The Role of Specific Ion Effects in Ion Transport: The Case of Nitrate and Thiocyanate

Abstract

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

Anions Enhance Rare Earth Adsorption at Negatively Charged Surfaces

“…SCNbehaves very differently compared to NO3and other halides at positively charged surfaces. 28 The current data in Figure 4, strongly suggests that the way SCNions affect the orientational water ordering is very similar under positively and negatively charged monolayers, observed as a red-shifted and narrowed VSFG signal, in good agreement with the overall observation of that the non-Coulombic forces are very dominant in SCNadsorption.…”

“…In contrast, NO3ions are known to fit into existing hydrogen bonding network of water molecules, without effecting the VSFG signal significantly, even at positively charged surfaces. 28 Therefore, even a small amount of La 3+ adsorption lead to very dramatic changes in VSFG signal ( Figure S1b). These subtle differences are only visible in La 3+ systems.…”

“…This is a very rough calculation and probably an underestimate. Because, it is possible that SCNions push water molecules from the interface, 28 also XR cannot detect less organized ions in the diffuse layer, since they do not create a significant electron density gradient. 29 Our approximation does not take these into account.…”

“…The VSFG spectra of air/aqueous interface, under various conditions, have been reported in the literature. 20,28,34,[39][40][41] It is possible to identify a bimodal water signal around 3000-3500 cm -1 due to the orientational ordering of water molecules at bare air/water the interface. Presence of charged monolayers, such as DHDP, creates a fixed electric field and the resulting interfacial ordering increases the VSFG signal, usually known as  (3) effect.…”

Ion-Specific Clustering of Metal-Amphiphile Complexes in Rare Earth Separations

“…33 The role of the liquid-liquid interface in SX process is still an understudied area, in spite of the increasing efforts in recent years. [51][52][53] Ion-specific effects in bulk phase 54 and at the interfaces 50,55 can play a significant role in SX. 14,56,57 TOMA complexes are known to play important roles in the SX process.…”