# Production of Microfibrilated Cellulose from Fast-Growing
## 3 Results and Discussion
### Film Properties
#### 3.3.2 Wetting Properties  
Water vapor sorption isotherms were collected to characterize the wetting behavior of the three different MFC films (Figure 3). The isotherms are similar for all samples showing a maximum water sorption of 26-28 wt% at 95% relative humidity. This value is comparable to other MFC obtained by microfluidification [57]. At the low relative humidity range up to 30%, it appears that P-MFC adsorbs slightly more water than the other two MFC films. This range is dominated by the formation of an initial water mono- and multilayer on the sorbent surface and therefore, is typically taken as a measure of a material's hydrophilicity [58].  
More information on the wetting behavior can be extracted from modeling the water sorption isotherms. For cellulosic materials, these curves are commonly fitted with [PERSON]'s model that describes the sorption process by an initial Langmuir-type adsorption, an intermediate adsorption following Henry's law and water clustering at high water activity [59]. This is due to the compositional and microstructural heterogeneity of natural fibers, which require a multi sorption mode model [60]. Thus, in [PERSON]'s model, the quantity of adsorbed water (Q) is calculated as follows:  
\[\text{Q}= \frac{\text{A}_{\text{L}}\text{\text{\text{\text{\text{\text{L}}}}}} \text{\text{\text{\text{\text{L}}}}}\text{\text{\text{\text{\text{A}}}}}^{1- \text{c}}_{W}}{1+\text{\text{\text{\text{L}}}}\text{\text{\text{\text{\text{A}}}}} \text{\text{\text{\text{W}}}}^{1-\text{c}}}+\text{\text{\text{K}}}_{\text{H}} \text{\text{\text{\text{\text{\text{A}}}}}}\text{\text{\text{\text{\text{W}}}}} \text{\text{\text{\text{\text{\text{+K}}}}}}\text{\text{\text{\text{\text{C}}}}} \text{\text{\text{\text{\text{-A}}}}}\text{\text{\text{\text{\text{W}}}}}}, \tag{6}\]  
where Q is the quantity of adsorbed water, \(\text{\text{\text{\text{a}}}}_{\text{wW}}\) is the water activity, \(\text{A}_{\text{L}}\) is the Langmuir capacity constant, \(\text{\text{\text{\text{b}}}}_{\text{L}}\) is the Langmuir affinity constant, c is a fitting coefficient, \(\text{\text{\text{K}}}_{\text{H}}\) is [PERSON]'s solubility coefficient, \(\text{\text{K}}_{\text{C}}\) is the equilibrium constant for the cluster formation and n is the mean number of water molecules per cluster.  
The fitting Parameters in Table 5 corroborate that P-MFC is slightly more hydrophilic than the other two MFC films, as suggested by the higher Langmuir capacity constant \(\text{A}_{\text{L}}\) and the mean cluster number n. On the other hand, O-MFC has the highest Henry's law constant, \(\text{\text{K}}_{\text{H}}\), which is ascribed to the solubility of water within the cellulose fiber matrix interstices [60]. Hence, it may be speculated that O-MFC has a more open, accessible fiber matrix structure than the other MFC samples. Furthermore, the lower crystallinity index of this sample indicates a higher percentage of amorphous domains in which water can be adsorbed [45].  
\begin{table}
\begin{tabular}{c c c c} \hline  & **O-MFC** & **E-MFC** & **P-MFC** \\ \hline \(\text{A}_{\text{L}}\) & 14 & 15 & 20 \\ \(\text{K}_{\text{H}}\) & 8.6 & 0.9 & 0.2 \\ n & 8.3 & 9.3 & 9.6 \\ R2 & 0.9976 & 0.9979 & 0.9981 \\ \hline \end{tabular} O-MFC: olive microfibirllated cellulose; P-MFC: poplar microfibirllated cellulose; E-MFC: eucalypt microfibirllated cellulose.  
\end{table}
Table 5: Fitting Parameters of the water isotherms on MFC films according to [PERSON]’s model.  
Figure 3: Water sorption isotherms of P-MFC, O-MFC and E-MFC films including the fitting curves according to [PERSON]’s model.