The Solution Behavior of Dopamine in the Presence of Mono and Divalent Cations: A Thermodynamic Investigation in Different Experimental Conditions

The interactions of dopamine [2-(3,4-Dihydroxyphenyl)ethylamine, (Dop(−))] with methylmercury(II) (CH(3)Hg(+)), magnesium(II), calcium(II), and tin(II) were studied in NaCl(aq) at different ionic strengths and temperatures. Different speciation models were obtained, mainly characterized by mononuclear species. Only for Sn(2+) we observed the formation of binuclear complexes (M(2)L(2) and M(2)LOH (charge omitted for simplicity); M = Sn(2+), L = Dop(−)). For CH(3)Hg(+), the speciation model reported the ternary MLCl (M = CH(3)Hg(+)) complex. The dependence on the ionic strength of complex formation constants was modeled by using both an extended Debye–Hückel equation that included the Van’t Hoff term for the calculation of enthalpy change values of the formation and the Specific Ion Interaction Theory (SIT). The results highlighted that, in general, the entropy is the driving force of the process. The sequestering ability of dopamine towards the investigated cations was evaluated using the calculation of pL(0.5) parameter. The sequestering ability trend resulted to be: Sn(2+) > CH(3)Hg(+) > Ca(2+) > Mg(2+). For example, at I = 0.15 mol dm(−3), T = 298.15 K and pH = 7.4, pL(0.5) = 3.46, 2.63, 1.15, and 2.27 for Sn(2+), CH(3)Hg(+), Ca(2+) and Mg(2+) (pH = 9.5 for Mg(2+)), respectively. For the Ca(2+)/Dop(−) system, the precipitates collected at the end of the potentiometric titrations were analyzed by thermogravimetry (TGA). The thermogravimetric calculations highlighted the formation of solid with stoichiometry dependent on the different metal:ligand ratios and concentrations of the starting solutions.