Preparation of Aqueous Dispersions of TEMPO-Oxidized Cellulose

Dec 8, 2016 - 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNs) with sodium carboxylate groups (TOCN-Na) were nanodi...
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Preparation of Aqueous Dispersions of TEMPO-Oxidized Cellulose Nanofibrils with Various Metal Counterions and Their Super Deodorant Performances Atsushi Sone,†,‡ Tsuguyuki Saito,† and Akira Isogai*,† †

Department of Biomaterials Science, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan ‡ Research and Development Center, Manufactured Products Development Laboratory, ZEON Co., 1-2-1 Yako, Kawasaki-ku, Kawasaki, Kanagawa 210-9507, Japan S Supporting Information *

ABSTRACT: 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNs) with sodium carboxylate groups (TOCN-Na) were nanodispersed in water. The Na+ ions of TOCN-Na were exchanged with other monovalent, divalent, and trivalent metal (M) ions. When an aqueous metal acetate or AlCl3 solution was added to a 0.1% TOCN-Na/water dispersion or TOCN-COOH gel/ water mixture under suitable conditions followed by sonication in water, individually nanodispersed TOCNs-M with divalent and trivalent metal counterions were obtained in yields of 24− 87%. The amount of metal introduced into TOCN-M was approximately 20−50% of the TOCN carboxylate content when divalent and trivalent metal ions were used. These results indicate that intrafibrillar ionic linkages are selectively formed and these TOCN-M species nanodisperse at the individual nanofiber level without agglomeration in water. Filter papers with adsorbed TOCN-Cu/water and TOCN-Ag/water dispersions efficiently decomposed or adsorbed H2S and CH3SH gases, but filter papers containing fibrous TEMPO-oxidized celluloses with Ag+ or Cu2+ counterions had much lower gas-decomposition efficiencies. TOCN-Na film in aqueous metal chloride solutions.4,5 However, no reports of the preparation of TOCN-M/water dispersions consisting of completely individual TOCN-M elements have been published. In modern society, good deodorants are required in various situations. Miki reported that materials containing Ag+ and other metal ions are effective against sulfur-containing compounds such as H2S and CH3SH, which cause serious odor problems.6 TOCN-M/water dispersions consisting of individualized TOCN-M elements immobilized on supporting materials such as cloth and paper sheets are expected to have high deodorant performances. In this study, a fibrous TOC-Na with a sodium carboxylate content of 1.4 mmol g−1 was used as the starting material and converted to TOCN-Na with ∼3 nm widths and aspect ratios of more than 300.1−3 TOCN-M/water dispersions were then prepared using an ion-exchange method. The TOCN-M/water dispersions were adsorbed on filter papers, and their deodorant

2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNs) are generally prepared from papermaking-grade fibrous softwood bleached kraft pulp with 20−40 μm widths by TEMPO-mediated oxidation and mechanical disintegration in water. Most of the C6-primary hydroxyl groups exposed on the crystalline plant cellulose microfibril surfaces are selectively oxidized to sodium C6carboxylate groups, while the original fibrous morphologies are maintained. When the sodium carboxylate content of the fibrous TEMPO-oxidized cellulose (TOC) is greater than 1 mmol g−1, completely nanofibrillated TOCNs with homogeneous widths of ∼3 nm and high aspect ratios, >300, can be quantitatively obtained by gentle mechanical disintegration in water.1,2 Sodium counterions in fibrous TOCs (TOC-Na) can be efficiently exchanged with other metal (M) ions to give TOCM through simple ion-exchange treatment in water.3 However, fibrous TOCs-M prepared with divalent or trivalent metal ions do not nanofibrillate to TOCN-M/water dispersions consisting of individual TOCN-M elements, even under harsh mechanical disintegration or sonication in water. Interfibrillar ionic linkages through metal ions probably cause this nondispersibility in water. The sodium counterions in TOCN-Na cast films can be completely converted to give TOCN-M films by immersing the © XXXX American Chemical Society

