CIS-01444; No of Pages 7 Advances in Colloid and Interface Science xxx (2014) xxx–xxx
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Brief overview on cellulose dissolution/regeneration interactions and mechanisms Bruno Medronho a,⁎, Björn Lindman b,c a b c
IBB-CGB, Faculty of Sciences and Technology, Ed. 8, University of Algarve, Campus de Gambelas, Faro 8005-139, Portugal Division of Physical Chemistry, Department of Chemistry, Center for Chemistry and Chemical Engineering, Lund University, Lund SE-221 00, Sweden Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
a r t i c l e
i n f o
Available online xxxx Keywords: Cellulose dissolution Cellulose regeneration Hydrophobic interactions Hydrogen bonding Solvents
a b s t r a c t The development of cellulose dissolution/regeneration strategies constitutes an increasingly active research field. These are fundamental aspects of many production processes and applications. A wide variety of suitable solvents for cellulose is already available. Nevertheless, most solvent systems have important limitations, and there is an intense activity in both industrial and academic research aiming to optimize existing solvents and develop new ones. Cellulose solvents are of highly different nature giving great challenges in the understanding of the subtle balance between the different interactions. Here, we briefly review the cellulose dissolution and regeneration mechanisms for some selected solvents. Insolubility is often attributed to strong intermolecular hydrogen bonding between cellulose molecules. However, recent work rather emphasizes the role of cellulose charge and the concomitant ion entropy effects, as well as hydrophobic interactions. © 2014 Published by Elsevier B.V.
Contents 1. Cellulose generalities . . . . . . . . . . 2. Solvents used in cellulose dissolution . . 3. Cellulose dissolution: on the mechanisms 4. Cellulose regeneration . . . . . . . . . 5. Final remarks . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . .
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1. Cellulose generalities Cellulose is a readily available and renewable biopolymer abundantly found in nature, typically combined with lignin and hemicelluloses in the cell wall of upper parts of plants [1]. This homopolysaccharide is formed by linearly connecting D-glucose units condensed through β(1–4) glycosidic bonds (Fig. 1). The degree of polymerization (DP) can vary considerably depending on the source (i.e., DP from 100 up to 20000) [2]. From the single anhydroglucopyranose unit, AGU, up to the micro and macro fibrils, cellulose organizes in a rather complex fashion where an extended intra- and intermolecular network of hydrogen bonds is indicated as the basis of cohesion between cellulose molecules (Fig. 1). It is believed that intramolecular hydrogen bonds provide ⁎ Corresponding author. Tel.: +351 289800910; fax: +351 289818419. E-mail address: [email protected] (B. Medronho).
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chain stiffness, while, on the other hand, intermolecular hydrogen bonds allow the linear polymer molecules to assemble in sheet-like structures [3]. A more recent perspective highlights the amphiphilic nature of cellulose (Fig. 2) [4–6]; the equatorial direction of a glucopyranose ring has a hydrophilic character because all three hydroxyl groups are located on the equatorial positions of the ring. On the other hand, the axial direction of the ring is hydrophobic since the hydrogen atoms of C–H bonds are located on the axial positions of the ring. Thus, cellulose molecules have an intrinsic structural anisotropy where the rather flat ribbons present sides with clear differences in polarity [7–9]. Such structural anisotropy is expected to considerably influence both the microscopic and macroscopic properties of cellulose but has largely been neglected in most discussions. Cellulose can be considered as a semi-crystalline polymer with amorphous regions, of low order, coexisting with higher order crystalline domains [10]. The origin and pretreatments of a cellulose sample
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Please cite this article as: Medronho B, Lindman B, Brief overview on cellulose dissolution/regeneration interactions and mechanisms, Adv Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cis.2014.05.004
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Fig. 1. Molecular structure of cellulose where the extended network of intra and inter-hydrogen bonding is represented. The anhydrocellobiose unit (i.e., disaccharide of two glucose molecules) is highlighted.
