Direct imaging of micrometer-thick interfaces in salt–salt aqueous biphasic systems

Abstract

Unlike the interface between two immiscible electrolyte solutions (ITIES) formed between water and polar solvents, molecular understanding of the liquid–liquid interface formed for aqueous biphasic systems (ABSs) is relatively limited and mostly relies on surface tension measurements and thermodynamic models. Here, high-resolution Raman imaging is used to provide spatial and chemical resolution of the interface of lithium chloride - lithium bis(trifluoromethanesulfonyl)imide - water (LiCl–LiTFSI–water) and HCl–LiTFSI–water, prototypical salt–salt ABSs found in a range of electrochemical applications. The concentration profiles of both TFSI anions and water are found to be sigmoidal thus not showing any signs of a positive adsorption for both salts and solvent. More striking, however, is the length at which the concentration profiles extend, ranging from 11 to 2 µm with increasing concentrations, compared to a few nanometers for ITIES. We thus reveal that unlike ITIES, salt–salt ABSs do not have a molecularly sharp interface but rather form an interphase with a gradual change of environment from one phase to the other. This knowledge represents a major stepping-stone in the understanding of aqueous interfaces, key for mastering ion or electron transfer dynamics in a wide range of biological and technological settings including novel battery technologies such as membraneless redox flow and dual-ion batteries.

Elucidating the Size and Shape of Individual Clinker Phases via Raman Imaging: Impact on Cement Hydration

“…Raman phase images can be used to obtain the PSD, shape, and quantity of every phase present in a cement individually, thereby adding a new dimension, composition, to the PSD of cements. Recently, Raman imaging has been performed in multiple dimensions to study various systems such as living mice, cells with acid–receptor interactions, Li metal anodes, and aqueous bi-phasic interfaces . In this study, in Figure , we report phase images and the corresponding PSDs of the major cementitious phases such as alite, belite, aluminate, and ferrite to illustrate the variation in PSDs (shift of the rapid growth segment) along the x -axis.…”

“…Recently, Raman imaging has been performed in multiple dimensions to study various systems such as living mice, 64 cells with acid−receptor interactions, 65 Li metal anodes, 66 and aqueous bi-phasic interfaces. 67 In this study, in Figure 2, we report phase images and the corresponding PSDs of the major cementitious phases such as alite, belite, aluminate, and ferrite to illustrate the variation in PSDs (shift of the rapid growth segment) along the x-axis. Further, we illustrate the variation in PSDs for all phases (both major and minor) in Movie S1.…”

Two-Step Mechanism for Halogen Intercalation in Graphite Enabled by Aqueous Biphasic Systems

“…Phase diagrams were measured using the cloud point titration method previously described. − Briefly, starting from a known mass of a concentrated solution of one component (LiBr 3 or LiTFSI), a concentrated solution of the second component is weighed while being added dropwise and vortexed until the solution turns cloudy. Then, Milli-Q water is weighed while being added dropwise and vortexed until the solution turns clear again (even though still showing the red color).…”

“…Our recent studies on ABS show that the halide-rich (top) phase contains a small amount of LiTFSI, while the TFSI-rich (bottom) phase contains a small amount of halogens. − To evaluate the potential impact of LiTFSI and LiCl on Br – oxidation, solutions containing both lithium halides (LiBr and LiCl) or LiBr with LiTFSI were studied. Addition of a large amount of LiCl (5m to 8m) or a small amount (0.1m) of LiTFSI in concentrated LiBr solutions does not lead to substantial changes in Br – oxidation (Figure S5).…”

Fine tuning of electrosynthesis pathways by modulation of the electrolyte solvation structure

“…Knowing that some supporting salts are known to induce demixing of ACN and water 64 while others maintain the nanoheterogeneity without inducing phase separation, fundamental work is currently devoted to understanding salt-induced liquid–liquid phase separation phenomena. 65,66 Such phenomena are ubiquitous, with implications in various fields such as biology, chemical extraction or batteries. However, not all synthetically relevant mixtures of solvents exhibit structuring, like mixtures of N , N -dimethylformamide (DMF) and water.…”