Received: October 16, 2016 Accepted: December 6, 2016

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inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the yields were determined from the dry weights of the dispersions (see Supporting Information). Although the original fibrous TOC had a carboxylate content of 1.4 mmol g−1, the TOCN-Na prepared from the TOC by mechanical disintegration had a lower Na+ content, that is, 1.1 mmol g−1. There may therefore be discrepancies between the carboxylate contents of TOCNs determined using an electric conductivity method and those of Na+ (counterion of the carboxylate groups) determined using ICP-AES with a calibration line. When the monovalent ion Ag+ was used, TOCN-Ag yields of almost zero and 13% were obtained using methods A and B, respectively. TOCN-Ag is intrinsically nanodispersible in water; therefore, most of the TOCN-Ag formed using method A may have been removed as a water-dispersible fraction during washing. Although fibrous TOC-M/water slurries were not converted to nanofibrillated TOCN-M/water dispersions, even using harsh mechanical treatment, nanodispersed TOCN-Cu, TOCN-Co, and TOCN-Al were obtained in 30, 43, and 87% yields, respectively, from a 0.1% TOCN-Na/water dispersion using method A. Nanodispersed TOCN-Cu, TOCN-Zn, TOCN-Ca, TOCN-Co, and TOCN-Al were obtained in 24, 31, 66, 57, and 67% yields, respectively, using method B (Figure 2). The effect of the initial TOCN-Na concentration on the yield or weight fraction of nanodispersed TOCN-Cu obtained using method A is shown in Figure S1 in the Supporting Information. The lower the initial TOCN-Na concentration in the dispersion is, the higher the yield of nanodispersed TOCN-Cu. The initial TOCN-Na concentration is therefore a key factor in the preparation of nanodispersed TOCN-M/water dispersions in high yields. It is not yet known why the yields of TOCN-Zn and TOCNCa were almost zero when method A was used. The results in Figure 2 show that almost all the TOCN-Zn and TOCN-Ca were removed during repeated washing in method A. Figure 3 shows the absorption bands arising from CO stretching vibrations in the FT-IR spectra of cast and dried films of TOCN-Na, TOCN-H, and TOCN-M (prepared using method B in Figure 1). The TOCN-Na film had one absorption band, at ∼1600 cm−1, from dissociated carboxyl groups, whereas the TOCN-H film had a band at ∼1730 cm−1 from protonated carboxyl groups, together with a small band at 1650 cm−1 from residual water in the film. When the TOCNH/water dispersion was counterion-exchanged with metal ions using method B, the TOCN-Ag, TOCN-Cu, TOCN-Zn, and TOCN-Ca films had one CO band at ∼1600 cm−1, showing that the protonated carboxyl groups in TOCN-H were almost completely converted to the corresponding metal carboxylate groups. When aqueous AlCl3 was used in the ion-exchange treatment, the films had a large CO band at ∼1600 cm−1 from aluminum carboxylate groups and a small band from protonated carboxyl groups. The pH of the AlC3/TOCN-Na mixture used in the ion-exchange treatment was ∼3.8, therefore some of the carboxylate groups were protonated during the procedure. Because the pH values of the mixtures in the other counterion-exchange treatments were in the range 5−8, no carboxyl groups protonation occurred in the TOCN-M films. Ion chromatography showed that none of the TOCN-M/ water dispersions obtained contained either acetate or chloride

efficiencies for H2S and CH3SH gases were evaluated and compared with those of fibrous TOC-M-added filter papers. Two methods for preparing TOCN-M/water dispersions from 0.1% TOCN-Na/water dispersions were used in this study (Figure 1). In method A, the sodium counterions in

Figure 1. Preparation scheme of TOCN-M/water dispersions from TOCN-Na/water dispersion by counterion exchange using methods A and B.

