Cellulose Dissolution in Ionic Liquid: Ion Binding Revealed by Neutron Scattering Vikram Singh Raghuwanshi, † Yachin Cohen,* ,‡ Guillaume Garnier, † Christopher J. Garvey, § Robert A. Russell, § Tamim Darwish, § and Gil Garnier* ,† † Bioresource Processing Research Institute of Australia (BioPRIA), Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia ‡ Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 3200003, Israel § Australian Nuclear Science and Technology Organisation (ANSTO), New Illawarra Rd., Lucas Heights, NSW 2234, Australia * S Supporting Information ABSTRACT: Dissolution of cellulose in 1-ethyl-3-methyl- imidazolium acetate (EMIMAc) ionic liquid (IL) was investigated by small-angle neutron scattering (SANS) with contrast variation. Cellulose and EMIMAc of different deuteration levels provide sufficient contrast in revealing the cellulose dissolution processes. Two experiments were performed: hydrogenated microcrystalline cellulose (MCC) was dissolved in deuterated IL (IL-D14), and deuterated bacterial cellulose (DBC) was dissolved in hydrogenated IL (IL-H14). Contrary to the expectation of high contrast between MCC and IL-D14, a dramatic reduction of the measured intensity (scattering cross section) was observed, about 1/3 of the value predicted based on the scattering length density (SLD) difference. This is attributed to the tight binding of acetate ions to the cellulose chains, which reduces the SLD difference. Measurements using small-angle X-ray scattering (SAXS) corroborate this effect by indicating increased contrast due to ion adsorption resulting in enhanced SLD difference. The experiments performed with DBC dissolution in IL-H14 suggest the presence of fractal aggregates of the dissolved cellulose, indicating lower solubility compared to the MCC. Contrast variation SANS measurements highlight tight ion binding of at least one acetate ion per anhydroglucose unit (AGU). EMIMAc is a successful cellulose solvent, as in addition to disrupting intermolecular hydrogen bonding, it imparts effective charge to the cellulose chains hindering their agglomeration in solution. ■ INTRODUCTION Cellulose is the most abundant and renewable polymer; it is used in many industrial and technological applications ranging from biomedical to nanomaterials and encompassing tradi- tional products in textile and paper. A main hindrance for successful applications has been the lack of an efficient and environmentally benign method to dissolve cellulose chains from their tight binding in the native crystal. The discovery of ionic liquids (IL) as a solvent by Rogers and co-workers has opened novel ways to dissolve cellulose. 1 However, applica- tions of IL for cellulose are still limited due to technical and economic issues. The search for new improved solvents is of current interest. It relies, in part, on obtaining a better understanding of the dissolution mechanism. 2 It has been suggested that IL ions adsorb onto the cellulose surface, disrupt the interchain hydrogen bonding network within the crystalline fibrillar structure, and release the chains into the IL matrix. Studies by molecular dynamics simulations 3 and NMR spectroscopy 4 have indicated that dissolution involves formation of strong association of IL anions capable of hydrogen bonding to the hydroxyl groups of cellulose. These may be augmented by the van der Waals interactions of the conjugated ring structures in IL cations in close contact with sugar rings. 5 Recent studies of cellulose/IL solutions by small-angle X-ray scattering (SAXS) provided evidence of a solvation sheath of the IL around the dissolved cellulose chain. SAXS measure- ments and molecular simulations of cellulose dissolved in a mixture of tetrabutylammonium acetate and dimethyl sulfoxide indicated that approximately one acetate ion binds to each anhydroglucose unit (AGU). 6 Another recent SAXS study and simulation on cellulose dissolved in 1-ethyl-3-methyl- immidazolium methylphosphonate came to a similar con- clusion. 7 The SAXS patterns in these studies were analyzed using a persistent worm-like chain model, in which a core− shell structure was evaluated for the electron density profile in a cross section perpendicular to the chain segment, indicating IL solvation of the cellulose chain. 6,7 Received: July 4, 2018 Revised: September 5, 2018 Article pubs.acs.org/Macromolecules Cite This: Macromolecules XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.macromol.8b01425 Macromolecules XXXX, XXX, XXX−XXX Macromolecules Downloaded from pubs.acs.org by WESTERN UNIV on 09/21/18. For personal use only.