Order in crystalline cellulose detected by atomic force microscopy (AFM)

In this work we applied AFM for the surface study of cellulose microcrystals using highly crystalline Valonia macrophysa vesicles as the substrate. Vesicles were pretreated with dilute alkali and an excess of distilled water prior to imaging with AFM. An AFM image of a vesicle showed a group of closely packed microcrystals. A more detailed image of the membrane was also obtained. In the area of 14 × 14 nm2 the surface roughness was typically 1 nm. We calculated the two-dimensional fast fourier transform (FFT) of the raw data and obtained crystal parameters as well as a highly filtered inverse-transformed image. This image showed a periodicity of 1.01 nm, which may correspond to the fibre repeat unit length. Molecular modelling was used to generate a Connolly model surface of the crystalline cellulose. The data created thereby was converted to a format identical with the AFM image. The experimental images were compared with the synthetic images made by molecular modelling. Crystal surface 110 was found to fit very well to the experimental image.