TOCN-Na were directly exchanged with other metal ions. In method B, the TOCN-Na was first converted to protonated TOCN (TOCN-H) gel particles in water by adding a dilute HCl solution; the subsequent procedure was the same as that in method A. Details of the two methods for TOCN-M preparation are given in the Supporting Information. The metal contents and yields of TOCN-M prepared from TOCN-Na using methods A and B are shown in Figure 2. The metal contents in the dispersions were determined using

Figure 2. Metal ion contents and yields of TOCN-M prepared from 0.1% TOCN-Na/water dispersion by counterion exchange using methods A and B in Figure 1. 1403

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dispersion, in which no acetate or chloride ions are present. Thus, nanofibrillated TOCN-M/water dispersions are obtained in various yields. In contrast, interfibrillar metal-ion linkages with divalent or trivalent metal ions are preferentially formed when fibrous TOC-Na/water slurries or nanofibrillated TOCNNa/water dispersions with high TOCN concentrations are used in the counterion exchange. In this case, almost no nanofibrillation of the TOCN-M gels formed in water is achieved, even by harsh mechanical disintegration treatment (Figure S2, Supporting Information). As a reference, we studied the dissolution behavior in water of the poly(acrylic acid) (PAA) copper salt and carboxymethyl cellulose (CMC) copper salt formed by counterion exchange with copper(II) acetate, washing with water, and sonication, according to method A. However, the PAA-Cu and CMC-Cu gels formed were insoluble in water, although the initial PAANa and CMC-Na concentrations in the solutions were less than 0.1%. These results show that intermolecular ionic linkages through Cu2+ are preferentially formed under these conditions, clearly different from those for TOCNs. The nanodispersibilities in water of TOCNs-M prepared using methods A or B are, therefore, characteristic of TOCN-Na with abundant sodium carboxylate groups densely and selectively positioned on the crystalline nanofibril surfaces. All the TOCN-M/water dispersions obtained in this study were transparent and showed birefringence when observed between cross-polarizers (Figure 5A,B), as in the case of

Figure 3. FTIR spectra of TOCN-Na, TOCN-H, and various TOCNM films.

ions indicating that all the counterions of metal ions in the TOCN-M/water dispersions were TOCN carboxylate groups. Although the metal ion contents of the TOCN-M/water dispersions determined by ICP-AES (Figure 2) did not completely correspond to the original carboxylate content of TOCN-Na, it can be concluded that one divalent or trivalent metal ion formed a metal carboxylate with two or three carboxylate groups in the TOCN-M/water dispersions. Because the yield of TOCN-Cu decreased as the initial concentration of the TOCN-Na/water dispersion increased (Figure S1, Supporting Information), TOCN-M/water dispersions consisting of nanofibrillated TOCN-M elements can be prepared from a dilute TOCN-Na/water dispersion (0.1% TOCN-Na in this study). Figure 4 shows a schematic diagram of a possible mechanism for the formation of TOCN-M/water dispersions containing divalent metal ions from TOCN-Na/ water dispersions using method A. Intrafibrillar metal-ion linkages (not interfibrillar metal-ion linkages) are formed during counterion exchange in a dilute TOCN-Na/water

Figure 5. TOCN-Cu/water dispersions observed (A) with or (B) without cross-polarizers and (C) atomic force microscopy image of TOCN-Cu.

TOCN-Na/water dispersions.7,8 These transparency and birefringence are characteristic of completely nanofibrillated TOCNs without agglomeration. The atomic force microscopy (AFM) image of TOCN-Cu in Figure 5C shows mostly individualized TOCN elements of widths ∼3 nm, determined from the AFM height images. The zeta potential of TOCN-Cu in a dilute dispersion was −30.14 mV. Each TOCN-Cu element had an anionic surface charge, resulting in complete nanodispersion of the TOCN-Cu elements in water (without agglomeration), based on electrostatic repulsion. The zeta potential of the original TOCN-Na was −51.02 mV; this shows that the anionic charge on TOCN-Na was reduced by counterion exchange of Na+ for Cu2+. TOCN-M/water dispersions and TOC-M/water slurries were adsorbed on filter papers, and their deodorant performances were evaluated by determining H2S or CH3SH concentrations in air containers. It has been reported that Ag+ and Cu2+ ions deodorized two malodorous gases;9,10 therefore, TOCN-M and TOC-M with ∼3 nm and 20−30 μm widths, respectively, containing Ag+ and Cu2+ were used in this study. The Ag+ or Cu2+ content of each filter paper sample was

