The systematic preparation of ILs and low-melting organic salts is a viable approach for obtaining tailored task-specific reaction media but it is only applicable within certain limitations. As an example, the viscosities of cellulose dissolving ILs are very high, compared to molecular solvents but also other ‘low-viscous’ ILs. It is not likely that a reduction of two or three orders of magnitudes, which seems to be required for certain derivatization reactions, can be achieved by modification of the molecular structure of ILs. One main reason for the high viscosity is the strong coulomb interaction between anions and cations, which is an ‘intrinsic feature’ of ILs and salts in general. Another factor is the formation of internal hydrogen bonds. Reduction in viscosity can be achieved by decreasing the strength of hydrogen

bonding between the two ionic species. Reducing the hydrogen bond donor ability (в) of an IL, which is mainly determined by the nature of the anion, can result in a significant decrease in viscosity (Table 5.4). Imidazolium salts with more hydro­phobic cations, e. g., dicyanamide or bis(trifluoromethylsulfonyl)imide, are less viscous compared to the corresponding chlorides, carboxylates, and phosphates that are rather strong hydrogen bond acceptors. However, these low viscous ILs also lack the ability to dissolve cellulose. It has been demonstrated that cellulose is only soluble in ILs with rather high в values above >0.8, i. e., those that exhibit strong hydrogen bonding between anions and cations and consequently possess higher viscosities.

The addition of co-solvents is an efficient way to diminish ‘intrinsic limita­tions’ of ILs that are predetermined in a narrow frame by the molecular structure, such as viscosities, polarities, densities, and melting points [128]. It has been demonstrated that the viscosity of cellulose/IL solutions can be reduced drasti­cally by addition of DMSO or DMF (Fig. 5.9) [33]. The decrease proceeds nearly exponentially with the weight fraction of dipolar aprotic co-solvent in the mixtures; at a typical mixing ration of 1 g co-solvent per g cellulose/IL solution, the viscosity is two order of magnitudes lower compared to an undiluted cellu­lose/IL solution of the same polymer content. Pyridine has been utilized as well to reduce the viscosity of cellulose/IL solutions [69]. For dilute and concentrated cellulose solutions in BMIMCl and AMIMCl it was demonstrated that the conformation of the dissolved polysaccharide is not impaired upon the addition of DMSO as a co-solvent [68]. The viscosity of the IL/DMSO mixtures as a function of the mole fraction of DMSO (xDMSO) could be predicted precisely by the following simple equation; with k being a constant (0.12 for BMIMCl and

0. 15 for AMIMCl):

П xDMSO

ln = ————- (5.3)

Піь k

Co-solvents can also be applied to tailor hydrophobicity of ILs along with their hydrogen bond donor/acceptor properties [129—131]. Thus, miscibility of hydro­philic cellulose/IL solutions with hydrophobic derivatization reagents could be improved by adding nonpolar co-solvents such as chloroform [41].

The addition of a solvent to cellulose/IL solutions can result in complete miscibility but also phase separation might occur. Moreover, it might induce precipitation of cellulose. Semi-quantitative predictions on these phenomena can be gained based on the solvatochromic parameters of potential co-solvents (Table 5.5) [132]. Only compounds with a high normalized empirical polarity (Etn > 0.3) were found to be miscible with the relatively hydrophilic ILs used as cellulose solvents. Precipitation of cellulose is induced by solvents with a strong hydrogen bond donor acidity (a < 0.5). In contrast, compounds that are strong hydrogen bond donor (в > 0.4) are very efficient co-solvents for cellulose/IL solutions; the amount of co-solvent that is tolerated before precipitation increases with increasing в-value. Solvatochromic measurements have been employed to characterize IL/co-solvent mixtures of different compositions that were employed for direct dissolution of cellulose [133, 134]. It could be demonstrated that the molar fraction of ILs in the mixtures can be reduced drastically (<0.3) if suitable dipolar aprotic co-solvents such as DMSO and amides are applied. In these mix­tures, the high в-value of the IL, which seems to be important for the dissolution of cellulose, is almost unaffected even in case of an excess of co-solvent [133].

aSolvent parameters: ETN: normalized solvent polarity, a: hydrogen bond donor ability, p: hydro­gen bond acceptor ability

bAMIMCl: 1-allyl-3-methylimidazolium chloride, BMIMCl: 1-butyl-3-methylimidazolium chlo­ride, EMIMAc: 1-ethyl-3-methylimidazolium acetate

c+: miscible, —: immiscible, +/p transition from miscible to precipitation, +/g transition from miscible to gelation, +/— transition from miscible to immiscible

dAmount of co-solvent (g per g cellulose/IL solutions) that can be added without permanent

precipitation/gelation/immiscibility

eWater, methanol, ethanol, and 2-propanol

fEthyl acetate, tetrahydrofuran, dioxane, diethyl ether, toluene, and hexane

In contrast, alcohols that are used for precipitation significantly decreased the hydrogen bond acceptor basicity. For IL/water mixtures it has been demonstrated that cellulose dissolves if the ‘net basicity’ (0.35 < в — а < 0.90) of the solvent falls into a specific range [134].

