The interaction of long chain sodium carboxylates and sodium dodecylsulfate with lead(II) ions in aqueous solutions

• Lead carboxylate (soap) formation in aqueous solutions has been revisited.
• Lead soap precipitation mechanism depends strongly on alkyl chain length.
• The surfactant head group acidity affects the mechanism of soap formation.
• Stability constants depend on alkyl chain length and other factors.
• Covalency of metal–oxygen bonds plays an important role in the association process.

The interaction of sodium octanoate, decanoate or dodecanoate with lead(II) has been studied in aqueous solutions using potentiometry, electrical conductivity, turbidity and ICP–OES measurements. These show an alkyl chain length dependence on the behavior. At the lead(II) concentration used (1.0 mM), relatively strong interactions are observed with the decanoate and dodecanaote, leading to formation of the lead carboxylates (soaps) as insoluble complexes. All techniques show 1:2 (metal:carboxylate) stoichiometry corresponding to charge neutralization. With sodium octanoate and lead(II), a rather weaker interaction is seen, and complexation is only observed at metal:carboxylate ratios > 0.5. However, in contrast to our previous work on octanoate and calcium(II) in aqueous solutions [1], precipitation does occur at higher concentrations. This difference between the behavior of the metal ions is probably due to the more covalent nature of the bonds of the carboxylate with Pb2+ than with Ca2+. Association constants of the complexes have been determined from potentiometric measurements and are consistent with data on solubility products. A comparison is made of the effect of surfactant head group on the interactions with lead(II) using two surfactants with the same chain length: dodecanoate and dodecylsulfate. Differences in their interactions with this metal ion in aqueous solutions are interpreted in terms of greater covalency of the bond between the metal and the carboxylate than with the sulfate group.

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