Figure 4. Schematic diagram of nanofibrillated TOCN-M/water dispersion prepared from TOCN-Na/water dispersion at low TOCN concentration using method A in Figure 1, that is, counterion exchange of Na+ for M2+, followed by mechanical disintegration in water. The widths of TOCNs are ∼3 nm. 1404

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neously formed in the filter paper with H2S, and TOCNCOOH and TOCN-COOCH3 should be simultaneously formed in the filter paper with CH3SH. These points will be discussed in a following full paper.

adjusted to ∼0.002 mmol. Figure 6 shows that TOCN-Cu achieved the highest decomposition or adsorption of both H2S

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00786. Details of experiments, yields of TOCN-Cu prepared from TOCN-Na/water dispersions at various TOCN concentrations (Figure S1), schematic illustration of the reason why fibrous TOC-M/water slurries and TOCNM/water dispersions could not be nanodispersed at high TOCN concentrations (Figure S2), and gas-adsorption behavior of TOCN-Cu/filter paper prepared at room temperature (Figure S3; PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81 358415269. ORCID

Akira Isogai: 0000-0001-8095-0441 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. A.S. performed the experiments and wrote the manuscript with contributions from all the authors. Funding

This study was supported in part by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (JST).

Figure 6. Time-dependent adsorption of H2S (A) or CH3SH (B) gas by TOCN-M/filter paper and TOC-M/filter paper samples, at 23 °C.

Notes

and CH3SH gases; TOCN-Ag gave the next-best performance. The decomposition of the two gases achieved using nanofibrillar TOCN-M was clearly higher than that achieved using fibrous TOC-M. The TOCN-M, of widths that are ∼3 nm, have surface areas that are much larger than those of fibrous TOC-M, of width ∼30 μm. Much larger amounts of metal ions can therefore be supported on the TOCN surfaces in the filter papers and can make contact with gas molecules (although the absolute metal contents of the tested filter papers were the same), resulting in higher deodorant efficiencies. The TOCN-M/filter paper samples used to obtain the results shown in Figure 6 were prepared by drying wet filter papers at 105 °C for 3 h. When the wet TOCN-M/filter papers were dried at room temperature for 2 d, similar gas-adsorption behavior was observed (Figure S3, Supporting Information). The drying conditions of the wet TOCN-M/filter papers therefore had almost no effect on the gas-adsorption behavior. The sulfur contents of the TOCN-Cu/filter papers before and after gas treatment for 180 min were determined using an X-ray fluorescence analyzer. The intensities of the sulfur peaks increased by 16 and 340%, respectively, for CH3SH and H2S. Although there was a large difference between the increases in the amounts of the two gases, these results indicate that these gases were decomposed by Cu2+ ions to form CuS in the TOCN-Cu-containing filter papers, resulting in decreases in the gas concentrations in the air in the test container. If this assumption is correct, TOCN-COOH should be simulta-

The authors declare no competing financial interest.

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REFERENCES

(1) Isogai, A.; Saito, T.; Fukuzumi, H. Nanoscale 2011, 3, 71−85. (2) Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Biomacromolecules 2007, 8, 2485−2491. (3) Saito, T.; Isogai, A. Carbohydr. Polym. 2005, 61, 183−190. (4) Homma, I.; Fukuzumi, H.; Saito, T.; Isogai, A. Cellulose 2013, 20, 2505−2515. (5) Shimizu, M.; Saito, T.; Isogai, A. J. Membr. Sci. 2016, 500, 1−7. (6) Miki, Y. Kagakukougaku 2007, 71 (9), 27−31. (7) De Souza Lima, M. M.; Borsali, R. Macromol. Rapid Commun. 2004, 25, 771−787. (8) Okita, Y.; Fujisawa, S.; Saito, T.; Isogai, A. Biomacromolecules 2011, 12, 518−522. (9) Shirai, H. Shinsozai 1992, 4, 75−81. (10) Shirai, H. Sen’i Gakkaishi 1994, 50 (6), 388−391.

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