Selective transport of trivalent rare earth metals from aqueous into organic environments with the help of amphiphilic "extractants" is an industrially important process. When the amphiphilic extractant is positively charged or neutral, the co-extracted background anions are not only necessary for the charge balance but also have a large impact on the extraction efficiency and selectivity. Particularly, the opposite selectivity trends observed throughout the lanthanides series in the presence of nitrate and thiocyanate ions have not been explained. To understand the role of background anions in the phase transfer of lanthanide cations, we use a positively-charged longchain aliphatic molecule, modeling a common extractant, and gain molecular level insight into interfacial headgroup-anion interactions. By combining surface sensitive sum frequency generation spectroscopy with x-ray reflectivity and grazing incidence x-ray diffraction, we observed qualitative differences in the orientational and overall interfacial structure of nitrate and thiocyanate solutions at a positively charged Langmuir monolayer. Though nitrate adsorbs without dramatic changes to the solvation structure at the interface or the monolayer ordering, thiocyanate significantly alters water structure and reduces monolayer ordering. We suggest that these qualitatively different adsorption trends help explain a reversal in system selectivity towards lighter or heavier lanthanides in solvent extraction systems in the presence of nitrate or thiocyanate anions.The effect of ions at interfaces has been of interest since Franz Hofmeister observed the differential abilities of ions to precipitate proteins. 1-3 The implications of the Hofmeister series extends well beyond biological systems; specific ion effects (SIE) are important in a wide range of fields, including geochemistry, 4-5 atmospheric chemistry, 6-8 and chemical separations, [9][10][11][12][13] where ion adsorption and/or transfer at aqueous interfaces play significant roles. Due to this broad applicability, the Hofmeister series and similar empirical trends in SIE have been extensively studied to understand the effects of ions that cannot be simply explained by their charge and ionic concentration. [3][4][5][14][15][16][17] The majority of these studies use the concept of "series" to explain certain physicochemical effects of ions being "more" or "less" for one ion compared to another one.Although useful in many cases, this language inevitably implies a possible single underlying mechanism to explain SIE. However, with the advancement of molecular-scale probes that can directly observe SIE at interfaces, it has become evident that such simple and universal explanation, possibly, does not exist. [2][3][18][19] Instead, SIE needs to be considered in a multidimensional parameter space, considering all the possible factors, such as surface functionalization and hydrophobicity, and ion-ion and ion-solvent correlations that are enhanced at interfaces and in confinement. 3,7
The nanoscale structure of a complex fluid can play a major role in the selective adsorption of ions at the nanometric interfaces, which are crucial in industrial and technological applications....
Effective and energy efficient separation of precious and rare metals is very important for a variety of advanced technologies. Liquid-liquid extraction (LLE) is a relatively less energy intensive separation technique, widely used in separation of lanthanides, actinides, and platinum group metals (PGMs). In LLE, the distribution of an ion between an aqueous phase and an organic phase is determined by enthalpic (coordination interactions) and entropic (fluid reorganization) contributions. The molecular scale details of these contributions are not well understood. Preferential extraction of an ion from the aqueous phase is usually correlated with the resulting fluid organization in the organic phase, as the longer-range organization increases with metal loading. However, it is difficult to determine the extent to which organic phase fluid organization causes, or is caused by, metal loading. In this study, we demonstrate that two systems with the same metal loading may impart very different organic phase organization; and investigate the underlying molecular scale mechanism. Small angle X-ray scattering shows that the structure of a quaternary ammonium extractant solution in toluene is affected differently by the extraction of two metalates (octahedral PtCl 6 2and square-planar PdCl 4 2-), although both are completely transferred into the organic phase. The aggregates formed by the metalate-extractant complexes (approximated as reverse micelles) exhibit more long-range order (clustering) with PtCl 6 2compared to that with PdCl 4 2-. Vibrational sum frequency generation spectroscopy, and complimentary atomistic molecular dynamics simulations on model Langmuir monolayers, indicate that the two metalates affect the interfacial hydration structures differently. Further, the interfacial hydration is correlated with water extraction into the organic phase. These results support a strong relationship between the organic phase organizational structure and different local hydration present within the aggregates of metalate-extractant complexes, which is independent of metalate concentration. File list (2) download file view on ChemRxiv Origins_of_clustering_acsami_revised.pdf (1.94 MiB) download file view on ChemRxiv SI_Origins of clustering_acsami_revised.pdf (1.82 MiB)
Controlled self-assembly of nanoparticles into ordered structures is a major step in fabricating nanotechnology based devices. Here, we report on the self-assembly of high quality superlattices of nanoparticles in aqueous suspensions induced via interpolymer complexation. Using small angle X-ray scattering, we demonstrate that the NPs crystallize into superlattices of FCC symmetry, initially driven by hydrogen bonding and subsequently by van der Waals forces between the complexed coronas of hydrogenbonded polymers. We show that the lattice constant and crystal quality can be tuned by polymer concentration, suspension pH and the length of polymer chains. Interpolymer complexation to assemble nanoparticles is scalable, inexpensive, versatile and general.
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