Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA.
Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA.
Phys Rev Lett. 2019 Feb 8;122(5):058001. doi: 10.1103/PhysRevLett.122.058001.
Rare earths, which are fundamental components of modern technologies, are often extracted from aqueous solutions using surfactants at oil-water interfaces. Heavier lanthanides are more easily extracted, even though all lanthanides are chemically very similar. Using x-ray fluorescence measurements and theoretical arguments, we show that there is a sharp bulk-concentration-dependent transition in the interfacial adsorption of cations from aqueous solutions containing Er^{3+} or Nd^{3+} in contact with a floating monolayer. The threshold bulk concentration of erbium (Z=68) is an order of magnitude lower than that of neodymium (Z=60), and erbium is preferentially adsorbed when the solution contains both ions. This implies that elemental selectivity during separation originates at the surfactant interface. Electrostatic effects arising from the interface dielectric mismatch, ionic correlations, and sizes of the ions explain the sharp adsorption curve and selectivity.
稀土元素是现代技术的基础组成部分，通常在油水界面使用表面活性剂从水溶液中提取。尽管所有的镧系元素在化学上都非常相似，但较重的镧系元素更容易被提取。通过 X 射线荧光测量和理论论证，我们表明，在含有 Er3+或 Nd3+的水溶液与漂浮单分子层接触时，阳离子在界面处的吸附存在一个明显的体浓度相关的转变。铒（Z=68）的体浓度阈值比钕（Z=60）低一个数量级，当溶液中同时含有两种离子时，铒优先被吸附。这意味着分离过程中的元素选择性起源于表面活性剂界面。界面介电失配、离子相关性和离子大小引起的静电效应解释了吸附曲线和选择性的陡峭。
