## 3 Results and Discussion
### Characterization of Nanocellulose Gels and Films
#### 3.2.7 Contact Angle and Surface Energy of Films  
Contact angle and surface free energy are fundamental parameters that play a crucial role in understanding the interactions between liquids and solid surface. The water contact angle refers to the angle formed between the interface of a liquid and the surface of the film and reveals its hydrophilicity. Surface free energy is a fundamental property that describes the tendency of a surface to interact with other materials, representing the amount of energy required to create a unit surface area on a material. Surface free energy influences crucial properties such as wettability, adhesion and interfacial behavior. Understanding and controlling this characteristic is essential in several applications.  
The results obtained for water contact angle, total energy, dispersive and polar components are presented in Table 3. The hydrophilic character of CNF-based films is confirmed. However, a considerable improvement in the hydrophobicity of CNF films is achieved after  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Properties** & **CNF\({}_{1}\)** & **WC CNF\({}_{1}\)** & **CNF\({}_{2}\)** & **WC CNF\({}_{2}\)** \\ \hline WVTR & & & & \\ (g.m\({}^{-2}\).day\({}^{-1}\)) & 264.0 & 49.2 & 219.2 & 77.8 \\ WVP & & & & \\ (g.Pa\({}^{-1}\).day\({}^{-1}\).m\({}^{-1}\)) & 7.03 \(\times\) 10\({}^{-6}\) & 1.39 \(\times\) 10\({}^{-6}\) & 6.03 \(\times\) 10\({}^{-6}\) & 3.0 \(\times\) 10\({}^{-6}\) \\ \hline \end{tabular}
\end{table}
Table 2: WVTR and WVP for CNF films (results presented as mean \(\pm\) standard deviation).  
wax coating, exhibiting a water contact angle higher than 100\({}^{\circ}\). The wax coating significantly increases the water contact angle, which means a drastic increase in hydrophobicity.  
The values obtained for CNF films are corroborated by previous studies [11; 36; 37; 38], where the contact angle varies between 30\({}^{\circ}\) and 50\({}^{\circ}\) for CNF films without any additives.  
Before coating, the CNF\({}_{2}\) film exhibits a higher polar component in contrast with CNF\({}_{1}\), suggesting a stronger hydrogen bonding capacity and thus a more hydrophilic character. This result can be confirmed by the FTIR spectrum shown in Figure 6, where a high intensity peak can be observed at 3300 cm\({}^{-1}\). Furthermore, this result can also be explained by the type of pulp, since sulfate (kraft) pulp tends to preserve or expose more polar functional groups, such as hydroxyls and carboxyl groups on the nanocellulose surface. In contrast, sulfite pulp may result in fewer of these groups due to differences in the chemical treatment, leading to a lower polar component on surface films.  
After coating, the CNF films exhibit lower surface energy than the uncoated films. About a 40% decrease in total surface energy is achieved. Since the dispersive component remains almost unchanged in all samples, we can state that the reduction in the total surface energy is determined by polar interactions. A reduction of almost 100% in the polar component is observed. Effectively, the wax coating virtually eliminates the polar component, which is consistent with the presence of a hydrophobic non-polar wax layer. Therefore, WC CNF films are more water repellent and consequently more suitable for applications requiring moisture resistance. These results are in agreement with those reported previously for nanocellulose papers with different lignin contents [39].