Molecular Clustering in Formaldehyde-Methanol-Water Mixtures Revealed by High-Intensity, High-q Small-Angle Neutron...

Area of Science:

• Physical Chemistry
• Materials Science
• Chemical Physics

Background:

• Methanol-water mixtures exhibit nonideal thermodynamic behavior.
• Microphase structures in these mixtures are difficult to observe at the molecular scale.

Purpose of the Study:

• To investigate the presence and structure of molecular clusters in methanol-water (mw) and formaldehyde-methanol-water (fmw) mixtures.
• To correlate experimental observations with molecular dynamics simulations.

Main Methods:

• Small-angle neutron scattering (SANS) experiments.
• Molecular dynamics (MD) simulations.

Main Results:

• Observed hydrophobic clusters of methanol in mw and formaldehyde-methanol in fmw at low methanol compositions (x_m ≤ 0.3).
• Identified a 3D hydrogen-bonded network of water with solutes at x_m = 0.5.
• Detected linear chains of methanol surrounding formaldehyde and water at high methanol compositions (x_m ≥ 0.7).
• SANS data and MD simulations showed close agreement for cluster size (r ≈ 0.5 nm, spherical) and volume fraction.

Conclusions:

• Molecular clusters are present in mw and fmw mixtures.
• The structure of these clusters is composition-dependent.
• SANS and MD simulations are effective tools for characterizing microphase structures in liquid mixtures.