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　π-π　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　γcmc　(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　(Amin).　The　increase　in　aliphatic　chain　length　led　to　a　reduction　of　the　cmc　and　the　C20　(concentration　of　surfactant　required　to　reduce　20　mN·　m-1　surface　tension　of　water),　but　had　little　effect　on　the　γcmc.　©　Editorial　office　of　Acta　Physico-Chimica　Sinica.