Guangming Luo

Communications: Monovalent ion condensation at the electrified liquid/liquid interface

X-ray reflectivity studies demonstrate the condensation of a monovalent ion at the electrified interface between electrolyte solutions of water and 1,2-dichloroethane. Predictions of the ion distributions by standard Poisson-Boltzmann (Gouy-Chapman) theory are inconsistent with these data at higher applied interfacial electric potentials. Calculations from a Poisson-Boltzmann equation that incorporates a nonmonotonic ion-specific potential of mean force are in good agreement with the data.

X-ray studies of the interface between two polar liquids: neat and with electrolytes

We demonstrate the use of X-ray reflectivity to probe the electron density profile normal to the interface between two polar liquids. Measurements of the interfacial width at the neat nitrobenzene/water and the neat water/2-heptanone interfaces are presented. These widths are consistent with predictions from capillary wave theory that describe thermal interfacial fluctuations determined by the tension and bending rigidity of the interface. Variation of the temperature of the water/nitrobenzene interface from 25 degrees C to 55 degrees C indicates that the role of the bending rigidity decreases with increasing temperature. X-ray reflectivity measurements of the electrified interface between an aqueous solution of BaCl2 and a nitrobenzene solution of TBATPB demonstrate the sensitivity of these measurements to the electrolyte distribution at the interface. A preliminary analysis of these data illustrates the inadequacy of the simplest, classical Gouy-Chapman theory of the electrolyte distribution.

Observation of a Rare Earth Ion–Extractant Complex Arrested at the Oil–Water Interface During Solvent Extraction

Selective extraction of metal ions from a complex aqueous mixture into an organic phase is used to separate toxic or radioactive metals from polluted environments and nuclear waste, as well as to produce industrially relevant metals, such as rare earth ions. Selectivity arises from the choice of an extractant amphiphile, dissolved in the organic phase, which interacts preferentially with the target metal ion. The extractant-mediated process of ion transport from an aqueous to an organic phase takes place at the aqueous-organic interface; nevertheless, little is known about the molecular mechanism of this process despite its importance. Although state-of-the-art X-ray scattering is uniquely capable of probing molecular ordering at a liquid-liquid interface with subnanometer spatial resolution, utilizing this capability to investigate interfacial dynamical processes of short temporal duration remains a challenge. We show that a temperature-driven adsorption transition can be used to turn the extraction on and off by controlling adsorption and desorption of extractants at the oil-water interface. Lowering the temperature through this transition immobilizes a supramolecular ion-extractant complex at the interface during the extraction of rare earth erbium ions. Under the conditions of these experiments, the ion-extractant complexes condense into a two-dimensional inverted bilayer, which is characterized on the molecular scale with synchrotron X-ray reflectivity and fluorescence measurements. Raising the temperature above the transition leads to Er ion extraction as a result of desorption of ion-extractant complexes from the interface into the bulk organic phase. XAFS measurements of the ion-extractant complexes in the bulk organic phase demonstrate that they are similar to the interfacial complexes.

Ion Distributions at the Water/1,2-Dichloroethane Interface: Potential of Mean Force Approach to Analyzing X-ray Reflectivity and Interfacial Tension Measurements

We present X-ray reflectivity and interfacial tension measurements of the electrified liquid/liquid interface between two immiscible electrolyte solutions for the purpose of understanding the dependence of interfacial ion distributions on the applied electric potential difference across the interface. The aqueous phase contains alkali-metal chlorides, including LiCl, NaCl, RbCl, or CsCl, and the organic phase is a 1,2-dichloroethane solution of bis(triphenylphosphor anylidene) ammonium tetrakis(pentafluorophenyl)borate (BTPPATPFB). Selected data for a subset of electric potential differences are analyzed to determine the potentials of mean force for Li(+), Rb(+), Cs(+), BTPPA(+), and TPFB(-). These potentials of mean force are then used to analyze both X-ray reflectivity and interfacial tension data measured over a wide range of electric potential differences. Comparison of X-ray reflectivity data for strongly hydrated alkali-metal ions (Li(+) and Na(+)), for which ion pairing to TPFB(-) ions across the interface is not expected, to data for weakly hydrated alkali-metal ions (Rb(+) and Cs(+)) indicates that the Gibbs energy of adsorption due to ion pairing at the interface must be small (<1 k(B)T per ion pair) for both the CsCl and RbCl samples. This paper demonstrates the applicability of the Poisson-Boltzmann potential of mean force approach to the analysis of X-ray reflectivity measurements that probe the nanoscale ion distribution and the consequences of these underlying distributions for thermodynamic studies, such as interfacial tension measurements, that yield quantities related to the integrated ion distribution.

