied cations was evaluated by using a Boltzmann-type equation and the calculation of pL(0.5) parameter. The sequestering ability was quantified at different ionic strengths, temperatures and pHs, and this resulted, in general, that the pL(0.5) trend was always: UO(2)(2+) > Cu(2+) > Cd(2+).