Quantitative modelling of water transport in ultrathin cellulose nanofibril films

Cellulose nanoparticles exhibit distinct water uptake behaviour in response to changing humidity conditions. Models that describe the associated phenomena, such as diffusivity and permeability, are needed for fundamental understanding of behaviour of cellulose nanoparticles. Cellulose thin films combined with surface-sensitive methods provide a unique platform for studying the fundamental aspects of water transport in cellulose nanofibril films quantitatively. We use the quartz crystal microbalance with dissipation monitoring (QCM-D) equipped with humidity uptake module to determine the water diffusivity and permeability of water in ultrathin cellulose nanofibril films. Additionally, we evaluate the relevance of the Langmuir-Flory-Huggins bimodal sorption model with regard to cellulose nanofibril films over the whole range of water activity. The model provides new insights into the peculiar behaviour of cellulose nanofibrils with respect to applications, such as membranes, barriers and sensors.