Structure of Molten Alkali Chlorides at Charged Interfaces and the Prediction and Interpretation of Their X-ray Reflectivity

Abstract

The fundamental properties of molten salts have been the subject of research that spans a century. Yet, in the past few years, there has been an unprecedented surge in interest for these systems in the bulk and under confinement by walls and interfaces including under applied potentials. This is driven by the prospect of exciting and very practical energy technologies, including those in the solar and nuclear fields. This article sets to answer two simple but fundamental questions. How does the liquid structure of alkali chlorides change at a real interface when it is charged? Also, how would such changes on the liquid side of the interface be detected in X-ray reflectivity experiments? We use an interface mimicking conductive diamond, which because of its lattice spacing, is an excellent choice for reflectivity experiments. The reason for our interest in X-ray reflectivity is that, as opposed to electrochemical measurements alone, this is likely the only technique in which atomic level information at the liquid side of the interface can be gained under the extreme temperature environments of molten salts. As it will become apparent, the interpretation of reflectivity results in terms of atomic positions is complex when multiple species with different X-ray contrasts on the liquid side are considered. A theoretical scheme termed “the peaks and antipeaks analysis of reflectivity” originally introduced in our prior work (J. Phys. Chem. C 2019, 123 (8), 4914–4925) is expanded to interpret the structural changes at the interface as a function of applied electrical bias.

Vacuum Interfacial Structure and X-ray Reflectivity of Imidazolium-Based Ionic Liquids with Perfluorinated Anions from a Theory and Simulations Perspective

“…Second, we comment here on our choice 10 for analysis based on number density profiles and not electron density profiles or the actual charge density profiles of the liquid. In the XRR literature it is often the electron density profile that is plotted because the technique is sensitive to that; two systems can have identical or nearly identical atomic density profiles but very different X-ray reflectivities.…”

“…In this work we will use two different splitting schemes for the species giving rise to ; the first one is based on correlations between polar and apolar subcomponents as in eq 1 (here polar refers to the combined contribution of cation and anion heads whereas apolar refers to the combined contribution of cation and anion tails all as defined in Figure 1 ), and the second is based on correlations between each of the different subionic subcomponents (positive, negative, alkyl, and fluoroalkyl) as in eq 2 . The reader is referred to refs ( 9 ) and ( 10 ) for the analytical derivation of equations associated with the peaks and antipeaks analysis of reflectivity that we seek not to repeat here. The case of the C[2]-mim + -based ILs is the simplest to understand; Figure 7 shows the polar–polar, polar–apolar, and apolar–apolar components of for C[2]-mim + /BETI – , C[2]-mim + /BSI(1,4) − , C[2]-mim + /BSI(1,6) − , and C[2]-mim + /BSI(1,8) − .…”

“…Even when reasonable inversion of the reflectivity to obtain an average electron density at and away from the interface can be accomplished, assigning contributions to each of the multiple different subspecies is challenging. We present in this work a practical implementation of theoretical methodology we recently developed , to better understand X-ray reflectivity from molecular dynamics (MD) simulations in the hope to encourage more colleagues working on ILs to attempt the technique as part of their structural studies.…”

“…The interfacial structure of C[2]-mim + /NTf 2 – at a sapphire surface was also investigated by using X-ray reflectivity in conjunction with MD simulations . Our group has also investigated ILs under confinement ,− using MD simulations, and in ref we developed the peaks and antipeaks analysis for reflectivity; the ideas introduced in ref were later expanded and applied to the study of diamond-confined molten alkali chloride salts . The literature on the structure of ILs using other surface sensitive techniques is vast, and a full set of citations is beyond the scope of this work, but particularly important has been the pioneering work by Baldelli et al using sum frequency generation − as well as Perkin et al , and Atkin et al using force microscopy. − …”

Quantification of Stern Layer Water Molecules, Total Potentials, and Energy Densities at Fused Silica:Water Interfaces for Adsorbed Alkali Chlorides, CTAB, PFOA, and PFAS

“…Given the large amount of information available on ion-specific effects from electrokinetic measurements, X-ray spectroscopy, − and atomistic simulations − ,, on fused silica:aqueous interfaces, we study the fused silica:water interface at several selected pH values and ionic strengths using alkali chlorides LiCl, KCl, NaCl, RbCl, and CsCl. We hypothesize that even though the cations have different hydration chemistries, hyperpolarizabilities, and sizes, the ions’ surface coverages are not high enough to result in distinguishable ion-specific outcomes in χ (2) and Φ(0) tot .…”