title: X-ray studies of interfacial strontium-extractant complexes in a model solvent extraction system.
abstract: The interfacial behavior of a model solvent extraction liquid-liquid system, consisting of solutions of dihexadecyl phosphate (DHDP) in dodecane and SrCl2 in water, was studied to determine the structure of the interfacial ion-extractant complex and its variation with pH. Previous experiments on a similar extraction system with ErCl3 demonstrated that the kinetics of the extraction process could be greatly retarded by cooling through an adsorption transition, thus providing a method to immobilize ion-extractant complexes at the interface and further characterize them with X-ray interface-sensitive techniques. Here, we use this same method to study the SrCl2 system. X-ray reflectivity and fluorescence near total reflection measured the molecular-scale interfacial structure above and below the adsorption transition for a range of pH. Below the transition, DHDP molecules form a homogeneous monolayer at the interface with Sr(2+) coverage increasing from zero to saturation (one Sr(2+) per two DHDP) within a narrow range of pH. Experimental values of Sr(2+) interfacial density determined from fluorescence measurements are larger than those from reflectivity measurements. Although both techniques probe Sr(2+) bound to DHDP, only the fluorescence provides adequate sensitivity to Sr(2+) in the diffuse double layer. A Stern equation determines the Sr(2+) binding constant from the reflectivity measurements and the additional Sr(2+) measured in the diffuse double layer is accounted for by Gouy-Chapman theory. Above the transition temperature, a dilute concentration of DHDP-Sr complexes resides at the interface, even for temperatures far above the transition. A comparison is made of the structure of the interfacial ion-extractant complex for this divalent metal ion to recent results on trivalent Er(3+) metal ions, which provides insight into the role of metal ion charge on the structure of interfacial ion-extractant complexes, as well as implications for extraction of these two differently charged ions.
authors.affiliation: Department of Physics and ‡Department of Chemical Engineering, University of Illinois at Chicago , Chicago, Illinois 60607, United States.
authors.forename: Wei
authors.initials: W
authors.lastname: Bu
authors.forename: Miroslav
authors.initials: M
authors.lastname: Mihaylov
authors.forename: Daniel
authors.initials: D
authors.lastname: Amoanu
authors.forename: Binhua
authors.initials: B
authors.lastname: Lin
authors.forename: Mati
authors.initials: M
authors.lastname: Meron
authors.forename: Ivan
authors.initials: I
authors.lastname: Kuzmenko
authors.forename: L
authors.initials: L
authors.lastname: Soderholm
authors.forename: Mark L
authors.initials: ML
authors.lastname: Schlossman