Colloidal behavior of highly charged and highly anisotropic cellulose nanofibrils

Highly charged and highly anisotropic cellulose nanofibrils (CNFs) have emerged as a new, renewable and biodegradable nanomaterial . They have been used in numerous applications in new devices and materials. In order to liberate these fibrils from macroscopic, cellulose-rich fibres and to create stable colloidal dispersion of these nanoparticles, allowing for a bottom-up engineering, they need to be highly oxidized which in turn results in a very high surface charge of the fibrils. This also means that in addition to their nano-dimension, which as such leads to a violation of the basic assumptions for the DLVO theory, the particles will also carry a high surface charge which induces additional interactions with multivalent ions which cannot be described with standard theories for colloidal interactions. However, we have developed a semi-quantitative understanding of how these extra interactions should be considered and with this concept it is possible to for example forecast the properties of films made from the CNFs . In addition to these special interactions, in aqueous dispersions, their extraordinarily high anisotropy complicates the dispersion properties of the CNFs even further. An anisotropy between 300 and 500 means that the overlap concentration, i.e. the concentration where the spherical orbit inscribed by one particle will start to interfere with another similar particle, is between 25 and 9 mg/l. Since the stiffness of the fibrils is also very high this means that the CNFs will interact strongly with each other at very low concentration and they will start to form volume spanning arrested states/colloidal glasses at very low volume fractions . By using a combination of very careful rheological investigations and a new network model describing the structure of these fibrillar dispersions it has been possible to clarify how the fibrils are interacting in colloidal gels made from very dilute dispersions, by neutralizing the charges, and how the storage modulus of the gels can be linked to the structure and the inherent properties of the fibrils. Due to the very high anisotropy of the particles it has been possible to form stable gels at CNF volume fractions around 0.0002 which we actually have termed “solid water” at room temperature. With a simple technique it has also been possible to create stabile channels insides these gels by a Layer-by-Layer coating of the walls of these channels with polyallylamine hydrochloride and montmorillionite clay.