Orientation of the Azobenzene Spacer of Carboxylic Methyl Ester Gemini Surfactants in Langmuir Monolayer

Abstract

The Langmuir monolayer of carboxylic methyl ester Gemini surfactants with the azobenzene spacer, referred to as MC m (azo)MC m , was prepared and the π-A isotherms were measured. The result revealed an orientational picture of the azobenzene spacer at the air/water interface. Before irradiation, the planar trans-azobenzene group adopted an approximate configuration parallel to the interface and lay on the air/water interface. After UV-light irradiation, the spacer became the twisted cis-one. Because of the location of the polar headgroups at the air/water interface, the two phenyl rings were also forced to take a near-parallel orientation with the interface to reduce the free energy. As a result, the spacer thin-layer was "thickened", which caused an elevation in collapse pressure.

Photoisomerization in different classes of azobenzene

“…ABn derivatives includes ABs with alkyl, 146,150 aryl, 151 halide, 151 carbonyl, 152 amide, 18,153,154 substituents as well as nitro, 146,152 and 3-amino, 156 and 3-alkoxy 157,158 azobenzenes. The absorption spectra are similar to unsubstituted ABs.…”

Self‐Assembly of Photoresponsive Molecular Amphiphiles in Aqueous Media

“…Langmuir monolayers, formed by insoluble or sparingly soluble amphiphiles, have attracted great interest due to the intrinsic properties facilitating the formation of well‐organized layered films. Langmuir monolayers of photoresponsive amphiphiles containing azobenzene, spiropyran, stilbene, overcrowded alkene, and dithienylethene units, show controlled reversible surface pressure, and tightly packed thin films of assembled structures can be obtained by the Langmuir–Blodgett method, which indicates significant prospects for optical devices and sensing applications [96–115] . Gibbs monolayers (absorption monolayers), composed of water‐soluble amphiphiles directly distributed at the air–water interfaces without the use of organic solvents, show tight correlations between assemblies in aqueous solution and at the air–water interface.…”

“…Langmuir monolayers of photoresponsive amphiphiles containing azobenzene,s piropyran, stilbene,o vercrowded alkene,and dithienylethene units,show controlled reversible surface pressure,a nd tightly packed thin films of assembled structures can be obtained by the Langmuir-Blodgett method, which indicates significant prospects for optical devices and sensing applications. [96][97][98][99][100][101][102][103][104][105][106][107][108][109][110][111][112][113][114][115] Gibbs monolayers (absorption monolayers), composed of water-soluble amphiphiles directly distributed at the air-water interfaces without the use of organic solvents,s how tight correlations between assemblies in aqueous solution and at the air-water interface. In early studies of Gibbs monolayers of photoresponsive amphiphiles,s ome crucial equilibrium physical parameters, e.g.,e quilibrium surface tension and critical micelle concentration (CMC), were employed to reveal the self-assembly transformations at air-water interfaces and in solutions.…”