Solute aggregation in dilute aqueous solutions of tetramethylurea

Aqueous solutions of tetramethylurea (TMU) were studied by small-angle neutron and X-ray scattering at temperatures 10, 25 and 40 °°C in the TMU mole fraction range of 0.0125–0.025. The scattering data indicate preferential attractive interaction between the solute molecules, which becomes stronger as the temperature increases. From SANS data, concentration fluctuations and Kirkwood–Buff integrals have been calculated. The experimental data reveal a considerable solute–solute attraction above 0.01 mole fraction of TMU. These findings indicate that in addition to the usually assumed solute caging by water molecules, solute clustering of hydrophobic nature should also be considered for this system.