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Sakura Morimitsu, Ayato Nishimura, Kenzo Deguchi, [Yuuki Mogami](https://orcid.org/0000-0002-9807-3165), [Shinobu Ohki](https://orcid.org/0000-0002-7357-3833), [Kenjiro Hashi](https://orcid.org/0000-0002-0320-4768), [Atsushi Goto](https://orcid.org/0000-0002-9472-4098), Kazuhiko Yamada

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This version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1007/s10570-025-06609-5.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Sulfated cellulose pulp with ultra-high water retention characteristics](https://mdr.nims.go.jp/datasets/c1008004-261e-4a9d-8930-d98d63c5094b)

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Microsoft Word - Manuscript5_final1 Sulfated cellulose pulp with ultra-high water 1 retention characteristics 2 Sakura Morimitsu1,2,*, Ayato Nishimura1, Kenzo Deguchi3, Yuuki Mogami3, 3 Shinobu Ohki3, Kenjiro Hashi3, Atsushi Goto3, Kazuhiko Yamada4 4 1Research and development Innovation Headquarters, Marusumi Paper Co., Ltd., 5 826 Kawanoe-cho, Shikokuchuo-shi, Ehime 799-0196, Japan 6 2Graduate School of Kuroshio Science, Kochi University, Monobe, Kochi, Japan 7 3National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003, 8 Japan. 9 4Interdisciplinary Science Unit, Multidisciplinary Sciences Cluster, Research and 10 Education Faculty, Kochi University, Nankoku 783-8505, Japan 11 Corresponding author: b24d6c05@s.kochi-u.ac.jp 12 ORCID: Sakura Morimitsu (https://orcid.org/0009-0002-5922-7309) 13  14 Acknowledgments 15 This work was supported by the "Advanced Research Infrastructure for Materials and 16 Nanotechnology in Japan (ARIM)" of the Ministry of Education, Culture, Sports, Science, and 17 Technology (MEXT).  18 2 Abstract: A sulfated cellulose pulp (SCP) exhibiting ultra-high water retention 19 properties has been developed, and its water retention mechanisms have been 20 analyzed based on molecular structures determined by solid-state 13C nuclear 21 magnetic resonance (NMR). Cellulose pulp (CP) from wood is sulfated using a 22 sulfamic acid and urea reaction system with varying concentrations of sulfamic 23 acid. SCP has 1.04–1.93 mmol/g of sulfate groups and exhibits a high water 24 retention value (WRV) of 16000%, which is approximately 100 times greater than 25 that of CP. After the sulfate reaction, new peaks at 69 ppm are observed in the 1H-26 13C cross-polarization/magic angle spinning (CPMAS) and dipolar 27 decoupling/MAS (DDMAS) 13C NMR spectra, which are assigned to the sulfated 28 C6. Quantitative evaluation of area ratios of the corresponding peaks in the 13C 29 DDMAS NMR spectra indicates that approximately 18% of C6 in CP is maximally 30 converted to sulfated C6 when R = 4.19. The present work demonstrates that the 31 WRV is related to the amount of sulfated C6, which guides the design of future 32 absorbent materials based on CP. 33  34 Keywords: Cellulose, Sulfated cellulose pulp, Water retention, Solid-state 13C 35 NMR, Sulfate group  36 3 Introduction 37 Sulfated cellulose pulp (SCP) has attracted considerable attention in recent years 38 because it affords cellulose nanofibers, which are fibers isolated to the smallest unit 39 that constitutes a cellulose pulp (CP). Several synthetic methods for SCPs have been 40 proposed, such as sulfuric acid hydrolysis (Rånby et al. 1949), the 41 HSO3Cl/(CH3CO)2O system (Zhang et al. 2011), the sulfamic acid/urea/N,N-42 dimethylformamide system (Huang and Zhang 2010), the HSO3Cl/N,N-43 dimethylformamide system (Zhao MW et al. 2007), and the deep eutectic solvent 44 system (DES, CP/sulfamic acid/urea system without water) (Sirviö et al. 2019). In 45 a similar manner to CP modified by carboxyl (Saito et al. 2007) and phosphate 46 groups (Ghanadpour et al. 2015; Noguchi et al. 2017; Zhao M et al. 2021), SCP is 47 also defibrillated via mechanical treatment in a high-pressure homogenizer, 48 affording in nanofibers. Nanofibers have been applied to functional materials such 49 as O2 gas barrier films (Fukuzumi et al. 2009), rheology control agents (Wang et al. 50 2019), and metal ion adsorbents (Liu et al. 2015; Liu et al. 2016; Riva et al. 2024; 51 Lehtonen et al. 2020). Thus, cellulose nanofibers possess various characteristics 52 that allow them to be used in various industries with expanding applications. 