Cationic surfactant influence on ternary complexes of CoII, NiII, CuII and ZnII ions with aspartic and citric acids

A potentiometric titration study has been undertaken to determine the cationic surfactant influence on mixed ligand complexes in ternary systems consisting of cobalt (II), nickel (II), copper (II), and zinc (II) with aspartic and citric acids at various concentrations (0.0–2.5% w/v) of the CTAB (Cetyltrimethyl ammonium bromide)-surfactant solution while maintaining an ionic strength of 0.16 mol L-1 (NaCl) at 30 °C. Titrations were performed in the presence of different ratios (M:L:X = 1:2:2, 1:4:2, and 1:2:4) of metal (M) and aspartic acid (L) to citric acid (X) using sodium hydroxide. Model systems were developed based on statistical parameters and residual analysis. For Co(II), Ni(II), Cu(II), and Zn(II), the species detected were MLXH2, MLX2H5, and MLX3. The electrostatic relationship of the ligands' side chains, charge neutralization, chelate effect, stacking interactions, and hydrogen bonding are used to explain the trend in the variation of logarithm of stability constants (log β) values with changing dielectric constant and mole fraction of the surfactant. The variation in species distribution as a function of pH and surfactant composition is also presented and discussed, as are plausible equilibria for the formation of the species and structures of the ternary complexes are also presented and discussed.