Kinetics of adsorption of azacrown ether and fatty acid at the toluene–water interface in the presence of metal ions

The dynamic interfacial tension (IT) of N,N-didecyl-diaza-18-crown-6 ether (22DD) and lauric acid (LAH) at the toluene–aqueous interface in the presence of different cations, i.e. H+, Cu(II), Pb(II), Cd(II), Ni(II) and Li(I) was studied. At least two relaxation processes were identified as being responsible for the observed slow non-monotonic decays of IT, at timescales <1 s and minutes-hours, respectively. The presence of cations influences the first, diffusion-controlled stage of adsorption. However, the adsorbed monolayer is not in an equilibrium state, and IT(t) continues to change, possibly due to complexation/decomplexation reactions at the interface. At higher concentrations of cations, non-monotonic IT(t) curves were obtained. The relaxation processes are very similar for 22DD and mixed 22DD–LAH monolayers, even though the mass transfer of cations through the interface is very different in both cases. The slow IT(t) changes are thus not related to mass transfer, but to internal monolayer relaxations. The surface elasticity of the studied monolayers is more sensitive than IT(t) to the presence of small amounts of metal ions, especially in the case of Cu(II). On the other hand, at higher concentrations of metal ions, IT(t) shows more variations as a function of metal ion concentrations.