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

Abstract

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 we...