温度升高如何引起酸性水两相溶液的相分离
How temperature rise induces phase separation in acidic aqueous biphasic solutions
摘要：
基于离子液体的酸性水两相溶液（AcABS）最近在金属回收领域提供了突破。实际上，三丁基十四烷基氯化鏻（P $_{44414}$ Cl）和水含量大于60%的酸的混合物呈现出对金属离子具有非常好的萃取效率的相分离。此外，该三元溶液呈现下溶液临界温度（LCST），这意味着相图的两相区域随着温度的升高而增加，换句话说，从均匀液体的相分离可以通过温度的升高（通常几十度）来诱导。我们在这里解决的微观机制驱动相分离。小角中子散射为我们提供了不同酸含量和温度下的结构信息。我们的特点是球形胶束形成的二元离子液体/水溶液和胶束聚集时，加入酸，由于静电排斥的屏蔽。如果添加盐导致溶液中相同的转变，则离子强度不是相关参数，并且必须考虑更微妙的影响，例如离子大小或极化率，以使相图合理化。酸浓度和/或温度的增加最终导致胶束絮凝和相分离。这最后一步是通过氯离子在胶束表面的吸附实现的，吸附焓为$\sim$ 12 kJ/mol。胶束之间的吸引力可以很好地理解在DLVO电位。这种放热吸附补偿了熵成本，导致系统的反直觉行为。
英文摘要：
Ionic-liquid based acidic aqueous biphasic solutions (AcABS) recently offered a breakthrough in the field of metal recycling. Indeed, the mixture of tributyltetradecylphosphonium chloride (P$_{44414}$Cl) and acid with water content larger than 60 \% presents a phase separation with very good extraction efficiency for metallic ions. Moreover, this ternary solution presents a Lower Solution Critical Temperature (LCST), meaning that the biphasic area of the phase diagram increases upon increase of temperature, in other terms the phase separation from a homogeneous liquid can be induced by an elevation of temperature, typically a few tens of degrees. We address here the microscopic mechanisms driving the phase separation. Small Angle Neutron Scattering provides us with structural information for various acid content and temperature. We characterized the spherical micelle formation in the binary ionic liquid/water solution and the micelle aggregation upon addition of acid, due of the screening of electrostatic repulsion. If addition of salt leads to identical transitions in the solution, the ionic strength is not a relevant parameter and more subtle effects such as ion size or polarizability have to be taken into account to rationalize the phase diagram. The increase of both acid concentration and/or temperature eventually leads to the micelle flocculation and phase separation. This last step is achieved through chloride ion adsorption at the surface of the micelle with an enthalpy of adsorption of $\sim$ 12 kJ/mol. The attraction between micelles can be well understood in terms of DLVO potential. This exothermic adsorption compensates the entropic cost, leading to the counter-intuitive behavior of the system.
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