## 2 Materials and Methods  
Two ILs, tricaprylmethylammonium chloride (C\({}_{25}\)H\({}_{55}\)N\({}^{+}\)Cl\({}^{-}\) or AliquatCl) and 1-butyl-3-metylimidazolium hexafluorophosphate (C\({}_{8}\)H\({}_{15}\)N\({}_{2}\)\({}^{+}\)PF\({}_{6}\)\({}^{-}\) or BMIMPF\({}_{6}\)), both from Sigma-Aldrich (St. Louis, MO, USA), were selected for cellulosic film modification and were used without further purification. AliquatCl is a quaternary ammonium salt (a 2:1 mixture of methyl trioctyl- and methyl trioctylammonium chloride), while BMIMPF\({}_{6}\) is an imidazole-based IL exhibiting an aromatic ring and shorted aliphatic chain, as can be seen in the Supplementary Information (Figure S1) where the structure of both ILs and cellulose is indicated. The molecular mass, density and viscosity of the ILs were: 284.2 g/mol, 1380 kg/m\({}^{3}\) and 267 mPa.s for BMIMPF\({}_{6}\), but 432 g/mol, 886 kg/m\({}^{3}\) and 1500 mPa.s (at 25 \({}^{\circ}\)C) for AliquatCl [39]. A highly swelling and transparent film of regenerated cellulose (RC) from Cellophane Espanola S.A. (Burgos, Spain) was selected for ILs modification via the dip-coating process, and samples of the RC film were submerged for 1 h in an open glass flask containing the corresponding IL. These samples will hereafter be called the RC /AliquatCl and RC/BMIMPF\({}_{6}\) films. A chemical surface characterization of the films was performed by a Physical Electronics spectrometer (PHI 5700) with X-ray Mg K\({}_{\alpha}\) radiation (300 W, 15 kV, 1253.6 eV) as the excitation source. High-resolution spectra were recorded by a concentric hemispherical analyzer operating in the constant pass energy mode at 29.35 eV, using a 720 \(\upmu\)m diameter analysis area. Although the optimum equipment take-off angle was 45\({}^{\circ}\), angle resolved XPS measurements (ARXPS) were performed for an in-depth surface analysis (without sample damage) by using five different take-off angles: \(\phi\) = 15\({}^{\circ}\), 30\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\) and 75\({}^{\circ}\), taking into account the relation between the escape depth (d) and the photoelectron mean free path (\(\lambda\)) [40,41], d \(\leq\) 3 \(\uplambda\) sin \(\phi\), which enabled us to obtain chemical information from depths ranging between 2.5 nm (\(\phi\) = 15\({}^{\circ}\)) and 9.3 nm (\(\phi\) = 75\({}^{\circ}\)), assuming the mean free path (\(\lambda\)) in polymers for the C 1\(s\) excited photoelectrons was 3.2 nm [42]. Samples were kept overnight at a high vacuum (preparation chamber) and were then transferred to the analysis chamber for testing, and each spectral region was scanned several times until a good signal-to-noise ratio was observed. Binding energies were determined with respect to the position of the adventitious C 1\(s\) peak at 285.0 eV (accurate \(\pm\)0.1 eV), and the residual pressure in the analysis chamber during the data acquisition was maintained below 5 \(\times\) 10\({}^{-7}\) Pa. The PHI ACCESS ESCA-V6.0 F software package was used for acquisition and data analysis. To accurately determine the binding energy (BE) of the different element core levels, recorded spectra were fitted using Gauss-Lorentz curves as was already described in detail [43]. Atomic concentration percentages (A.C. %) of the characteristic elements found on the surface of the analyzed samples were determined taking into account the corresponding area sensitivity factor for each measured spectral region [44]. Films elastic measurements were performed at laboratory atmospheric conditions with a force digital gauge (Mark-T, ES20 model) connected to a computer, with a maximum tension of 100 N, length accuracy of \(\pm\)0.01 mm and a strength rate of 0.167 mm/s. Light transmittance and reflection measurements were carried out with a Varian Cary 500 spectrophotometer (Agilent Technologies, Santa Clara, CA, USA) provided with an integrating sphere of Spectralon for wavelengths ranging between 200 nm and 2000 nm, which allowed us to obtain information on visible and near-infrared (nir) regions.