Interactions of divalent metal cations with headgroups of monomers, dimers, and trimers of anionic surfactant

Abstract

The goal of this work was to investigate the interaction and binding of the metal divalent cations, Me2+, with the headgroups of monomers, dimers, and trimers of anionic surfactant, for the alkaline earth metal cations (Me2+ = Be2+, Mg2+, Ca2+, Sr2+, Ba2+), the transition metal ions (Me2+ = Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+), and Sn2+ and Pb2+ ions. Herein, sodium dodecyl sulfate, SDS, was applied as an anionic surfactant. In this study mass spectrometry with electrospray ionization was used, which allowed determination of the preference of Me2+ binding to DS monomers (PMemon) and dimers (PMedim) and the total energies of interaction (bonding), Ebind, of divalent metal ions with DS trimers. The PMemon, PMedim, and Ebind parameters were correlated with different ion properties to characterize the interactions between Me2+ ions and a given anionic surfactant. The metal ions were ordered according to their affinity to DS monomers and dimers and their strength of binding with DS trimers. As follows from the results presented, from among the alkaline earth metal cations, the Ca2+ ions are the easiest to bind with DS monomers, dimers, and trimers and make the strongest bonds with them, while among the other Me2+ ions considered, the Hg2+, Sn2+, and Pb2+ cations interact very weakly and poorly with DS monomers, dimers, and trimers. From the results obtained in this work, it can be concluded which divalent metal ions should be used to introduce more or less significant changes in the aggregate structures of anionic surfactants.