Preparation of Langmuir–Blodgett films of aligned sepiolite fibers and orientation of methylene blue molecules adsorbed on the film

Sepiolite is a clay mineral with a fibrous morphology; the fibers possess rectangular channels along the fiber direction, and have cation-exchange ability. Amphiphilic octadecylammonium cations (ODAH+) spread at an interface between the air and a dispersion of sepiolite fibers have hybridized with the fibers to form a floating monolayer. Surface pressure–molecular area isotherms did not indicate clear collapse points of the hybrid monolayers. Observation of the hybrid films on glass plates by atomic force microscopy revealed that the sepiolite fibers preferred to be aligned perpendicularly to the compression direction, and that some fibers overlapped upon compression or even before compression. Out-of-plane and in-plane X-ray diffraction patterns of the hybrid film showed that the fibers were not only aligned by the compression but oriented around the fiber direction. Electronic spectra of methylene blue cations (MB+) adsorbed on the hybrid film indicated the presence of monomers and dimers of MB+. The absorption intensity due to the monomer was dependent on the polarization direction of the incident beam; interestingly, the monomers were spontaneously oriented along the sepiolite fiber direction but the dimers were not oriented. By comparison with the results of rhodamine 6G adsorbed on the film, the monomers of MB+ would probably be present in the channels of sepiolite fibers, while the dimers would be adsorbed on the external surface of the fibers.