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distinct degrees of oxidation, were washed with water and characterized for their aldehyde content, as described elsewhere [39]. The DACs were dispersed in acidified water and a certain molar ratio of GT/aldehyde was added. The cationic celluloses were vacuum filtered and thoroughly washed with a mixture of isopropanol/water (9:1 v/v ),with the exception of sample GT1.15_P. This sample, due to its high degree of substitution and consequent high solubility, was washed in successive centrifugation cycles [37]. The produced cationic celluloses were labeled according to the cationizing agent used, with the acronyms CH or GT, followed by the degree of substitution. Some of the cationic celluloses were further processed to produce cationic MNFCs. For that purpose, they were subjected to a pass at 500 bar and a second pass at 700 bar in a high-pressure homogenizer (HPH; GEA Niro Soavi, model: Panther 115 NS3006L). These samples were also labeled with the letter F (F stands for fibrillated). More details about the preparation of these samples are available elsewhere [23]. The samples that resulted in fully soluble materials, due to their high DS or the combination of cationization with HPH, were labeled with the letter P (P stands for polyelectrolyte). 2.3. Cationic Cellulose Characterization The CCs were characterized by elemental analysis, potentiometric titration, ZP mea- surements, FTIR-ATR and optical microscopy [23]. Elemental analysis was performed in an EA 1108 CHNS-O analyzer from Fisons (Italy) to quantify the nitrogen content of the samples, which was used to calculate the corresponding DS of the quaternary ammonium groups. The CD was assessed using potentiometric titration. In a typical titration, a cationic cellulose suspension is adjusted to pH 11 with a NaOH aqueous solution and then titrated with 0.01 M HCl until the inflection point of the pH versus HCl volume curve is reached. The ZP of the diluted CCs suspensions (ca. 0.1 wt %) was determined via ELS in a Zetasizer NanoZS device from Malvern Instruments. The FTIR-ATR spectra were obtained on a Bruker Tensor 27 spectrometer, using 128 scans and a resolution of 4 cm − 1 , in the range of 650–4000 cm − 1 . Polarized light optical microscopy images were acquired using an Olympus BH-2 KPA microscope from the Olympus Optical Co., Ltd., equipped with an Olympus ColorView III high-resolution CCD color camera. The cationic MNFCs were further characterized in terms of the yield of fibrillation (YF) and soluble fraction (SF). The YF was determined via centrifugation of 40 mL of CCs aqueous suspensions (0.2 wt %) at 9000 rpm for 30 min, using a Universal 32 Hettich centrifuge. The weight percentage of CC remaining in the supernatant was considered as the yield of fibrillation. The SF was determined via the vacuum filtration of 3 mL of the original cationic MNFCs suspension (ca. 1 wt %) through a cellulose acetate membrane filter with a 0.2 μ m pore size. The weight percentage of CC on the filtrate was considered as the soluble fraction. More detailed information regarding the characterization of these cationic MNFCs is available elsewhere [23]. The weight average molecular weight (avgMw) of the cellulose polyelectrolytes was determined by size exclusion chromatography (SEC) in an Agilent 1260 Infinity II High- Temperature GPC System that was equipped with two PL aquagel-OH Mixed-H 8 μ m (300 × 7.5 mm) columns and a PL aquagel-OH 8 μ m(50 × 7.5 mm) guard column [23]. 2.4. LDS Flocculation Tests To access the performance of the produced cationic celluloses to flocculate PCC, the evolution of the size distribution of the PCC flocs was monitored in a Mastersizer 2000 device (from Malvern Instruments) equipped with the Hydro 2000 module, in a similar way to that proposed by Rasteiro et al. [31,32,35] for the screening of synthetic polyelectrolytes. Previously to the measurements, a 1 wt % aqueous suspension of PCC was subjected to magnetic stirring for 30 min and ultrasonicated for 15 min at 50 kHz to help disaggregate the PCC particles. The suspension was then maintained under magnetic stirring. For the
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