Liquid-liquid separation of aqueous solutions: A molecular dynamics study

Area of Science:

• Physical Chemistry
• Materials Science
• Thermodynamics

Background:

• Supercooled water exhibits a liquid-liquid phase transition (LLPT) below its second critical point.
• This transition involves separation into high-density liquid (HDL) and low-density liquid (LDL) phases.
• Understanding solute effects on LLPT is crucial for water science.

Purpose of the Study:

• To investigate the influence of hydrophilic (NaCl) and hydrophobic (Ne) solutes on the LLPT of water.
• To determine how these solutes affect the coexistence region and phase diagram of supercooled water.
• To elucidate the mechanisms of solute-induced phase separation.

Main Methods:

• Molecular dynamics simulations were employed to model supercooled aqueous solutions.
• The local density of solute particles was quantified to analyze phase separation.
• Pressure-temperature phase diagrams were constructed to map the liquid-liquid coexistence region.

Main Results:

• NaCl solutions separated into solute-rich HDL and solute-poor LDL at low pressures.
• Ne solutions separated into solute-rich LDL and solute-poor HDL at high pressures.
• Both solutes expanded the temperature range of liquid-liquid separation.

Conclusions:

• Solute type dictates preferential solvation and influences phase separation behavior.
• Hydrophilic ions (NaCl) are preferentially solvated in HDL, while hydrophobic solutes (Ne) are preferentially solvated in LDL.
• Solutes modify the LLPT phase diagram of water through preferential solvation effects.