Cationic Gemini surfactant at the air/water interface

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The surface properties and structures of a cationic Gemini surfactant with a rigid spacer, p-xylyl-bis(dimethyloctadecylammonium bromide) ([C 18H 37(CH 3) 2N + CH 2 C 6H 4 CH 2 N +(CH 3) 2C 18H 37],2Br −, abbreviated as 18-Ar-18,2Br −1), at the air/water interface were investigated. It is found that the surface pressure–molecular area isotherms observed at different temperatures do not exhibit a plateau region but display an unusual “kink” before collapse. The range of the corresponding minimum compressibility and maximum compressibility modulus indicates that the monolayer is in the liquid-expanded state. The monolayers were transferred onto mica and quartz plates by the Langmuir–Blodgett (LB) technique. The structures of monolayers at various surface pressures were studied by atomic force microscopy (AFM) and UV–vis spectroscopy, respectively. AFM measurements show that at lower surface pressures, unlike the structures of complex or hybrid films formed by Gemini amphiphiles with DNA, dye, or inorganic materials or the Langmuir film formed by the nonionic Gemini surfactant, in this case network-like labyrinthine interconnected ridges are formed. The formation of the structures can be interpreted in terms of the spinodal decomposition mechanism. With the increase of the surface pressure up to 35 mN/m, surface micelles dispersed in the network-like ridges gradually appear which might be caused by both the spinodal decomposition and dewetting. The UV–vis adsorption shows that over the whole range of surface pressures, the molecules form a J-aggregate in LB films, which implies that the spacers construct a π – π aromatic stacking. This π – π interaction between spacers and the van der Waals interaction between hydrophobic chains lead to the formation of both networks and micelles. The labyrinthine interconnected ridges are formed first because of the rapid evaporation of solvent during the spreading processes; with increasing surface pressure, some of the alkyl chains reorient from tilting to vertical, forming surface micelles dispersed in the network-like ridges due to the strong interaction among film molecules.