X‐Ray Studies of Surfactant Ordering and Interfacial Phases at the Water‐Oil Interface

X‐ray reflectivity studies of surfactants at the water‐oil interface yield structural information with sub‐nanometer resolution. In this presentation, we reviewed recent X‐ray reflectivity measurements of the interface between water and a hexane solution of the hydrocarbon alkanol CH3(CH2)19OH and fluorocarbon alkanol CF3(CF2)7(CH2)2OH. The mixed system exhibits three monolayer phases, two of which are similar to single surfactant phases. A transition from a liquid monolayer to a solid monolayer occurs with increasing temperature. This unusual phase transition and the qualitative features of the phase diagram are predicted by an appropriate superposition of the behavior of the two single surfactant systems.

Erbium(III) Coordination at the Surface of an Aqueous Electrolyte

Grazing-incidence (GI) X-ray absorption spectroscopy (XAS) under conditions of total external reflection is used to explore the coordination environment of the trivalent erbium ion, Er(3+), at an electrolyte-vapor interface. A parallel study of the bulk aqueous electrolyte (1 M ErCl3 in HCl at pH = 1.54) shows that the Er(3+) ions have a simple hydration shell with an average Er-OH2 bond distance of 2.33(1) Å, consistent with previous descriptions of the aquated cation, [Er(OH2)8](3+). No other correlations are observed in the electrolyte EXAFS (extended X-ray absorption fine structure) data acquired at room temperature. In contrast, the coordination of the Er(3+) ions at the electrolyte-helium interface, as interrogated by use of electron-yield detection, reveal correlations beyond the Er-OH2 hydration shell that are unexpectedly well-defined. Analyses show an environment that consists of a first coordination sphere of 6-7 O atoms at 2.36(1) Å and a second one of 3 Cl atoms at 2.89(2) Å, suggesting the formation of a neutral [(H2O)6-7ErCl3] entity at the surface of the electrolyte. The presence of a third, distant peak in the Fourier transform data is attributed to Er-Er correlations (in possible combination with contributions from distant Er-O and Er-Cl interactions). The best-Z and -integer fits reveal 3 Er atoms at 3.20(2) Å, confirming the near-surface-enrichment of Er(3+) as revealed previously by use of X-ray reflectivity measurements (J. Phys. Chem. C 2013, 117, 19082). Here, the strong associations between the Er-aqua-chloro entities at the electrolyte-vapor interface are shown to be consistent with the formation of domains of polynuclear cluster motifs, such as would arise through hydrolysis reactions of the aquated Er(3+) cations. The local structural results and the calculated surface coverage are of relevance to understand the myriad reactions involved in the hydrometallurgical process of solvent extraction (SX) for metal purification, which involves the transfer of a selected metal ion, like Er, across an interface from an aqueous electrolyte to an organic phase.

X-ray fluorescence from a model liquid/liquid solvent extraction system

X-ray fluorescence near total reflection (XFNTR) is measured from the liquid/liquid interface between dodecane and an ErCl3 aqueous solution by monitoring L shell Er emission lines. A custom sample cell is used to minimize absorption of the fluorescence x-rays that pass through dodecane on their way to the detector. The Er3+ concentration near the interface is related to the fluorescence intensity by a scale factor that is extracted by fitting the incident-angle dependent Er Lα emission line intensities for different ErCl3 bulk concentrations. As an application, we present the use of XFNTR to monitor the growth of interfacial crud in a model solvent extraction system consisting of an interface between a dodecane solution of bis(2-ethylhexyl) phosphate (HDEHP) and an ErCl3 aqueous solution.

X-RAY ANALYSIS OF ZNSE/ZNSTE QUANTUM STRUCTURE

A novel microstructure of an A1BA2‐type ZnSTe/ZnSe quantum structure has been investigated by high‐resolution x‐ray two‐axis diffraction, reflectivity, and x‐ray topography which offer a nondestructive, high‐strain‐sensitive method for analyzing low‐dimensional structures. The results show that the molecular‐beam‐epitaxy growth condition was well controlled to suppress dislocations extending from the substrate and the epitaxial layers have high crystalline quality. The ZnS0.665Te0.335 epilayer undergoes a tensile strain with eT∥=2.5633×10−2 and eT⊥=8.8254×10−2 while a compressed strain with eB∥=2.7864×10−3 and eB⊥=9.5061×10−3 exists in the first layer of ZnSe. The interfacial roughness is about 5 A and the lateral correlation length ≥2000 A.