Tuning Low-Density Liquid Water with MgCl₂.

Area of Science:

• Physical Chemistry
• Solution Chemistry
• Spectroscopy

Background:

• Water exhibits a liquid-liquid critical point (LLCP) and complex phase behavior.
• Understanding water's structure under varying conditions is crucial for fundamental science.
• The influence of dissolved salts on water's unique properties remains an active research area.

Purpose of the Study:

• To investigate the structural changes in light and heavy water with MgCl2 addition.
• To explore the impact of salt concentration on low-density liquid (LDL) water domains.
• To elucidate the relationship between salt concentration and water's liquid-liquid phase transition (LLPT).

Main Methods:

• Excess infrared spectroscopy to differentiate water structures.
• Density functional theory (DFT) calculations for structural analysis.
• Systematic variation of MgCl2 concentrations in H2O and D2O.

Main Results:

• Excess spectroscopy successfully identified and quantified LDL water structures.
• MgCl2 addition initially decreases then increases LDL water populations.
• Significant disruption of LDL structures occurred at 0.4 M H2O and 0.6 M D2O.
• Threshold concentrations (1 M H2O, 1.3 M D2O) were found to increase LDL populations.

Conclusions:

• The study provides insights into how MgCl2 influences water's structural domains.
• Findings shed light on the complex liquid-liquid phase transition (LLPT) of water.
• This research contributes to understanding the factors governing water's LLCP.