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

Area of Science:

• Physical Chemistry
• Electrochemistry
• Materials Science

Background:

• Molecular understanding of aqueous biphasic systems (ABSs) interfaces is limited, relying on indirect methods.
• Existing models often assume sharp interfaces, which may not reflect reality.

Purpose of the Study:

• To investigate the molecular structure of salt-salt ABS interfaces using high-resolution Raman imaging.
• To compare the interface characteristics of ABSs with traditional immiscible electrolyte solutions (ITIES).

Main Methods:

• High-resolution Raman imaging was employed to analyze LiCl-LiTFSI-water and HCl-LiTFSI-water systems.
• Concentration profiles of anions and water were mapped at the interface.

Main Results:

• ABS interfaces exhibit sigmoidal concentration profiles for TFSI anions and water, indicating no significant adsorption.
• Interface thickness extends to micrometers, significantly broader than the nanometer-scale ITIES interfaces.
• This reveals a gradual transition zone, not a sharp molecular boundary.

Conclusions:

• Salt-salt ABSs form extended interphases, not sharp interfaces, challenging previous assumptions.
• This diffuse interface understanding is vital for optimizing electrochemical applications like batteries.
• Advances understanding of aqueous interfaces for ion and electron transfer processes.