Nonionic urea surfactants: formation of inverse hexagonal lyotropic liquid crystalline phases by introducing hydrocarbon chain unsaturation.

The homo-interaction between urea moieties residing in close proximity to each other generally results in very strong intermolecular hydrogen bonding. The bifurcated hydrogen bonding exhibited by n-alkyl substituted ureas means that for those urea surfactants possessing medium and long hydrocarbon chain substituents the crystal to isotropic liquid melting point is high and the solubility in water is very low, compared to other similar chain length nonionic surfactants. In addition, saturated n-alkyl urea surfactants do not form lyotropic liquid crystalline phases in water. In this work the strong intermolecular hydrogen bonding of the urea headgroup has been ameliorated through the introduction of unsaturated hydrocarbon chains, viz., oleyl (cis-octadec-9-enyl), linoleyl (cis, cis-octadec-9,12-dienyl), and linolenyl (cis, cis, cis-octadec-9,12,15-trienyl) with one, two, and three carbon double bonds, respectively. Unsaturation in the C18 urea surfactants lowers the melting point and promotes an inverse hexagonal phase, in oleyl urea-water and linoleyl urea-water systems, which is thermodynamically stable in excess water. As the degree of unsaturation is increased to three in linolenyl urea, there is a tendency for autoxidation/polymerization. The occurrence of an inverse hexagonal phase in the nonionic urea surfactant-water systems has been rationalized in terms of both local molecular and global self-assembled aggregate packing constraints.