[show abstract][hide abstract] ABSTRACT: During cellulose dissolution in non-derivatizing solvents, the inter-and intramolecular hydrogen bonds of the polymer are deconstructed. This occurs either by hydrogen bond formation between one or more components of the solvent systems and the hydroxyl groups of the cellulose or by coordination bond formation between the metal ion present in the medium and the hydroxyl group of cellulose molecules. None of the polymer molecules are actually chemically modified during dissolution. In the first part of this review, we examine the literature pertaining to the different interaction mechanisms between cellulose and non-derivatizing solvent systems with emphasis on the inorganic molten salt hydrates. In the second part of this effort, we further review inorganic molten salt hydrates from the point of view of the changes they impart to the physical properties of the cellulose and the various chemical reactions that can be performed in it. ■ INTRODUCTION Cellulose is a homopolysaccharide that is formed from linearly connecting D-glucose units condensed through the β(1−4) glycosidic bonds. 1,2 This natural polymer has a 2-fold screw axis along the chain direction. The degree of polymerization (DP) of the macromolecule can vary from 100 to 20,000 depending on the sources. 3 Cellulose possesses a highly crystalline structure due to the presence of extensive intra-and intermolecular hydrogen bonding, 4−8 which has been examined in great detail. 9−12 Consequently this natural polymer is insoluble in water and typical organic solvents and can only be dissolved if the intra-and intermolecular hydrogen bonds are effectively disrupted. Cellulose dissolution processes can be broadly classified in two categories as will be discussed below: 13 Cellulose Dissolution with Chemical Modification. A well-known method of cellulose dissolution is by prior chemical modification of the macromolecule. The main objective of this procedure is to functionalize the hydroxyl groups so as to disrupt the intra-and intermolecular hydrogen bondings but with minimal chain degradation. Functionalization reactions of cellulose include nitration, 14,15 xanthation, 7,16 esterifica-tion, 7,17,18 and etherification. 7,17 Though the solubility of the derivatized cellulose depends on the type and degree of derivatization, most of the derivatives are soluble in common polar organic solvents like DMF, DMSO, dioxane etc. 17,19 Detailed descriptions of such process are beyond the scope of this review. Cellulose Dissolution without Chemical Modification. Solvents capable of dissolving cellulose without prior chemical modification are frequently described as non-derivatizing solvents. Such cellulose solvent systems are known to include ionic liquids, 5,7,20 organic solvents in the presence of an inorganic salt, 7,21−23 amine oxides, 7,24,25 aqueous alkali solutions, 7,26 aqueous complex solutions, 7,27 and inorganic molten salt hydrates. 7,28−30 Ionic Liquids. Ionic liquids are one promising set of non-derivitaizing solvents of cellulose. Ionic liquids generally consist of large low charge density organic cations with low charge density inorganic or organic anions. 13,31,32 The low charge density and frequent size mismatch between the ions cause the salt to remain in the liquid phase at relatively low temperatures (<100 °C). 33 Some of them are non-flammable liquids even at room temperature, 5 which when coupled with their docu-mented thermal and chemical stability and recyclability offer attractive characteristics for cellulose processing. Moreover, due to their low vapor pressures and reasonable ease of recovery, compared to other organic solvents, ionic liquids are promoted as an ideal green material. 5,20 There have been reports regarding ionic liquid processing of cellulose as early as 1934. 34,35 However, in early 2000, Swatloski and co-workers reintroduced the concept of ionic liquids as potential solvents for cellulose. 36 Remsing et al. studied the 35/37 Cl NMR spectra relaxation times of cellobiose (the smallest possible repeat unit of cellulose that is also used as a model compound for this natural polymer) solvated in 1-n-butyl-3-methyl-imidazolium chloride ([C 4 mim]Cl) at 90 °C to understand the interaction between cellulose and this solvent system. 37 This effort shows that
[show abstract][hide abstract] ABSTRACT: Thermophysical properties of two pyridinium-based ionic liquids, 1-ethyl-2-methylpyridinium bis(trifluoromethylsulfonyl)imide and 1-propyl-2-methylpyridinium bis(trifluoromethylsulfonyl)imide, have been measured from 278.15 to 323.15 K, with a temperature step of 2.5 K. The properties measured were: densities, speeds of sound, refractive indices, surface tensions, isobaric molar heat capacities, electrical conductivities and viscosities. Thermal properties were also recorded in the temperature range from 100 to 320 K. From the experimental results coefficients of thermal expansion, isentropic compressibilities, molar refraction and surface enthalpies and entropies have been determined. Moreover, a theoretical study has been performed using ab initio calculations, level of theory HF/6-31G(d). From this study, we have obtained qualitative information about the magnitude and the directionality of the cation–anion interactions which allows a better understanding of the physico-chemical properties of these ionic liquids.
Journal of Solution Chemistry 41(10). · 1.13 Impact Factor
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