Surface Micelles of CF3(CF2)7(CH2)10COOH on Aqueous La3+ Subphase Investigated by Atomic Force Microscopy and Infrared Spectroscopy

Abstract

CF3(CF2)7(CH2)10COOH spontaneously assembles into monodispersed nanometer-sized micelles after spreading onto the aqueous lanthanum acetate subphase in a Langmuir trough at 283 K, according to the atomic force microscope (AFM) images. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and ordinary polarized infrared spectroscopy are applied to study the surface micelles in situ on the aqueous subphase and ex situ on the solid substrate, respectively. The long axes of the −(CH2)7− helix and −(CH2)10−chain are slightly inclined, with tilt angles of 25 ± 3°, on the solid substrate and basically so inclined on the aqueous subphase. The micelle formation comes from a subtle interplay of the steric hindrance imposed by the bulky −(CH2)7− helix and the van der Waals interaction between the underlying −(CH2)10− chains. The micelle has a CH2 antisymmetric stretching [νas(CH2)] frequency of 2918.0 ± 0.1 cm-1 on the solid substrate and 2916.4 ± 0.2 cm-1 on the aqueous subphase, corresponding to a predominantly trans zigzag planar chain conformation. This trans zigzag chain has a hexagonal packing in the micelle. Elevating the subphase temperature from 283 to 303 K and keeping the temperature at 303 K for some time (30−120 min) causes the micelles to fuse into each other, according to the AFM images. In the fused monolayer, the hydrocarbon segment has a higher νas(CH2) frequency of ∼2920 cm-1, implying that some gauche kinks appear during the thermal treatment. It is found that micelle fusion occurs preferentially at zero surface pressure in loosely packed monolayers rather than at 20 mN/m in tightly packed monolayers. Finally, the combined in situ and ex situ infrared characterization of the surface micelles eliminates transfer artifacts concerning the headgroup orientation.