Adsorption of Photosensitive Quaternary Ammonium Gemini Surfactant a4-6-m at the Air/Water Interface

Three dissymmetric quaternary ammonium Gemini surfactants (referred to as a4-6-m) were synthesized. One of the two hydrophobic chains of a4-6-m was a butyl chain terminated by an azobenzene group and the other was a conventional aliphatic chain of different length (m=12, 14, 16). The results showed that the trans-a4-6-m molecules adsorbed at the air/water interface with a vertical orientation of the two hydrophobic chains. The pi-pi interaction between azobenzene groups resulted in a denser arrangement of the surfactants. The azobenzene groups toward the air-side in the adsorption layer yielded a higher gamma(cmc) (surface tension value corresponding to the critical micelle concentration (cmc)) by comparison to 12-6-12. After UV-light irradiation, the trans-azobenzene groups converted into twisted cis-forms with a large dipole moment. These cis-azobenzene groups lay among the vertically oriented aliphatic chains and their relatively free location enhanced their dipole-dipole interactions, which promoted the tight packing of adsorbed molecules and slightly reduced the minimum molecular occupation area (A(min)). The increase in aliphatic chain length led to a reduction of the cmc and the C-20 (concentration of surfactant required to reduce 20 mN . m(-1) surface tension of water), but had little effect on the gamma(cmc).