53 Superabsorbent polymers (SAPs), known for their high absorbency, are a widely 54 recognized functional material. They are commonly used in products such as 55 disposable nappies (Zhang et al. 2021), concrete admixtures (He et al. 2019), and 56 soil conditioners (Hou et al. 2018). These polymers can absorb more than 1,000 57 times their own weight in water (Meshram et al. 2020). However, the components 58 of SAPs, such as polypropylene, polyester, and polyacrylic acid, are not 59 biodegradable, posing significant environmental and health issues. Therefore, there 60 has been a growing demand for environmentally friendly and highly absorbent 61 materials. 62 Recently, our group has presented the synthesis of SCP using a CP/sulfamic 63 acid/urea system, affording a water retention value (WRV) of approximately 64 3000% (Nishimura and Otsuka 2024), with enhancing water dispersion stability and 65 transparency in aqueous dispersions (submitted). Furthermore, the mechanism by 66 which SCP appears transparent in water is attributed to the swollen fiber structure 67 of the SCP fibers (Nishimura and Otsuka 2024). The molecular structure of SCP 68 has been analyzed using 13C solid-state nuclear magnetic resonance (NMR), which 69 shows that the sulfation reaction occurs at the hydroxy groups in the C2, C3, and 70 4 C6 positions. However, the relationship between its molecular structure, fiber 71 swelling, and WRV is not completely clear. While SCP has the potential to solve 72 the environmental issues of conventional petroleum-derived SAPs, further 73 improvements in its water retention capacity are required. Therefore, understanding 74 the molecular structure of SCP, particularly the influence of the distribution of 75 sulfate groups on WRV, is important for enhancing its water retention capacity. 76 The sulfate group of SCP was previously investigated using the 1H-13C cross-77 polarization/magic angle spinning (CPMAS) method (Zhang et al. 2011), which 78 can provide useful structural information. CPMAS is a technique that enhances 79 sensitivity by transferring the magnetization of 1H, which has a natural abundance 80 of 99%, to 13C, which has a natural abundance of 1%. However, it is well known 81 that its quantitative reliability is low because the signal intensities are affected by 82 the distance to the 1H and molecular mobility of the measurement. On the other 83 hand, dipolar decoupling/magic angle spinning (DDMAS) is highly quantifiable 84 because 13C can be measured directly without passing through 1H magnetization. 85 One of the disadvantages of DDMAS is that it takes a long analysis time because 86 five times the relaxation time T1 is required for the recycle time to make the 87 quantification reliable (Snape et al. 1989). To the best of our knowledge, no studies 88 have reported 13C DDMAS experiments on SCP. 89 In this study, a novel SCP exhibiting an ultra-high water retention capacity was 90 synthesized using the CP/sulfamate/urea reaction system. The molecular structure 91 of SCP was examined via CPMAS, while DDMAS was employed for the 92 quantification of the sulfate groups, elucidating the relationship between the water 93 retention capacity and the molecular structure of SCP. Our findings provide 94 essential information for the development of environmentally friendly absorbent 95 materials that can be used in sustainable products such as agricultural water 96 retention materials and biodegradable hygiene products. 97  98 Materials and methods 99 Materials 100 The cellulose pulp (CP) used in this study was never-dried softwood-bleached kraft 101 pulp (Marusumi Paper Co., Ltd., Japan). The concentration of the CP slurry was 102 5 adjusted to 25 wt%, and the water used for the CP slurry was replaced from 103 industrial to pure water. Sulfamic acid (H2NSO3H), urea ((H2N)2C=O), sodium 104 hydroxide (NaOH), and sodium hydrogen carbonate (NaHCO3) were purchased 105 from FUJIFILM Wako Pure Chemical Co. All the materials were used without 106 further purification. The ion-exchange resin (Amberlite HPR1024H) was purchased 107 from Organo Co., Japan. Pure water (distilled by Marusumi Paper Co., Ltd., Japan, 108 conductivity < 0.1 mS/m, pH 5–8) was used in all the experiments. 