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 tempe rature range bf 20 degrees C to the melting point, have been determined. Th e melting points of the three n-alkyl ureas, and the enthalpies for this cr ystalline solid to isotropic liquid transition, are similar. The urea headg roup interaction clearly dominates the melting behavior with only a minor c ontribution from alkyl chain interaction. The n-alkyl urea solubility in wa ter, partial binary surfactant-water phase diagrams, and surface activity a t the air-water interface have also been determined. The strong intermolecu lar hydrogen bonding existing in the crystalline state results in a relativ ely low solubility and a high crystal solubility boundary for each of the n -alkyl ureas. In surfactant-water mixtures, in ther temperature range of 20 -98 degrees C, three phases have been observed, viz., crystalline solid, a dilute aqueous solution of n-alkyl urea, and an isotropic liquid that is co nsidered to be rich in n-alkyl urea but contains water as well. The isotrop ic liquid phase forms in the n-alkyl urea-water mixtures at temperatures th at are ca. 30-40 degrees C lower than the melting point of the neat surfact ant. The isotropic liquid coexists with the dilute aqueous solution of n-al kyl urea in the low surfactant region of the phase diagram. These surfactan ts do not appear to form micelles in aqueous solution. In terms of solubili ty and air/water surface activity, the medium chain length n-alkyl ureas ar e similar to medium chain length n-alkanols.