A Langmuir monolayer with a nontraditional molecular architecture

In the traditional concept, a stable Langmuir monolayer can be formed from amphiphilic molecules at the air-water interface with their hydrophobic alkyl chains oriented toward the air and the polar moieties embedded in the water. The intermolecular interactions between the polar moieties and the water subphase as well as the van der Waals interactions between the alkyl chains are requisite to hold together the molecules of the ordered Langmuir monolayers. Pure hydrocarbon chains without any polar moieties cannot form a Langmuir monolayer. In contrast to this traditional concept, we now report the discovery of an unusual Langmuir monolayer formed from a disubstituted urea lipid molecule (PDA-UR). The unique property of this monolayer exists in the fact that the polar moiety of the lipid molecule is actually suspended in the air phase while one of the hydrophobic tails is in contact with water. The existence of such a nontraditional monolayer is attributed to the strong hydrogen bonding network formation between the urea functional groups in the air phase. This unusual Langmuir monolayer model is soundly supported by various experimental investigations, which include the surface pressure-area isotherm and ellipsometry measurements, FT-IR and UV-vis absorption spectroscopic studies, as well as Brewster angle and scanning tunneling microscopic observations.