In situ study of the competitive adsorption of ions at an organic-aqueous two-phase interface: The essential role of the Hofmeister effect

Understanding of the microcosmic essence of the competitive adsorption of different ions at liquid/liquid interfaces is of crucial importance for the elucidation of the unique chemical reactivities or selectivities of ions in numerous heterogeneous chemical processes. However, the knowledge of the microscopic mechanism behind the competitive adsorption of ions at the liquid/liquid interface is lacking. Herein, the competitive adsorption of various inorganic salt anions at organic-aqueous two-phase interfaces has been investigated as compared to that of the CrO₄^2- ions by total internal reflection UV-visible (TIR-UV) spectroscopy since CrO₄^2- ions are detectable by UV-visible spectroscopy and have a relatively poor interface propensity as compared to other chaotropic ions. Experimental results indicate that the interface propensities of different salt anions to the organic/aqueous phase interface follow the Hofmeister series. Molecular dynamics simulations further provided molecular-level evidence for role of the Hofmeister series of ions in the competitive adsorption of salt anions at organic-aqueous two-phase interfaces; the present study provided the first-hand experimental evidence demonstrating the occurrence of the Hofmeister series effect at the organic/aqueous two-phase interfaces, influencing the competitive adsorption of different salt ions; moreover, it is expected to offer a basis for the development of new strategies for the regulation of the chemical reactivity and selectivity of ions at organic/aqueous phase interfaces by introduction of other ions for competitive adsorption.