SMALL-ANGLE NEUTRON-SCATTERING IN AQUEOUS-SOLUTIONS OF TETRAMETHYLUREA

Three types of small-angle neutron-scattering experiments in solutions of tetramethylurea (TMU) in water are reported: the dependence of the scattering pattern on temperature and on concentration, the contrast variation controlled by the isotopic composition of solvent (H2O to D2O ratio), and measurements of the single-particle form factor by using appropriate mixtures of tetramethylurea and deuterated tetramethylurea dissolved in two different mixtures of heavy and light water. Despite the rather small cross section (typically smaller than 0.05 cm-1 sr-1), good statistical accuracy was achieved. The results show that some structural changes take place in the aqueous TMU solution at an aquamolality of about 0.7, that the hydrophobic interaction between the TMU molecules becomes more attractive at higher temperatures, that contact pairs are present in the relatively dilute solutions, and that there is preferential orientation of water molecules in the hydration sphere. Incoherent mixtures of isotopically substituted solute molecules seem to be a good tool for the absolute intensity calibration of small-angle neutron-scattering spectrometers.