## 3 Ice-Templating of Nanocellulose Suspensions
### Effect of the Suspension Formulation  
#### 4.2.1 Suspending Fluid  
The characteristics and properties of the fluid that is used to disperse the colloids are crucial in the ice-templating process [64; 65; 66; 67]. The fluid properties can affect (i) the crystal morphology, (ii) the state of dispersion of the suspended particles, and (iii) the mechanisms of particle segregation and redistribution occurring during the solidification phase. If these phenomena and their effects on the final microstructures have been investigated for many ice-templating systems, much less is known concerning nanocellulose systems. [PERSON] et al. [139] produced thin nanoporous membranes (Figure 12a) from CNF dispersions in ter-butyl alcohol (TBA). Aqueous CNF suspensions were completely solvent exchanged to TBA through ethanol. The CNF membranes that were obtained after freezing and sublimation showed fine porous microstructures with nanometric pore sizes (average pore size of about 24 nm) and also the presence of domains with aggregated CNFs. [PERSON] et al. [140] also produced thin nanoporous filter membranes using TBA. However, in that study, CNFs were dispersed in various water/TBA mixtures at TBA concentrations up to 60 wt% before freezing at T \(\approx-20\)\({}^{\circ}\)C and sublimation. Interestingly, the authors observed that the materials prepared without adding TBA exhibited columnar structures with micrometer-sized large pores, whereas those obtained from CNF dispersions in water/TBA mixtures at TBA concentrations ranging from 20 to 50 wt% displayed fine porous structures, in the form of CNF networks, with nanometer-sized fine pores (Figure 12b). In contrast, the CNF filters prepared from CNF/water/TBA dispersions with 60 wt% TBA showed heterogeneous microstructures with large pores and film-like structures together with fine porous structures. The role of TBA in the formation  
Figure 11: SEM micrographs of CNF foams obtained after freezing at various temperatures and then sublimation. The images inset are SEM micrographs of CNF foams obtained at lower (**a**) or higher (**b–d**) magnifications (Adapted from [PERSON] et al. [56] with permission from Elsevier). of fine porous structures after ice-templating can be related to the two following scenarios. Firstly, the presence of TBA molecules in water changes the morphology and growth kinetics of ice-crystals. A finer analysis of the phase diagram of the binary mixture of water/TBA reveals that two eutectic compositions exists, respectively, on the water-rich and TBA-rich sides of the phase diagram [141]. In addition, various TBA hydrates can also be formed, depending on the TBA concentration and freezing temperature [141]. Hence, a wide diversity of crystal morphologies can be observed during the solidification of a binary mixture of water/TBA [142]. Secondly, TBA molecules can attach to the surface of CNFs through hydrogen-bonding interactions which leads to hydrophobic surfaces of the fibrils by introducing tert-butyl groups [143]. This can considerably disturb the redistribution of solid particles by the solidification front. Indeed, the chemical steric hindrance effect of the tert-butyl groups can restrict the self-association behaviour of CNFs and the formation of continuous cell walls as well. It is also worth noting that the addition of TBA can disturb the colloidal stability of nanocellulose systems. This last point is presumably at the origin of the heterogeneous porous microstructures observed by [PERSON] et al. [140] for filters prepared with 60 wt% TBA (Figure 12b). The effect of the colloidal stability on the microstructure of ice-templated materials is discussed further in the next subsection. Figure 12: **(a)** Photograph of a porous CNF film of high specific surface area obtained after freezing a CNF dispersion in ter-butyl-alcohol (TBA) and then sublimation. (Adapted with permission from [PERSON] et al. [139]. Copyright 2011 American Chemical Society). **(b)** SEM micrographs of CNF filters obtained from various CNF/water/TBA dispersions with 0–60 wt% TBA.