*2.5. Cellulose Nanofiber Characterization*

In order to evaluate the suitability of the different pretreatments and the effect of the residual lignin in the final products, the CNFs/LCNFs obtained were deeply characterized. The nanofibrillation yield, which determines the nanometric fraction of the CNF suspension by the separation of the non-nanometric material by centrifugation, was determined according to the methodology described by Besbes et al. [35]. For this, a 0.1% cellulose nanofiber suspension was centrifuged at 11,000 × *g* for 12 min. The dry weight of the non-nanometric material precipitated during centrifugation compared to the dry weight of the initial suspension was used to inversely determine the nanofibrillation yield. The optical transmittance at 800 nm of the 0.1% cellulose nanofiber suspension was measured using a Lambda 25 UV-Spectrometer. The carboxyl content (CC) was determined using conductimetric titration as described by Besbes et al. [35]. The cationic demand (CD) was determined using a particle charge detector Mütek PCD 05 following the protocol described by Espinosa et al. [23]. The values of cationic demand and carboxyl content are used for the theoretical calculation of the specific surface area of the cellulose nanofibers assuming a simultaneous interaction between the hydroxyl and carboxyl groups of the cellulose nanofiber surface and PolyDADMAC in the monolayer coating [23]. Assuming the cylindrical geometry of the cellulose nanofiber and using the specific surface, it is possible to determine the width of the nanofibers. This method has been evaluated in previous publications, and the theoretical values are very good approximations to the values observed by electron microscopy [23].

### *2.6. Viscosity, Degree of Polymerization and Length*

The intrinsic viscosity ( ty (ɳ<sup>s</sup> −1 <sup>s</sup>) of the cellulose nanofibers was determined according to the ISO 5351:2010 standard. The degree of polymerization is related to the intrinsic viscosity (in mL·g −1 ) using the empirical relationship suggested by Marx-Figini [36]:

$$\text{LPP (<950): DP = (\text{\textquotesingles} \langle 0.42 \rangle)}\tag{1}$$

−1

−1

–

–

−1

−1

$$\text{DP (} > 950) \text{: } \text{DP}^{0.76} = \text{(} \text{ls/2.28)}\tag{2}$$

ɳ The length of the cellulose nanofiber was estimated from the degree of polymerization values using the equation proposed by Shinoda et al. [37]:

$$\text{Length (nm)} = 4.286 \times \text{DP-757} \tag{3}$$

ɳ

ɳ

–

ɳ

−1

−1

−1

–

– The measurements were made in triplicate, and the mean value and standard deviation were calculated.
