A Langmuir Monolayer with a Nontraditional Molecular Architecture

Abstract

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.

Dynamic Photoresponsive Functions in Organized Layer Systems Comprised of Azobenzene-containing Polymers

“…[60][61][62][63][64] Urea is chosen as the hydrophilic head because the monolayer of alkylureas shows interesting characteristic properties attributed to the formation of so-called bifurcated (double) hydrogen bond among the urea units on water. [65][66][67][68] When urea is connected with Az unit, further interesting behavior appears as follows. 1) The trans-to-cis photoisomerization of Az is completely hindered at the air-water interface despite the fact that the absorption maximum of the -Ã band almost coincides with that in solution.…”

Adsorption and micellar properties of a mixed system of nonionic–nonionic surfactants

“…Therefore, it is fundamentally important to study the 2-D surface phase behavior of mixed surfactant systems to select a suitable surfactant pair that could show desired performance properties. In this regard, a host of surface-sensitive techniques such as neutron reflectivity [12,13], sum frequency vibrational spectroscopy [14][15][16][17], ellipsometry [18,19], and infrared reflection absorption spectroscopy [20][21][22][23] have been widely used to gain insight into the phase behavior, the tilt orientation, the constituents of the adsorbed species, and the thickness of the adsorbed film at the air-water interface. Sensitive as the above-mentioned techniques may be, none of them can provide direct information as to changes in surface morphological features of the monolayer with changes in the surface concentration of the molecules.…”

Molecular Interactions at Interfaces

“…There is also a report on the formation of a non-traditional Langmuir monolayer of disubstituted urea lipid molecule where the hydrophilic part stays away from the water surface, whereas the hydrophobic part stays near to the water surface. The stabilization of such monolayer was attributed to the hydrogen bonding network of the urea moiety of the molecule Huo et al (2000).…”