109  110 Sulfation of cellulose pulp 111 The sulfate reaction solution was prepared by adding and stirring sulfamic acid 112 (1.25–5.00 g, 0.013–0.051 mol) and urea (2.50 g, 0.042 mol) with pure water (12 113 g) at room temperature (296 K). The 25 wt% CP (8 g, BD2 g) was immersed in the 114 sulfate reaction solution for 10 min, spread thinly and uniformly on an acrylic plate, 115 and dried at 358 K for 3 h using a constant-temperature dryer (DKN602, Yamato 116 Scientific Co. Japan). The dried CP reacted when heated to 413 K for 30 min. The 117 reactants were neutralized with NaHCO3 and washed with pure water on a 330-118 mesh sieve (Sampo Co., Japan). The sulfated cellulose pulp slurry was adjusted to 119 a concentration of 1.0 wt% with pure water and then refrigerated at 277 K. The 120 reaction conditions were determined at 5 different molar ratios (R), where R was 121 determined by dividing the number of moles of sulfamic acid by the number of 122 moles of the glucose residues units in CP. The moles of glucose residues units were 123 calculated from the dry weight of the pulp/162. 124  125 Water retention value 126 The water retention value (WRV) was measured using the TAPPI method and 10 g 127 of the 0.5 wt% SCP slurry. The SCP slurry was centrifuged at 3000 g for 15 min 128 and the WRV was calculated using the following Equation (1). 129 WRV(%) = × 100                                                           (1) 130 where Ww is the mass after centrifugation and Wd is the mass after drying the 131 sample at 358 K for 24 h. 132  133 6  134 X-ray diffraction measurements  135 The X-ray diffraction (XRD) patterns of samples were measured by a X-ray 136 generator (Empyrean, Spectris Co., Ltd. United Kingdom) at a power level of 45 137 kV and 40 mA with Cu-Kα radiation (λ = 1.54 Å) in the range 2θ = 10–30°. The 138 crystallinity index (CI) of CP and SCP was calculated according to the following 139 Equation (2): (Segal et al. 1959) 140 CI(%) =   × 100%                                                         (2) 141 where I200 is the overall intensity of the peak at 2θ about 22.4°, and Iam represents 142 the intensity of the minimum at 2θ about 18.5°  143  144 Scanning electron microscopy  145 The morphology observation of CP and SCP was observed by field emission-146 scanning electron microscopy (FE-SEM, JSM-6701F, JEOL Ltd., Japan) with an 147 acceleration voltage of 3 kV. The samples were freeze-dried and coated with 148 platinum. 149  150 Solid-state 13C NMR  151 Freeze-dried SCP was powdered using a blender (WB-1; Osaka Chemical Co., 152 Osaka, Japan). 1H and 13C NMR experiments were performed at 500.194 and 153 125.774 MHz, respectively, on an 11.7 T spectrometer (JEOL ECZ 500, JEOL Ltd., 154 Japan) using a 4 mm magic-angle spinning (MAS) probe at room temperature. 155 Potassium bromide was used to adjust the magic angle, and adamantane was used 156 as the reference. Standard ramped cross-polarization (CP), dipolar decoupling, and 157 Torchia sequences (Torchia 1978) were used with a MAS frequency of 15 kHz and 158 high-power irradiation to achieve heteronuclear decoupling during each detection 159 period. The broadening factor of 80Hz was applied to all the NMR spectra. For 13C 160 CPMAS, the following parameters were used: /2 for 1H, 3.3 s; mixing time, 5 161 ms; spectral width, 400 ppm; data points, 1024; recycle delay time (RD), 5 s; 162 7 accumulations, 1000–1200 scans. For 13C DDMAS, the following parameters were 163 used: /2 for 13C, 3.3 s; RD, 1200 s; accumulations, 260–300 scans; the other 164 parameters were the same as those of 13C CPMAS. For Torchia experiments of T1 165 measurements, the following parameters were used: RD, 4 s; accumulations, 700 166 scans; interval time, 1, 1.39, 1.95, 2.71, 3.79, 5.25, 7.37, 10.28, 14.34, 20.0, 50.0, 167 80.0, 125.0, and 200 s; the other parameters were the same as those of the 13C 168 CP/DDMAS experiments. All NMR spectra were processed and analyzed using 169 DELTA software (JEOL USA, Inc.). 170  171 Elemental analysis of sulfur contents  172 The amount of sulfate groups in SCP was determined by a CHNS/O elemental 173 analyzer (Flash EA1112, Thermo Finnigan, USA). The samples were used freeze-174 dried SCP (10 mg). The amount of sulfate groups in the samples (mmol/g) was 175 calculated from the sulfur content (%) in the sample and sulfur atomic mass. The 176 degree of substitution (DS) was calculated using the following Equation (3): (Sirviö 177 et al. 2019). 178 DS =×( × )                                                              (3) 179 where S is the sulfur content (%). 