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

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 ( Δ p K ai 0 ) are compared. For PADA/DTAC Δ p K a1 0 = 0.05 and Δ p K a2 0 = 0.6 . For PADA/SDS Δ p K a1 0 = 2.1 and Δ p K a2 0 = 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 ( Δ p K ai w ) have also been analysed. Concerning PADA/Triton X-100, the shifts Δ p K a1 w = − 0.1 and Δ p K a2 w = − 0.9 are rationalized in terms of dielectric constant reduction at the reaction sites. Concerning PADA/DTAC, Δ p K a1 w = − 0.05 and Δ p K a2 w = − 0.3 whereas, for PADA/SDS, Δ p K a1 w = 2.0 and Δ p K a2 w = 1.2 . The p K a2 w 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.