X-ray studies of interfacial strontium-extractant complexes in a model solvent extraction system

Area of Science:

• Physical Chemistry
• Surface Science
• Materials Science

Background:

• Solvent extraction involves liquid-liquid systems where ions transfer between phases.
• Understanding interfacial ion-extractant complex structure is crucial for optimizing extraction efficiency.
• Previous studies used adsorption transitions to immobilize and characterize complexes at interfaces.

Purpose of the Study:

• To determine the interfacial ion-extractant complex structure for SrCl2 extraction using dihexadecyl phosphate (DHDP).
• To investigate how pH affects the complex structure and metal ion coverage at the liquid-liquid interface.
• To compare the behavior of divalent Sr(2+) with trivalent Er(3+) ions at the interface.

Main Methods:

• Studied a model system of DHDP in dodecane and SrCl2 in water.
• Utilized X-ray reflectivity and fluorescence near total reflection to probe interfacial structure.
• Employed an adsorption transition method to immobilize complexes for characterization.
• Applied Stern equation and Gouy-Chapman theory for data analysis.

Main Results:

• DHDP formed a homogeneous monolayer at the interface below the adsorption transition, binding Sr(2+).
• Sr(2+) coverage increased with pH, reaching saturation (1 Sr(2+) per 2 DHDP).
• Fluorescence detected more Sr(2+) than reflectivity, indicating presence in the diffuse double layer.
• Above the transition, dilute DHDP-Sr complexes remained at the interface.

Conclusions:

• The study elucidated the molecular-scale structure of the interfacial DHDP-Sr complex.
• Metal ion charge significantly influences the structure of interfacial ion-extractant complexes.
• Findings provide insights into the extraction mechanisms of divalent and trivalent metal ions.