IL/co-solvent mixtures can be exploited for the homogeneous preparation of highly engineered cellulose derivatives. As described above, CS and TOSC can be prepared at low temperatures (<25 °C) in a completely homogeneous reaction in ILs but utilization of co-solvents is indispensable due to the high viscosity of the undiluted cellulose solutions [33, 35]. Homogeneous preparation of hydrophobic cellulose silyl ethers was realized in IL/chloroform mixtures [41]. Another example for the beneficial use of co-solvents is the hydroxyalkylation of cellulose in EMIMAc with gaseous oxiranes is a heterogeneous process that yields higher DS values if DMSO is added to decrease viscosity and increase solubility of the etherification reagents [37]. In fact, many of the derivatization reactions reported in literature have been carried out not in pure ILs but with the aid of co-solvents (see Tables 5.1 and 5.2) although it has not been mentioned explicitly in each case. Several patented procedures that are of interest for the preparation of commercially attractive cellulose derivatives were performed in IL/co-solvent mixtures [38, 54, 135—137].

In addition to physical and dissolution properties, co-solvents can also alter the chemical behavior of IL based reaction media by either facilitating or preventing specific derivatization reactions. As an alternative to commonly applied imidazolium chlorides, 1-allyl-3-methylimidazolium fluoride has been prepared as a novel solvent for cellulose [138]. Although the IL could be used for dissolution and acetylation of cellulose, the DS values of the products obtained were low because the fluoride anion induced cleavage of the ester bond. In the presence of DMSO, however, highly substituted cellulose esters could be obtained, presumably because of strong solvation of the anion by hydrogen bonding with the co-solvent. Pyridine, on the other hand, is an attractive co-solvent for derivatization reactions that require bases, e. g., tosylation, and tritylation, because it reduces viscosity of cellulose/IL solutions and likewise promotes the chemical conversion [35, 40]. Thus, no additional bases are required because the co-solvent is applied in a slight excess. An interesting approach is to exploit the co-solvent as derivatization reagent. A series of cationic cellulose esters has been prepared in mixtures of BMIMCl with different lactames, e. g., N-methyl-2-pyrolidine and e-caprolactam, as co-solvents [135, 139]. The cyclic amides were activated with tosyl chloride and subsequently reacted with the polysaccharide backbone under ring opining.

Combination of the above described aspects, i. e., synthesis of novel ILs or low melting salts and the use of co-solvents to improve dissolution and processing of cellulose, is a logical consequence with huge potential for a broad range of applications. Interestingly this approach has already been described in the early 1930s in a number of patents [140, 141]. N-Alkylpyridinium salts have been utilized therein as cellulose solvents and pyridine was utilized to decrease their melting points of 120-130 °C and achieve dissolution at ambient temperature. Mixtures of these pyridinium salts and co-solvents have also been applied for shaping and chemical derivatization of cellulose [142, 143]. Although, these pub­lications rose little interest during their time, they were rediscovered within the frame of the increasing interest for novel polysaccharide solvents. These reports are frequently considered as the first attempts to use ILs for dissolution of cellulose which is strictly speaking incorrect regarding the fact that the high melting points of the salts applied lie above 100 °C.

It is self-evident that co-solvents represent additional components that need to be considered in the recycling process. Volatile co-solvents, e. g., pyridine and chlo­roform, can be recovered by evaporation. Subsequently, the crude IL can be purified using suitable techniques. In contrast, co-solvents with relatively low vapor pressures, in particular dipolar aprotic ones like DMF, DMSO, and DMI, will remain in the crude IL after evaporating the non-solvent used for precipitation of the cellulose derivative. After removal of impurities that are harmful for disso­lution or chemical modification of cellulose, the polysaccharide can be dissolved directly in the recycled IL/co-solvent mixture.

5.3 Conclusion

Compared to other cellulose solvents that are useful for cellulose derivatization yet restricted to small lab-scale synthesis, ILs bear huge potential for the homogeneous preparation of highly engineered cellulose derivatives on a commercially attractive scale. They can rapidly dissolve large amounts of cellulose and can be utilized as reaction media for numerous derivatization reactions. Nevertheless, it has been demonstrated in this chapter that several very specific issues that are related to the unique physical and chemical properties of ILs need to be considered. Regarding the limitations of currently applied ILs, it can be expected that a ‘next generation’ of task-specific ILs and related reaction media will be advanced in the near future, e. g., by including alternative types of cations and anions as well as IL/co-solvent systems. Another open question is IL recycling. In this context, the thermal behavior and chemical reactivity of ILs during dissolution and chemical derivati — zation of cellulose must be considered even more. If the specific side reaction of ILs are recognized and fully understood, they can be avoided or even exploited for preparing novel types of cellulose derivatives. As concluding remark it should be pointed out that research on the use of ILs for processing of cellulose will benefit a lot from interdisciplinary contributions from areas such as general organic, phys­ical, and theoretical chemistry, chemical — and process engineering, biochemistry, toxicology, material testing, and other related fields.