## 3 Results and Discussion
### Physicochemical Characterization of the RC/IL Films  
Changes in the elastic characteristics of the RC support film as a result of the IL modification was expected, taking into account the results previously obtained using other modifying agents (lipid layer coverage, dendrimer inclusion/coating, silver or lipid nanoparticles inclusion,... [34; 35]); Figure 4 shows the normal tensile stress (F/S) versus strain (AL/Lo) curves obtained for both RC/IL films and the RC support, where significant differences in the elastic characteristics of the films can be observed. Although the three samples exhibited a rather similar value for the elastic limit (e.l\(\sim\)0.03), the curves were clearly different: the strain-stress curve shown by the RC support was similar to that reported by regenerated fibers (viscose or cellulose acetate) and different RC membranes [48; 49], with a first part associated with an elastic behavior plus a second and long plastic region, while the RC/IL films showed a higher slope and shorted elongation at break. Since the plastic behavior of RC films can be an impediment for some applications (compaction under pressure), the mechanical properties of cellulose fibers and polymer films can be improved by using silane compounds or other materials [50; 51]. In this context, it might be of interest to indicate the significant effect of water content on the elastic behavior of the RC-support, associated with its high hydrophilic character [52], as can be observed in the stress-strain curves obtained for the dry and wet samples of the RC film presented in the Supplementary Information (Figure S3); since elastic measurements were performed at atmospheric conditions, the presence of the ILs might cause a reduction in the water content of the RC/IL films. The elastic (or Young) modulus was determined from the slope of the initial linear dependent part, and the following values were determined: Y(RC/BMIMPF\({}_{6}\)) = 3.34 GPa and Y(RC/AliquatCl) = 1.52 GPa, which are in the order of values determined for RC films containing ILs [53]; these results indicate an increase of around 372% and 95%, respectively, with respect to the value determined for the RC-support film. Light transmission and reflexion are non-destructive measurements which provide information of interest on optical characteristics of thin films or layers deposited on a solid matrix [54; 55; 56]. Figure 5 shows the transmittance spectra for the two RC/IL films studied, showing high transmittance values (>91%), but slight differences depending on the IL can also be observed in both the visible (400-800 nm, between the two vertical dashed black lines) and the near-infrared (nir) regions, with the light transmittance % being slightly lower for the RC/BMIMPF\({}_{6}\) film than for the RC/AliquatCl one, as can be observed in Figure 5a,b. The surface coverage of the RC-support by both ILs seemed to reduce the oscillation in the wavelength interval 1600-2000 nm, moving the RC peak at 1927 nm to 1937 nm and shielding the peak at 1975 nm. Figure 4: Normal tensile stress versus strain for the RC/BMIMPF\({}_{6}\) film (\(\bullet\)), the RC/AliquatCl film (\(\blacktriangledown\)) and the RC-support film (\(\square\)). The wavelength dependence of the light reflexion percentage for both RC/IL films and the RC-support is shown in Figure 6, where slight differences in reflexion % in the nir region (14% reduction for the RC/BMIMPF\({}_{6}\) film and 6% for the RC/AliquatCl film with respect to the RC-support at 1400 nm) can be observed, but practically similar reflexion percentage values were obtained in the visible region. Consequently, the inclusion of the ILs does not seem to cause changes in the transmission/reflection/absorption characteristics of the RC-support. Figure 5: Light transmission as a function of wavelength: (**a**) for the whole range studied; (**b**) for the near-infrared region, for the RC/BMIMPF\({}_{6}\) film (red solid line), the RC/AliquatCl film (blue solid line) and the RC-support (black dashed line).