180  181 Fourier-transform infrared spectroscopy  182 The Fourier-transform infrared (FTIR) spectra of CP and SCP were recorded on a 183 Fourier transform infrared spectrometer (IRTracer-100, Shimadzu Corp., Japan) in 184 transmission mode, with a resolution of 4 cm−1. 185  186 Results and discussion  187 Figure 1 shows the relationship between the R value and the degree of substitution 188 (DS). DS increased as the R value increased. The sulfated cellulose pulp (SCP) with 189 a degree of substitution ranging from 0.20 to 0.40 was obtained by adjusting the 190 amount of sulfamic acid used in the reaction with cellulose pulp, showing the 191 significant role of sulfamic acid in this modification process. 192 8  193 Fig. 1 Relationship between the molar ratio (R) and the degree of substitution (DS). R is the molar 194 of sulfamic acid divided by the molar of cellulose pulp (CP) 195  196 Figure 2 shows the WRV results of SCP for different R values. WRV increased 197 slightly as the R value increased from 0 to 1.05, and increased almost linearly as 198 the R value increased above 1.05. The maximum WRV reached approximately 199 16000% when the R value was 4.19, which is approximately 5 times greater than 200 the value reported by Nishimura and Otsuka for SCP and approximately 100 times 201 greater than that of CP. Furthermore, previous studies have shown that the WRV of 202 TEMPO-oxidized CP is approximately 400% at about 1.5 mmol/g of carboxyl 203 groups (Saito et al. 2007) and phosphorylated CP has a WRV of approximately 204 900% at about 1.9 mmol/g of phosphorus groups (Hou et al. 2022). Such a result 205 suggests that the developed SCP in this study has a higher WRV than other 206 9 chemically modified cellulose pulps with a similar amount of functional groups.207  208 Fig. 2 Water retention value (WRV) of sulfated cellulose pulp (SCP) with different R values 209  210 The crystallinity of SCP was investigated by XRD, and the fiber morphology was 211 examined using FE-SEM to elucidate the cause of this hydration phenomenon. 212 Figure 3(a) shows the XRD pattern of CP and SCP, denoted as i and ii, respectively. 213 After the sulfation of CP, the peak corresponding to the (200) plane decreased 214 slightly, and the line shape broadened slightly. However, the typical cellulose I 215 peaks 1-10, 110, and 200 were observed at 2θ angles of 14.8°, 16.6°, and 22.6° 216 (with the 1-10 and 110 peaks overlapping), respectively, indicating that the crystal 217 structure was almost maintained (French 2014). The CI of CP and SCP was 218 estimated using the Segal equation, based on unprocessed raw XRD profiles. 219 Although the absolute values may be affected by the baseline contribution, the CI 220 of SCP decreased slightly from approximately 85% for CP to approximately 73%, 221 indicating a relative trend of reduced crystallinity following sulfation. No baseline 222 correction or smoothing was applied to the XRD profiles used for this calculation. 223 FE-SEM was used to investigate the micromorphology of SCP to understand this 224 decrease in crystallinity. Figure 3(b) shows the FE-SEM images of CP and SCP 225 with R = 4.19, denoted as i and ii, respectively. The widths of both fibers are 226 approximately the same, at around 100 µm. In contrast, SCP exhibited different 227 10 surface morphologies than CP, whose fiber surface was ragged. These morphology 228 results clarified that despite the intense reaction with strong acid and heat, SCP 229 maintained almost the same fiber morphology as CP before the reaction, and the 230 fiber surface was rough. It is considered that the increase in the amorphous region 231 on the fiber surface made it easier for water to access in fiber. In addition to these 232 results of surface observations, SCP was assessed at the molecular level to clarify 233 the mechanism of WRV increase further. 