Effects of micelle nature and concentration on the acid dissociation constants of the metal extractor PADA

•SDS (−), DTAC (+) or Triton X (non-ionic) micelles differently affect PADA's pKas.•In the SDS negative micelle, PADA is located between the fixed and the shear layer.•In the DTAC positive micelle, PADA protrudes towards the bulk solvent.•In Triton X, dielectric constant reduction at the reaction sites occurs.•NaCl induces reduction of surface potential (SDS) or of dielectric constant (others).

The pyridine-2-azo-p-dimethylaniline (PADA) ligand presents two acid dissociation constants, being pKa1 related to the pyridinium and pKa2 related to the anilinium residue. These have been measured by spectrophotometric titrations in aqueous solutions containing either the anionic (SDS), or the non-ionic (Triton X-100) or the cationic (DTAC) surfactants. The pKai shifts of the charged systems from that of the PADA/Triton X-100 reference (ΔpKai0) are compared. For PADA/DTAC ΔpKa10=0.05 and ΔpKa20=0.6. For PADA/SDS ΔpKa10=2.1 and ΔpKa20=2.1 both yielding the value of −126 mV for the surface potential (ψ) of SDS. The ψ value, lying between the calculated Stern potential and the zeta potential, indicates that the dye is located on the SDS micelles between the fixed and the shear layer. In contrast, the behaviour of PADA/DTAC is explained assuming that the positively charged deprotonation sites of PADA are forced to protrude towards the bulk solvent by the positive charges of DTAC micelles. The shifts of the apparent pKai from the aqueous values (ΔpKaiw) have also been analysed. Concerning PADA/Triton X-100, the shifts ΔpKa1w=−0.1 and ΔpKa2w=−0.9 are rationalized in terms of dielectric constant reduction at the reaction sites. Concerning PADA/DTAC, ΔpKa1w=−0.05 and ΔpKa2w=−0.3 whereas, for PADA/SDS, ΔpKa1w=2.0 and ΔpKa2w=1.2. The pKa2w values decrease on raising the surfactant concentrations for all the investigated systems. This behaviour is explained assuming that the increase of the overall micellar surface and, by consequence, of the reaction sites number, results in a site dilution effect which disfavours proton association. The addition of NaCl induces changes of pKa1 and pKa2 which are explained in terms of (large) reduction of ψ for PADA/SDS and of (small) reduction of the dielectric constant for the other systems.

Graphical abstractThe pKas of PADA dye shift in the presence of micelles, depending on equilibrium involved, charge and concentration of the micelles and added salt.Figure optionsDownload full-size imageDownload as PowerPoint slide