Phase behavior and molecular dynamics simulation studies of new aqueous two-phase separation systems induced by HEPES...

Area of Science:

• Physical Chemistry
• Solution Chemistry
• Materials Science

Background:

• Aqueous solutions of organic solvents are widely used but pose separation challenges.
• Developing efficient and environmentally friendly separation methods is crucial.

Purpose of the Study:

• To investigate the phase behavior of aqueous organic solvent mixtures with the addition of 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid (HEPES).
• To elucidate the molecular mechanisms driving the observed phase separation using molecular dynamics simulations.
• To explore the potential of HEPES as a benign separation auxiliary agent.

Main Methods:

• Experimental determination of phase diagrams for various aqueous organic solvent systems at ambient temperature.
• Molecular dynamics (MD) simulations of HEPES + water + tetrahydrofuran (THF) mixtures across different compositions.
• Analysis of hydrogen bonding, radial distribution functions, coordination numbers, and interaction energies from MD simulations.

Main Results:

• Addition of HEPES to aqueous solutions of THF, 1,3-dioxolane, 1,4-dioxane, propanols, tert-butanol, acetonitrile, and acetone induced liquid-liquid phase separation.
• MD simulations confirmed the formation of two distinct liquid phases, with organic solvents excluded from the water-rich phase.
• HEPES demonstrated insolubility in pure THF, aggregating entirely, and simulations revealed its role in mediating the phase separation through complex interactions.

Conclusions:

• HEPES acts as an effective inducer of phase separation in aqueous organic solvent mixtures.
• The study provides molecular-level insights into the mechanism of solvent exclusion driven by zwitterionic solutes.
• Findings suggest potential for designing novel, benign separation auxiliary agents based on zwitterions.