摘要/Abstract
液-液两相萃取过程中，有机磷类萃取剂分子的界面行为决定了其以何种形式参与到界面萃取反应中.为了阐明萃取剂分子界面行为的变化特点，采用Langmuir单分子膜技术研究了单分子膜中P507分子在气-液界面的吸附和聚集行为随亚相pH、有机溶剂极性的变化.通过测定表面压-分子面积等温线，并采用界面红外反射吸收光谱（IRRAS）分析表征气-液界面P507分子间相互作用，结果发现，以正己烷作铺展溶剂时，随亚相pH的降低，P507单分子膜质子化程度提高，P507分子极性端水化能力削弱，分子间相互作用增强，单分子膜中形成含有分子间氢键的聚集体.但采用极性有机溶剂（二氯甲烷和氯仿）铺展P507单分子膜，膜内P507分子界面聚集状态发生变化.铺展溶剂极性增强，单分子膜内会含有更多极性端水化能力强的P507分子单体，并且亚相pH降低，单分子膜不会出现类似正己烷条件下的π-A曲线收缩和P-O-H基团峰位红移现象.这证实了有机铺展溶剂极性可以改变P507单分子膜中分子界面存在形式和聚集状态.本工作为深入理解溶剂萃取过程中水油两相界面处酸性有机磷类萃取剂分子的聚集行为变化及其对界面反应活性的影响机制奠定了基础.
关键词: P507, 单分子膜, 气-液界面, 聚集行为, 表面压-分子面积等温线
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 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 π-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.
Key words: P507, monolayer, air-water interface, aggregation behavior, surface pressure-area isotherm
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