Article pubs.acs.org/Biomac
Facile Approach for the Dispersion of Regenerated Cellulose in Aqueous System in the Form of Nanoparticles Mukund Adsul, Sarvesh K. Soni, Suresh K. Bhargava,* and Vipul Bansal* School of Applied Science, RMIT University, GPO Box 2476 V, Melbourne, Victoria 3001, Australia
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ABSTRACT: This study reports a facile method to disperse cellulose in deionized water, wherein a critical condition of regenerated cellulose is discovered, where it completely disperses up to a maximum of 5 g L−1 concentration in deionized water with the help of ultrasonication. The dispersed cellulose is characterized by TEM and DLS, the latter among which shows 200 nm hydrodynamic radii of cellulose nanoparticles dispersed in deionized water. FTIR analysis of dispersed cellulose reveals that dispersed cellulose losses its crystallinity during regeneration and dispersion step employed in this study. The dispersed cellulose reported in this study is able to form free-standing, transparent films, which were characterized by SEM, XRD, TGA, EDX, and FTIR spectroscopy and show resistance against dissolution in water. Additionally, the dispersed cellulose is able to undergo at least three times faster enzymatic hydrolysis in comparison to pristine microcrystalline cellulose under similar reaction conditions. The dispersed cellulose reported here could be a better material for reinforcement, preparation of hydrogels, and drug delivery applications under physiological environment.
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INTRODUCTION Cellulose is the most abundant, natural, green, and inextinguishable biopolymer in nature.1,2 Facile manipulation of this biopolymer in an aqueous system is not possible because it is rather insoluble in water and therefore does not disperse well in aqueous solutions. Since its discovery about 150 years ago, researchers have been struggling to dissolve or disperse cellulose in water without any modification to its chemical structure either in the form of dispersed nanoparticles or as standalone molecules. To disperse cellulose particles or cellulose chains in an aqueous system, typically either surfactant are used or chemical modification of cellulose is carried out.3−5 However, the highly ordered crystalline nature of cellulose along with strong inter- and intramolecular hydrogen bonding makes it rather insoluble in water. Additionally, the lack of suitable solvent for this biomacromolecule is the major barrier for the preparation of cellulose hydrogels.6 To the best of the authors’ knowledge, not a single methodology is reported that allows dissolution or dispersion of cellulose in pristine water without its chemical modification. Previously, dissolution of cellulose has been demonstrated in the presence of a high percentage of NaOH/urea,7,8 various thiocyanates,9,10 and N-methylmorpholine-N-oxide.11 Additionally, direct dissolution of cellulose in solvents such as hydrophilic ionic liquids (e.g., [BMIM][Cl]),12,13 inorganic molten salts such as LiClO4·3H2O, NaSCN/KSCN/LiSCN·2H2O, and LiCl/ZnCl2/H2O14 and phosphoric acid15 has also been demonstrated. However, if cellulose dissolved using aforementioned methods is transferred to pristine water, it tends to precipitate out, thereby making it unsuitable for © 2012 American Chemical Society
generating a homogeneous solution of cellulose dispersed in pure water and, therefore, such cellulose cannot be employed or manipulated for further applications in aqueous systems. Another method to dissolve cellulose in water is the derivatization or chemical modification of its natural structure (e.g., −OH group modification leading to carboxymethyl cellulose) so that it reduces the hydrogen bonding within and between the cellulose chains, which makes it easily dispersible in water. However, such chemical modification methods are usually cost-intensive and, hence, limited to laboratory experiments.16 We recently demonstrated that the dispersion of cellulose in pristine water could be achieved without derivatization with the help of carbon nanotubes (CNTs).17 It was observed that, in the presence of CNTs, dissolution of cellulose increased due to the intermolecular hydrogen bonding between CNTs and cellulose chains. However, in that case, cellulose molecules were always found to be associated with CNTs and cellulose could not be separated free from CNTs. Also, the amount of cellulose dissolution was rather less (