234  235 11 Fig. 3 (a) X-ray diffraction patterns of CP and SCP with R values of 4.19, (b) SEM images of CP 236 and SCP with R values of 4.19. Labels i and ii correspond to CP and SCP, respectively 237  238 The molecular structure of SCP was investigated by solid-state 13C NMR 239 spectroscopy to elucidate the reason for the significant increase in WRV. Figure 4(a) 240 shows the further 13C CPMAS NMR spectrum of CP with the spectral assignment 241 based on the literature (Atalla and VanderHart 1999; El Hariri and El Nokab 2022; 242 Foston 2014; Solum 1996). C6cry and C6am denote the crystalline and non-243 crystalline C6 signals, respectively. The C6am peak was observed at 62 ppm, the 244 C6cry peak was at 65 ppm, the C2, C3, and C5 carbon peaks were located between 245 70 and 78 ppm, the C4am peak was at 84 ppm, the C4cry carbon peak was at 89 ppm, 246 and the C1 peak was at 105 ppm. Figure 4(b) shows the 13C CPMAS NMR spectrum 247 of SCP at R = 4.19. A new peak was observed in the spectrum of SCP at 69 ppm, 248 which was attributed to the sulfated C6 carbon (Zhang et al. 2011). The intensities 249 of the C6am peak changed, and a new peak appeared, suggesting that C6am was 250 involved in the reaction. In addition, the intensities related to C2, C3, and C5 were 251 also changed. However, these peaks overlapped and it was not easy to distinguish 252 between them. Therefore, this study focused on the peak associated with C6 which 253 12 is clearly distinguishable.254  255 Fig. 4 13C CPMAS NMR spectra of (a) CP and (b) SCP with an R value of 4.19 256  257 As was mentioned, the DDMAS method is valid for quantitative measurements of 258 the above structural changes if RD is set to more than five times that of T1. The 259 longest T1 in CP, which is believed to be the same as that in SCP, was 163 s for 260 C4cry. Under these experimental conditions, RD was set at 1200 s so that the 261 intensity of each signal was guaranteed to be quantitative. Figure 5 shows the solid-262 13 state 13C DDMAS NMR spectra of the SCP with R values ranging from 0 to 4.19. 263 Although the trend was not strictly linear, the peak intensity derived from the sulfate 264 groups of SCP at 69 ppm appeared to increase as the R value increased from 0 to 265 4.19. Furthermore, the C6am peak at 62 ppm decreased as R increased from 1.05 to 266 4.19, whereas the C6cry peak at 65 ppm did not exhibit any significant change. 267 Previous studies reported that CP consists of alternating crystalline and non-268 crystalline regions (Wickholm et al. 1998). The crystalline region of CP was hardly 269 accessible in the sulfated reaction because of its regularly lined and high-density 270 composition. Moreover, the non-crystalline sections of CP are irregular and 271 flexible, which enhances the reactivity of the chemicals (Yu and Wu 2010). The 272 hydroxy groups of C6 exposed on the surface of microfibrils in the amorphous 273 regions may be sulfated first. 274 This interpretation is further supported by NMR analysis. Horii et al. (1983) first 275 reported the relationship between solid-state 13C NMR chemical shifts and the 276 conformation of the hydroxymethyl group at C6, and Yoneda et al. (2008) 277 demonstrated that the gt conformation is the most abundant of the three 278 conformations (tg, gg and gt) in a simpler, more readily crystallisable cellulose-like 279 model compound. According to Horii et al., the tg conformation is characteristic of 280 the crystalline regions of cellulose I. In contrast, the gg and gt conformations are 281 typically observed at 60–62.6 ppm and 62.5–64.5 ppm, respectively, and are 282 predominant in amorphous regions. Furthermore, the gt and gg conformations are 283 also predominant in some disordered surface chains of native cellulose I, whereas 284 the tg conformation is largely absent (Viëtor et al., 2002). Therefore, the decrease 285 in the signal at 60–64.5 ppm and the appearance of a new peak at 69 ppm suggest 286 that hydroxy groups originally in the gt and gg conformations, which are typically 287 present in amorphous and some cellulose surface regions, were selectively sulfated. 288 The 69 ppm signal reflects the presence of newly introduced sulfate groups. This is 289 considered to result from both conformational changes and alterations in the 290 14 electronic environment introduced through sulfation.291  292 Fig. 5 13C DDMAS NMR spectra of SCP with R values of (a) 0, (b) 1.05, (c) 2.09, (d) 3.14, and (e) 293 4.19 294  295 Quantitative measurements of SCP focused on the peaks between 57 and 70 ppm 296 regarding C6. Figure 6 shows the deconvolution of sulfated C6 in the 13C DDMAS 297 NMR spectra of SCP with (a) R = 1.05 and (b) R = 4.19. Deconvolution of the 298 signals was performed using Voigt functions. The percentage of C6 converted to 299 15 sulfated C6 in CP was calculated by comparing the intensities of each peak. Figure 300 7 shows the correlation between the amount of sulfate groups at C6 in SCP and the 301 R values. The amount of sulfate groups increased as the R value increased from 302 1.05 to 2.09 and continued to increase slightly as the R value increased from 2.09 303 to 4.19. Sulfated C6 was almost absent when R = 1.05. The maximum value of 304 sulfated C6 was approximately 18% when R = 4.19. The results shown in Figures 305 2 and 7 indicate that WRV increased as the amount of sulfated C6 increased.    