Langmuir-Blodgett films of cellulose nanocrystals: preparation and characterization.

The goal of this work is the preparation of monolayers of cellulose I nanocrystals providing flat crystalline cellulose surfaces. Suspensions of cellulose nanocrystals were prepared by hydrolyzing ramie and tunicin fibers with sulfuric acid. Due to surface grafted sulfate groups, the negatively charged, rod-like cellulose nanocrystals were found to form stable layers at the air-water interface in the presence of a cationic amphiphilic molecule such as dioctadecyldimethylammonium (DODA) used in this work. These layers were formed at different cellulose-DODA weight ratios, compressed and analyzed by tensiometry, ellipsometry and Brewster angle microscopy. At low cellulose concentrations the layers are discontinuous, becoming dense and homogeneous upon reaching a critical weight ratio, which depends on the aspect ratio of the cellulose nanocrystals. After transfer onto silicon wafers, the surface composition and morphology as well as the thickness of the films were examined by X-ray photoelectron spectroscopy, ellipsometry and atomic force microscopy. The results indicate that they are monolayer films, well structured, relatively smooth and pure. These films offer a crystalline and easily reproducible model cellulose surface.