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

Methanol-Water (mw) mixtures, with or without a solute, display a nonideal thermodynamic behavior, typically attributed to the structure of the microphase. However, experimental observation of the microphase structures at the molecular length scale has been a challenge. We report the presence of molecular clusters in mw and formaldehyde-methanol-water (fmw) mixtures using small-angle neutron scattering (SANS) experiments and molecular dynamics (MD) simulations. Hydrophobic clusters of methanol in mw and formaldehyde-methanol in fmw mixtures were observed at low methanol compositions (x(m) <= 0.3). A three-dimensional hydrogen-bonded network of water with the solute is observed at x(m) = 0.5. Linear chains of methanol surrounding the formaldehyde and water molecules were observed at high methanol compositions (x(m) >= 0.7). The calculated size of the molecular clusters (r approximate to 0.5 nm, spherical) from the SANS data and their volume fraction closely matched the MD simulation results.