306 Some hydroxy groups extend outward onto the surface of microfibrils. Regarding 307 CP, some microfibrils are closely packed together via hydrogen bonds between the 308 exposed hydroxy groups. However, replacing the hydroxy groups with polar sulfate 309 groups results in electron repulsion and steric hindrance. Such findings suggest that 310 WRV increased because the space between the microfibrils expanded by sulfation 311 reaction, creating space for water to enter. Additionally, the hydroxy groups of C6 312 are located farther from the surface of microfibrils than those of C2 and C3, 313 16 suggesting that the quantity of sulfated C6 significantly impacts WRV.314  315 Fig. 6 Deconvolution of sulfated C6 in the 13C DDMAS NMR spectra of SCP with R values of (a) 316 1.05 and (b) 4.19 317  318 17  319 Fig. 7 Relationship between the amount of sulfate groups at C6 and the R value 320  321 To further consider the relationship between the amount of sulfated C6 and WRV, 322 the total amount of sulfate groups in SCP was measured. Figure 8 shows the 323 correlation between the total content of the SCP sulfate groups determined by the 324 elemental analysis method and the R values. The total amount of sulfate groups 325 increased from 0 to 1.04 mmol/g as the R value increased from 0 to 1.05, and 326 continues to increase slightly from 1.51 to 1.83 mmol/g as the R value increased 327 from 2.09 to 4.19. The maximum total amount of sulfate groups reaches 1.83 328 mmol/g when R = 4.19, suggesting that it is nearly equivalent to that of the amount 329 of modifiable hydroxy groups of C6 present on the surface of microfibrils (Isogai 330 2018). These results indicate that sulfate groups are mainly introduced to the 331 hydroxy groups at the C2 and C3 positions of CP via the sulfation reaction when 332 the amount of sulfamic acid reacting with cellulose pulp is small. In contrast, the 333 proportion of sulfated C6 relative to the total sulfate groups increases, with more 334 sulfamic acid reacting with cellulose pulp. Interestingly, at R = 1.05, although 335 sulfated C6 is almost absent (Figure 7), the total amount of sulfate groups is 1.04 336 mmol/g (Figure 8). This suggests that most of 1.04 mmol/g is sulfated C2 and C3. 337 In addition, Figure 2 shows that WRV is hardly enhanced at R = 1.05. In other 338 words, the sulfated C2 and C3 do not significantly enhance the water retention 339 18 capacity. This indicates that the amount of sulfated C6 is essential for improving 340 the WRV of CP materials.341  342 Fig. 8 Association between the amount of sulfate groups and the R value 343  344 FTIR spectroscopy was used to verify the sulfation of the hydroxy groups in CP. 345 Figure 9 shows the infrared spectra of (a) CP and SCP with R values of (b) 0, (c) 346 1.05, (d) 2.09, (e) 3.14, and (f) 4.19. The intensity of the peaks at 810 and 1230 347 cm−1 gradually increased as the R value increased. The peak at 810 cm−1 was 348 attributed to S=O vibrations and that at 1230 cm−1 was attributed to C-O-S 349 vibrations. However, S=O and C-O-S vibrations were almost nonexistent in the 350 spectra of CP and SCP with R = 0. Therefore, urea is not directly involved in the 351 sulfate reaction of CP, supporting the preferential sulfation of hydroxy groups.  352 19  353 Fig. 9 Fourier-transform infrared spectra of (a) CP and SCP with R values of (b) 0, (c) 1.05, (d) 354 2.09, (e) 3.14, and (f) 4.19 355  356 Conclusions 357 A novel SCP was developed using a sulfate reaction with CP, exhibiting a WRV of 358 16000% for R = 4.19. Solid-state 13C NMR was used to investigate and quantify 359 the molecular structure of SCP. After sulfation, the intensity of the C6am peak at 62 360 ppm decreased, whereas the new peak at 69 ppm increased. The other peaks 361 remained almost unchanged. The amount of sulfate at C6 increased with an increase 362 in R value. Under the presence of sulfamic acid in low quantities during the sulfate 363 reaction, sulfate groups are preferentially introduced into the hydroxy groups at C2 364 and C3 of CP. The sulfate reactions occur at C2, C3, and C6 for high quantities of 365 sulfamic acid. FT-IR spectroscopy confirmed that the hydroxy groups were mainly 366 sulfated. This study demonstrates the importance of the substitution rate of the 367 sulfated C6 in SCP to achieve ultra-high water retention in cellulose pulps.  