Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA.
X-ray Science Division, Argonne National Laboratory, Lemont, IL 60439, USA.
Phys Chem Chem Phys. 2023 Jun 21;25(24):16389-16403. doi: 10.1039/d3cp01029e.
Extractant aggregation in liquid-liquid extraction organic phases impacts extraction energetics and is related to the deleterious efficiency-limiting liquid-liquid phase transition known as third phase formation. Using small angle X-ray scattering, we find that structural heterogeneities across a wide range of compositions in binary mixtures of malonamide extractants and alkane diluents are well described by Ornstein-Zernike scattering. This suggests that structure in these simplified organic phases originates from the critical point associated with the liquid-liquid phase transition. To confirm this, we measure the temperature dependence of the organic phase structure, finding critical exponents consistent with the 3D Ising model. Molecular dynamics simulations were also consistent with this mechanism for extractant aggregation. Due to the absence of water or any other polar solutes required to form reverse-micellar-like nanostructures, these fluctuations are inherent to the binary extractant/diluent mixture. We also show how the molecular structure of the extractant and diluent modulate these critical concentration fluctuations by shifting the critical temperature: critical fluctuations are suppressed by increasing extractant alkyl tail lengths or decreasing diluent alkyl chain lengths. This is consistent with how extractant and diluent molecular structure are known to impact metal and acid loading capacity in many-component LLE organic phases, suggesting phase behavior of practical systems may be effectively studied in simplified organic phases. Overall, the explicit connection between molecular structure, aggregation and phase behavior demonstrated here will enable the design of more efficient separations processes.
萃取剂在液-液萃取有机相中聚集会影响萃取热力学，并与有害的、效率受限的液-液相间转变有关，这种转变称为第三相形成。我们使用小角 X 射线散射发现，在马来酰胺萃取剂和烷烃稀释剂的二元混合物中，广泛组成范围内的结构非均质性可以很好地用奥恩斯坦-泽尔尼克散射来描述。这表明这些简化有机相中的结构源于与液-液相变相关的临界点。为了证实这一点，我们测量了有机相结构随温度的变化，发现临界指数与 3D Ising 模型一致。分子动力学模拟也与这种萃取剂聚集机制一致。由于不存在形成反胶束状纳米结构所需的水或任何其他极性溶质，这些波动是二元萃取剂/稀释剂混合物固有的。我们还展示了萃取剂和稀释剂的分子结构如何通过改变临界温度来调节这些临界浓度波动：增加萃取剂烷基链长度或降低稀释剂烷基链长度会抑制临界波动。这与萃取剂和稀释剂的分子结构如何影响许多组分 LLE 有机相中金属和酸的负载能力一致，这表明实际体系的相行为可以在简化的有机相中有效地进行研究。总的来说，这里展示的分子结构、聚集和相行为之间的明确联系将使更有效的分离过程的设计成为可能。
