The extraction of lanthanides from aqueous nitrate solutions by quaternary ammonium nitrate ionic liquids (e.g., [A336][NO₃]) shows a negative sequence (i.e., light lanthanides are more efficiently extracted than heavy lanthanides), which conflicts with the lanthanide contraction. In this study, we explored the origin of the negative sequence by investigating the extraction of lanthanides from ethylammonium nitrate by [A336][NO₃]. The extraction shows a positive sequence, which is converted to a negative sequence with the addition of water. The transformation from positive to negative sequences reveals that the negative sequence is caused by the hydration of lanthanide ions: hydration of lanthanide ions counteracts the extraction. Therefore, the use of solvents that have weak solvation with lanthanide ions might enhance the separation of the elements by solvent extraction.

中文翻译：

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水合作用通过溶剂萃取抵消镧系元素的分离

_{硝酸季铵离子液体（如[A336][NO 3} ]）从硝酸盐水溶液中提取镧系元素呈现负序（即轻镧系元素比重镧系元素更有效地提取），这与镧系元素收缩相矛盾。在本研究中，我们通过研究 [A336][NO ₃]。萃取呈正序，加水后转为负序。从正序列到负序列的转换表明负序列是由镧系离子的水合引起的：镧系离子的水合抵消了萃取。因此，使用与镧系离子具有弱溶剂化作用的溶剂可能会增强溶剂萃取对元素的分离。