368   369 20 References 370 Atalla RH, VanderHart DL (1999) The role of solid state 13C NMR spectroscopy in studies of the 371 nature of native celluloses. Solid State Nucl Magn Reson 15:1-19. https://doi.org/10.1016/s0926-372 2040(99)00042-9 373 El Hariri El Nokab M, Habib MH, Alassmy YA, Abduljawad MM, Alshamrani KM, Sebakhy KO 374 (2022) Solid State NMR a Powerful Technique for Investigating Sustainable/Renewable Cellulose-375 Based Materials. Polymers 14:1049. https://doi.org/10.3390/polym14051049 376 French AD (2014) Idealized powder diffraction patterns for cellulose polymorphs. Cellulose 377 21:885–896. https://doi.org/10.1007/s10570-013-0030-4 378 Foston M (2014) Advances in solid-state NMR of cellulose. Curr Opin Biotechnol 27:176-184. 379 https://doi.org/10.1016/j.copbio.2014.02.002 380 Fukuzumi H, Saito T, Iwata T, Kumamoto Y, Isogai A (2009) Transparent and high gas barrier films 381 of cellulose nanofibers prepared by TEMPO-mediated oxidation. Biomacromolecules 10:162-165. 382 https://doi.org/10.1021/bm801065u 383 Ghanadpour M, Carosio F, Larsson PT, Wågberg L (2015) Phosphorylated cellulose nanofibrils: a 384 renewable nanomaterial for the preparation of intrinsically flame-retardant materials. 385 Biomacromolecules 16:3399-3410. https://doi.org/10.1021/acs.biomac.5b01117 386 He Z, Shen A, Guo Y, Liu Z, Li De, Qin X, Zhao M, Wang Z (2019) Cement-based materials 387 modified with superabsorbent polymers Constr Build Mater. 225:569-590. 388 https://doi.org/10.1016/j.conbuildmat.2019.07.139 389 Horii F, Hirai A, Kitamaru R (1983) Solid-state 13C-NMR study of conformations of 390 oligosaccharides and cellulose: Conformation of CH₂OH group about the exo-cyclic C–C bond. 391 Polym Bull 10:357-361. https://doi.org/10.1007/BF00281948 392 Hou G, Zhao S, Peng L, Fang Z, Isogai A (2022) A systematic study for the structures and properties 393 of phosphorylated pulp fibers prepared under various conditions. Cellulose 29:7365-7376. 394 https://doi.org/10.1007/s10570-022-04713-4 395 Hou X, Li R, He W, Dai X, Ma K, Liang Y (2018) Superabsorbent polymers influence soil physical 396 properties and increase potato tuber yield in a dry-farming region. J Soils Sediments 18:816-826. 397 https://doi.org/10.1007/s11368-017-1818-x 398 Huang X, Zhang WD (2010) Preparation of cellulose sulphate and evaluation of its properties. J 399 Fiber Bioeng Inform 3:32-39. https://doi.org/10.3993/jfbi06201006 400 Isogai A (2018) Development of completely dispersed cellulose nanofibers. Proc Jpn Acad Ser B 401 Phys Biol Sci 94:161-179. https://doi.org/10.2183/pjab.94.012 402 Lehtonen J, Hassinen J, Kumar AA, Johansson LS, Mäenpää R, Pahimanolis N, Pradeep T, Ikkala 403 O, Rojas OJ (2020) Phosphorylated cellulose nanofibers exhibit exceptional capacity for uranium 404 capture. Cellulose 27:10719-10732. https://doi.org/10.1007/s10570-020-02971-8 405 Liu P, Borrell PF, Božič M, Kokol V, Oksmana K, Mathew AP (2015) Nanocelluloses and their 406 phosphorylated derivatives for selective adsorption of Ag+, Cu2+ and Fe3+ from industrial effluents. 407 J Hazard Mater 294:177-185. https://doi.org/10.1016/j.jhazmat.2015.04.001 408 21 Liu P, Oksman K, Mathew AP (2016) Surface adsorption and self-assembly of Cu(II) ions on 409 TEMPO-oxidized cellulose nanofibers in aqueous media. J Colloid Interface Sci 464:175-182. 410 https://doi.org/10.1016/j.jcis.2015.11.033 411 Meshram I, Kanade V, Nandanwar N, Ingle P (2020) Super-Absorbent Polymer: A Review on the 412 Characteristics and Application. Int J Adv Res Chem Sci 7:8-21. https://doi.org/10.20431/2349-413 0403.0705002 414 Nishimura A, Otsuka S (2024) Sulfated cellulose pulp with high water retention and a swollen fiber 415 structure for the preparation of completely dispersed cellulose nanofibers. Cellulose 31:3561-3571. 416 https://doi.org/10.1007/s10570-024-05841-9 417 Noguchi Y, Homma I, Matsubara Y (2017) Complete nanofibrillation of cellulose prepared by 418 phosphorylation. Cellulose 24:1295-1305. https://doi.org/10.1007/s10570-017-1191-3 419 Rånby BG, Banderet A, Sillén LG (1949) Aqueous colloidal solutions of cellulose micelles. Acta 420 Chem Scand 3:649-650. https://doi.org/10.3891/acta.chem.scand.03-0649 421 Riva L, Dotti A, Iucci G, Venditti I, Meneghini C, Corsi I, Khalakhan I, Nicastro G, Punta C, 422 Battocchio C (2024) Silver Nanoparticles Supported onto TEMPO-Oxidized Cellulose Nanofibers 423 for Promoting Cd2+ Cation Adsorption. ACS Appl Nano Mater 7:2401-2413. 