Interfacial Behavior of Acidic Organophosphorus Extractant Monolayer at Air-Water Interface: Subphase pH and Spreading Solvent Effect

The interfacial properties of extractant molecules have a significant impact on their complexation reaction activity with rare earth ions at liquid-liquid interface during solvent extraction. Although it is known that acidic organophosphorus extractant exists mainly in the form of dimers in nonpolar organic solvent, the research on solvent extraction kinetics has pointed out that the extractant molecules should react with rare earth ions in the form of monomers at the interface. Therefore, understanding the existing forms of acidic organophosphorus extractant at the interface will help comprehend the interfacial reaction process in solvent extraction. Traditionally, the interfacial properties of the extractant molecules were investigated by measuring interfacial tension isotherms and calculating interfacial adsorption parameters. However, this method can not provide the information of interfacial active species and the aggregation behavior of them. In order to clarify the characteristics of the interfacial behavior of organic extractant molecules at the interface, the effect of subphase pH and the polarity of spreading organic solvent on the adsorption and aggregation behavior of P507 molecules at the air-water interface were investigated by surface pressure-area isotherms and infrared reflectance absorption spectroscopy (IRRAS) based on Langmuir monolayer technique. It was found that P507 monolayers spread by n-hexane at the air-water interface had a certain solubility in the subphase water due to the ionization of the polar groups of P507 molecules. And the solubility decreased as the subphase pH decreased. Thus, the surface pressure-area isotherms changed significantly due to the total amount of P507 molecules remaining on the surface of water changed with the subphase pH. When the subphase pH decreased below 2.0, the influence of the solubility of P507 molecules became inapparent and the amount of P507 molecules remaining on the surface of water was almost unchanged. The intermolecular hydrogen bonds formed between the polar groups due to the protonation degree of P507 monolayers improved and the hydration ability of P507 polar groups was weakened. The aggregates formed in the monolayer were confirmed by the red shift of P-O-H groups in IRRAS spectra. However, when the P507 monolayers were spread by polar organic solvent (dichloromethane and chloroform), the existing forms of P507 molecules in the monolayers were changed with the polarity of spreading solvent. And the p-A isotherms of P507 monolayers didn't exhibit the shrinkage of molecular area which existed in the monolayers spread by n-hexane when subphase pH decreased. It meant that the existing forms and aggregation behavior of P507 molecules in monolayers could be altered by the spreading solvent and more P507 monomers existed in the monolayer as the polarity of spreading solvent increased. The conclusion was confirmed by the shift of the peak positions of P-O-H with the spreading solvent in IRRAS spectra. The present work highlights the significant influence of the existing forms of P507 molecules on the interfacial properties of P507 monolayer at the air-water interface and the aggregation behavior in the monolayers can be changed by subphase pH and the spreading solvent.