On the external morphology of native cellulose microfibrils

This study is based on the working hypothesis that the external morphology of the cellulose microfibrils is correctly represented by a combination of eight surfaces issued from four lateral cleavage planes of the I-α and I-β allomorphs. Models of these surfaces have been generated and investigated before and after relaxation, thus allowing one to predict, for each of these, their roughness, the accessibility of the hydrophilic and hydrophobic groups as well as their surface and attachment energies. Results showed that the ensemble of eight surfaces could be divided into three families. The first family contains four hydrophilic and moderately rough surfaces, which dominate the external morphology of the microfibrils and are thus responsible for their macroscopic properties. Surfaces of the two other families are of minor importance in the external morphology as they are located at the corners of the cellulosic macrocrystals. They are either flat and hydrophobic or rough and hydrophilic. The flat surfaces are of high biological and technical significance as they are specifically recognized by hydrophobic substances, including the cellulose binding modules of cellulases. Relaxation resulted in a significant disorganization of the rough surfaces whereas the other surfaces remain close to their original organisation. The interpretation of the surface and attachment energies of the surfaces, evidences the major influence of the biosynthetic process in the design of the external morphology of the cellulose microfibrils, as opposed to the classical kinetic and thermodynamic crystal growth mechanism.