424 https://doi.org/10.1021/acsanm.3c06052 425 Saito T, Kimura S, Nishiyama Y, Isogai A (2007) Cellulose nanofibers prepared by TEMPO-426 mediated oxidation of native cellulose. Biomacromolecules 8:2485-2491. 427 https://doi.org/10.1021/bm0703970 428 Segal L, Creely JJ, Martin AE and Conrad CM (1959) An Empirical Method for Estimating the 429 Degree of Crystallinity of Native Cellulose Using the X-Ray Diffractometer. Textile Research 430 Journal 29:786-794. https://doi.org/10.1177/004051755902901003 431 Sirviö JA, Ukkola J, Liimatainen H (2019) Direct sulfation of cellulose fibers using a reactive deep 432 eutectic solvent to produce highly charged cellulose nanofibers. Cellulose 26:2303-2316. 433 https://doi.org/10.1007/s10570-019-02257-8 434 Snape CE, Axelson DE, Botto RE, Delpuech JJ, Tekely P, Gerstein BC, Pruski M, E. Maciel GE, 435 Wilson MA (1989) Quantitative reliability of aromaticity and related measurements on coals by 13C 436 n.m.r. A debate. Fuel 68:547-548. https://doi.org/10.1016/0016-2361(89)90142-7 437 Solum MS (1996). In: Grant DM, Harris RK (eds) Encyclopedia of nuclear magnetic resonance, vol 438 8. John Wiley & Sons, Chichester, pp 5047-5051 439 Torchia DA (1978) The measurement of proton-enhanced carbon-13 T1 values by a method which 440 suppresses artifacts. J Magn Reson 30:613-616. https://doi.org/10.1016/0022-2364(78)90288-3 441 Viëtor RJ, Newman RH, Ha M-A, Apperley DC, Jarvis MC (2002) Conformational features of 442 crystal-surface cellulose from higher plants revealed by solid-state ¹³C NMR. Plant J 30(6):721-443 731.https://doi.org/10.1046/j.1365-313X.2002.01327.x 444 Wang R, Rosen T, Zhan C, Chodankar S, Chen J, Sharma PR, Sharma SK, Liu T, Hsiao BS 445 (2019) Morphology and flow behavior of cellulose nanofibers dispersed in glycols. 446 Macromolecules 52:5499-5509. https://doi.org/10.1021/acs.macromol.9b01036 447 Wickholm K, Larsson PT, Iversen T (1998) Assignment of non-crystalline forms in cellulose I by 448 CP/MAS 13C NMR spectroscopy. Carbohydrate Research 312:123-129. 449 22 Yoneda Y, Mereiter K, Jaeger C, Brecker L, Kosma P, Rosenau T, French AD (2008) Van der Waals 450 versus hydrogen-bonding forces in a crystalline analog of cellotetraose: cyclohexyl 4′-O-cyclohexyl 451 β-D-cellobioside cyclohexane solvate. J Am Chem Soc 130:16678-16690. 452 https://doi.org/10.1021/ja805147t 453 Yu Y, Wu H (2010) Significant differences in the hydrolysis behavior of amorphous and crystalline 454 portions within microcrystalline cellulose in hot-compressed water. Ind Eng Chem Res 49:3902-455 3909. https://doi.org/10.1021/ie901925g 456 Zhang K, Brendler E, Geissler A, Fischer S (2011) Synthesis and spectroscopic analysis of cellulose 457 sulfates with regulable total degrees of substitution and sulfation patterns via 13C NMR and FT 458 Raman spectroscopy. Polymer 52:26-32. https://doi.org/10.1016/j.polymer.2010.11.017 459 Zhang K, Peschel D, Bäucker E, Groth T, Fischer S (2011) Synthesis and characterisation of 460 cellulose sulfates regarding the degrees of substitution, degrees of polymerisation and morphology. 461 Carbohydr Polym 83:1659-1664. https://doi.org/10.1016/j.carbpol.2010.10.029 462 Zhang S, Peng Y, Jiang R, Liu W, Yang H, Yun N, Chai X (2021) Predicting the Swelling Behavior 463 of Acrylic Superabsorbent Polymers Used in Diapers Adv Polym Tech 2021:7. 464 https://doi.org/10.1155/2021/9999826 465 Zhao M, Fujisawa S, Saito T (2021) Distribution and Quantification of Diverse Functional Groups 466 on Phosphorylated Nanocellulose Surfaces. Biomacromolecules 22:5214-5222. 467 https://doi.org/10.1021/acs.biomac.1c01143 468 Zhao MW, Lin L, Bi SZ, Nodirali N, Si YG (2007) Preparation and anticoagulation activity of 469 sodium cellulose sulfate. Int J Biol Macromol 41:376-382. 470 https://doi.org/10.1016/j.ijbiomac.2007.05.007  471 23 Statements & Declarations 472 Funding  473 The authors did not receive support from any organization for the submitted work. 474 Competing Interests 475 The authors have no conflicts of interest directly relevant to the content of this 476 article. 477 Compliance with Ethical Standards 478 Not applicable. 479