Laboratoire MSM, UMR 7177, Institut de Chimie, 1 rue B. Pascal, 67000 Strasbourg, France.
J Phys Chem B. 2014 Mar 20;118(11):3133-49. doi: 10.1021/jp411332e. Epub 2014 Mar 6.
We report a molecular dynamics (MD) study of biphasic systems involved in the liquid-liquid extraction of uranyl nitrate by tri-n-butylphosphate (TBP) to hexane, from "pH neutral" or acidic (3 M nitric acid) aqueous solutions, to assess the model dependence of the surface activity and partitioning of TBP alone, of its UO2(NO3)2(TBP)2 complex, and of UO2(NO3)2 or UO2(2+) uncomplexed. For this purpose, we first compare several electrostatic representations of TBP with regards to its polarity and conformational properties, its interactions with H2O, HNO3, and UO2(NO3)2 species, its relative free energies of solvation in water or oil environments, the properties of the pure TBP liquid and of the pure-TBP/water interface. The free energies of transfer of TBP, UO2(NO3)2, UO2(2+), and the UO2(NO3)2(TBP)2 complex across the water/oil interface are then investigated by potential of mean force (PMF) calculations, comparing different TBP models and two charge models of uranyl nitrate. Describing uranyl and nitrate ions with integer charges (+2 and -1, respectively) is shown to exaggerate the hydrophilicity and surface activity of the UO2(NO3)2(TBP)2 complex. With more appropriate ESP charges, mimicking charge transfer and polarization effects in the UO2(NO3)2 moiety or in the whole complex, the latter is no more surface active. This feature is confirmed by MD, PMF, and mixing-demixing simulations with or without polarization. Furthermore, with ESP charges, pulling the UO2(NO3)2 species to the TBP phase affords the formation of UO2(NO3)2(TBP)2 at the interface, followed by its energetically favorable extraction. The neutral complexes should therefore not accumulate at the interface during the extraction process, but diffuse to the oil phase. A similar feature is found for an UO2(NO3)2(Amide)2 neutral complex with fatty amide extracting ligands, calling for further simulations and experimental studies (e.g., time evolution of the nonlinear spectroscopic signature and of surface tension) on the interfacial landscape upon ion extraction.
我们报告了一个分子动力学（MD）研究，涉及双相系统，该系统参与硝酸铀酰通过三丁基磷酸酯（TBP）从“pH 中性”或酸性（3 M 硝酸）水溶液到正己烷的液 - 液萃取，以评估 TBP 本身、UO2(NO3)2(TBP)2 配合物以及未配位的 UO2(NO3)2 或 UO2(2+)的表面活性和分配的模型依赖性。为此，我们首先比较了 TBP 的几种静电表示方法，以了解其极性和构象特性、与 H2O、HNO3 和 UO2(NO3)2 物种的相互作用、在水或油环境中的相对溶剂化自由能、纯 TBP 液体的性质以及纯-TBP/水界面的性质。然后通过平均力势（PMF）计算研究 TBP、UO2(NO3)2、UO2(2+)和 UO2(NO3)2(TBP)2 配合物穿过水/油界面的转移自由能，比较了不同的 TBP 模型和两种硝酸铀酰电荷模型。结果表明，用整数电荷（分别为+2 和-1）描述铀酰和硝酸盐离子会夸大 UO2(NO3)2(TBP)2 配合物的亲水性和表面活性。使用更合适的静电势能（ESP）电荷，模拟 UO2(NO3)2 部分或整个配合物中的电荷转移和极化效应，可以使后者不再具有表面活性。这一特征通过 MD、PMF 和有无极化的混合-分相模拟得到了证实。此外，用 ESP 电荷将 UO2(NO3)2 物种拉到 TBP 相，可以在界面上形成 UO2(NO3)2(TBP)2，然后有利于其提取。因此，在提取过程中，中性配合物不应在界面上积累，而是扩散到油相。在具有脂肪酰胺萃取配体的中性 UO2(NO3)2(酰胺)2 配合物中也发现了类似的特征，这需要进一步的模拟和实验研究（例如，非线性光谱特征和表面张力的时间演化）来研究离子萃取时界面景观的变化。
