Fluctuations and micro-heterogeneity in aqueous mixtures

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Computer simulations of aqueous mixtures frequently overestimate water-water density correlations.
• Previous explanations suggested these overestimations were simulation or model artifacts.

Purpose of the Study:

• To investigate the cause of systematically overestimated water-water density correlations in computer simulations of aqueous mixtures.
• To determine if enhanced water correlations are a physical effect or a simulation artifact.

Main Methods:

• Extensive molecular dynamics simulations of aqueous mixtures.
• Utilized a mixture of the extended single point charge (SPC) water model and a weaker version with scaled partial charges.
• Analyzed density correlations, structure factors, and concentration fluctuations.

Main Results:

• Enhanced water correlations are a genuine physical effect, not a simulation artifact.
• Strongly correlated water domains form at higher concentrations of the weaker water model (mole fraction x > 0.4).
• These domains influence the spatial decay of density correlations and produce a prepeak in the structure factor, similar to micro-emulsions.

Conclusions:

• Aqueous mixtures can exhibit Lifshitz-type behavior with competing concentration fluctuations and water domain formation.
• A method to predict accurate correlation function decay was developed by reconciling thermodynamic and structural data.
• Findings offer new insights into experimental Kirkwood-Buff integrals for aqueous mixtures.