Nonionic n-Hexyl, n-Heptyl, and n-Octyl Urea Surfactants:  Some Physicochemical Properties

The thermal properties of n-hexyl, n-heptyl, and n-octyl urea, in the temperature range of 20 °C to the melting point, have been determined. The melting points of the three n-alkyl ureas, and the enthalpies for this crystalline solid to isotropic liquid transition, are similar. The urea headgroup interaction clearly dominates the melting behavior with only a minor contribution from alkyl chain interaction. The n-alkyl urea solubility in water, partial binary surfactant−water phase diagrams, and surface activity at the air−water interface have also been determined. The strong intermolecular hydrogen bonding existing in the crystalline state results in a relatively low solubility and a high crystal solubility boundary for each of the n-alkyl ureas. In surfactant−water mixtures, in the temperature range of 20−98 °C, three phases have been observed, viz., crystalline solid, a dilute aqueous solution of n-alkyl urea, and an isotropic liquid that is considered to be rich in n-alkyl urea but contains water as well. The isotropic liquid phase forms in the n-alkyl urea−water mixtures at temperatures that are ca. 30−40 °C lower than the melting point of the neat surfactant. The isotropic liquid coexists with the dilute aqueous solution of n-alkyl urea in the low surfactant region of the phase diagram. These surfactants do not appear to form micelles in aqueous solution. In terms of solubility and air/water surface activity, the medium chain length n-alkyl ureas are similar to medium chain length n-alkanols.