Physical Chemistry

Direct Imaging of Micrometer Thick Interfaces in Salt-Salt Aqueous Biphasic Systems

Authors

  • Damien Degoulange Chimie du Solide et de l’Energie, Collège de France, UMR 8260, 75231 Paris Cedex 05 & Sorbonne Université, 75006 Paris, France & Réseau sur le Stockage Electrochimique de l’Energie (RS2E), CNRS FR3459, 33 rue Saint Leu, 80039 Amiens Cedex, France ,
  • Raj Pandya Laboratoire Kastler Brossel, ENS-Université PSL, CNRS, Sorbonne Université, Collège de France, 24 rue Lhomond, 75005 Paris, France & Department of Physics, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom ,
  • Michael Deschamps CNRS, CEMHTI UPR3079, Univ. Orléans, 1D avenue de la Recherche Scientifique, 45071 Orléans, France & Réseau sur le Stockage Electrochimique de l’Energie (RS2E), CNRS FR3459, 33 rue Saint Leu, 80039 Amiens Cedex, France ,
  • Dhyllan Skiba Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA ,
  • Betar Gallant Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA ,
  • Sylvain Gigan Laboratoire Kastler Brossel, ENS-Université PSL, CNRS, Sorbonne Université, Collège de France, 24 rue Lhomond, 75005 Paris, France ,
  • Hilton de Aguiar Laboratoire Kastler Brossel, ENS-Université PSL, CNRS, Sorbonne Université, Collège de France, 24 rue Lhomond, 75005 Paris, France ,
  • Alexis Grimaud Chimie du Solide et de l’Energie, Collège de France, UMR 8260, 75231 Paris Cedex 05 & Sorbonne Université, 75006 Paris, France & Réseau sur le Stockage Electrochimique de l’Energie (RS2E), CNRS FR3459, 33 rue Saint Leu, 80039 Amiens Cedex, France & Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, MA 02467, USA

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 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, in agreement with an increasing surface tension as a function of concentration, both being typical of a negative adsorption mechanism. More striking, however, is the length at which the concentration profiles extend, ranging from 11 to 2 m 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.

Content

Thumbnail image of Interface_ABS_final.pdf

Supplementary material

Thumbnail image of Interface_ABS_SI_final.pdf
Supporting Information: Direct Imaging of Micrometer Thick Interfaces in Salt-Salt Aqueous Biphasic Systems
Supporting Information for Direct Imaging of Micrometer Thick Interfaces in Salt-Salt Aqueous Biphasic Systems (Supplementary discussion 1 and Fig. S1 to S13)