determines the degree of crystallinity of cellulose, which typically is found between 40% and 60%. Several suggestions on how these crystalline and non-crystalline regions are intermixed have been developed over the years, such as single crystals or uniform elementary fibrils, but nowadays, the so-called “fringed fibrillar model” is widely accepted [11–16]. In this model, the cellulose nanofibril is not regarded a single crystal but rather as a less structured arrangement of non-uniform crystalline segments accompanied by amorphous parts, both longitudinally and laterally displaced [12]. Apart from being used in unmodified forms, such as wood or cotton, cellulose can be extracted from its natural sources and either used in the paper industry or, in a smaller scale, in some specific applications such as regenerated fibers (e.g., Lyocell or viscose). In fact, it is believed that forest-based raw materials, such as cellulose, can play a major role in replacing fossil oil-based fibers and cotton by new ecological man-made fibers in both woven and non-woven end applications. The success of this replacement is intimately dependent on the develop of new solvents and strategies for dissolution and regeneration. Cellulose may also be modified by chemical, enzymatic or microbiological methods to obtain new valuable derivatives and materials. Large-scale production of cellulose derivatives (mainly ethers and esters) and regenerated materials (i.e., fibers, films, food casing, membranes, and sponges, among others) find applications in several important commercial areas such as the membrane, polymer and paint industries [3]. A challenging issue is the fact that many important applications of cellulose involve its dissolution. Due to the complexity of such a biopolymeric network, the partially crystalline structure and the extended non-covalent interactions among molecules, chemical processing of
cellulose is rather difficult. Cellulose does not melt nor is it soluble in common aqueous and organic solvents [2,17]. However, this polysaccharide is soluble in more exotic media with no apparent common properties [18]. The more consensual vision among leaders in the field has been that the key to dissolve cellulose resides in the solvent capacity to break the above mentioned intra- and intermolecular hydrogen bond network [19]. Other interactions among cellulose molecules have been mostly ignored. Recently, we have reanalyzed this problem and argued against this accepted picture [4–6]. Instead, we have concluded that cellulose has clear amphiphilic properties and a careful examination of the interactions involved suggests that hydrophobic interactions play a significant role in governing cellulose solubility. 2. Solvents used in cellulose dissolution Why is cellulose dissolution so important? The following reasons can be enumerated: preparation of regenerated and innovative materials such as fibers (e.g., textile applications) and films (e.g., packaging applications), production of valuable cellulose derivatives in a homogenous environment (note that typical solvents cannot penetrate inside crystalline regions of cellulose and heterogeneous modification is restricted only to the surface of the crystallites), and finally, to degrade cellulose more efficiently (e.g., important for, for instance, biorefinery purposes). Typically, cellulose dissolution is preceded by polymer swelling, which is defined as a process where the solvent molecules penetrate and labilize the cellulose structure to a certain extent, leaving the volume and physical properties of the biopolymer significantly altered
Fig. 2. Hydrophilic and hydrophobic parts of the cellulose molecule: left, top view of the glucopyranose ring plane highlighting hypothetic hydrogen bonding between the hydroxyl groups located on the equatorial positions of the intermediate ring; right, side view of the glucopyranose ring plane showing the hydrogen atoms of C–H bonds on the axial positions of the ring.
Please cite this article as: Medronho B, Lindman B, Brief overview on cellulose dissolution/regeneration interactions and mechanisms, Adv Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cis.2014.05.004
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while the solid, or semi-solid fractions, remain practically unchanged. On the other hand, complete dissolution is expected to fully destroy the supramolecular structure, resulting in a solution where the polymer is, at best, molecularly dispersed. Interestingly, the same solvent may act as a dissolving agent or merely as a swelling agent, depending on the conditions used in the experiment (i.e., concentrations, temperature, etc.) [6]. The dissolution of a polymer, such as cellulose, in a solvent is, of course, governed by the free energy of mixing [20]. The mixing process will occur spontaneously when the free energy change on mixing is negative. Otherwise, phase separation may result from the mixing process. The polymer molecular weight is a key parameter in dissolution; the higher the molecular weight, the weaker is the entropic driving force contribution for dissolution [21]. Under these conditions, the enthalpy term is crucial in determining the sign of the Gibbs free energy change. One should mention that polymer dissolution is often controlled by kinetics rather than by thermodynamics and cellulose is a very clear example of this. From a thermodynamic point of view, a reasonable solvent for cellulose dissolution must be able to overcome the low entropy gain by favorable solvent/polymer interactions. In practice, finding a non-derivatizing solvent that does not reduce the DP nor does react with the cellulose is one the most challenging parts in cellulose dissolution. From the traditional viscose route, which is generally a slow process and environmentally hostile (e.g., discharge of toxic gases), up to state-of-art ionic liquid systems, several derivatizing and non-derivatizing (aqueous and non- aqueous) solvents, with strikingly unrelated properties, have been developed [6]. The list is vast and includes quite unusual combinations and experimental conditions: simple or multicomponent mixtures, aqueous and organic media, inorganic and organic salts, high and low temperatures, high and low pHs, etc. A few classical examples of non-derivatizing solvents can be mentioned, such as aqueous inorganic complexes (e.g., cuprammonium hydroxide), concentrated salt solutions (e.g., zinc chloride, ammonium, calcium and sodium thiocyanate solutions), salts dissolved in organic solvents (e.g., lithium chloride/N,N-dimethylacetamide, ammonia/ ammonium salt, tetrabutylammonium fluoride/dimethyl sulfoxide), aqueous alkali (lithium hydroxide or sodium hydroxide solutions), etc. [22]. A more detailed and updated list can be find elsewhere [1,6,23]. What most of these solvents have in common, for different reasons, is the fact that they are not easily scaled up. Recently, developed processes for producing regenerated cellulose fibers are emerging as possible alternatives to the viscose process. Among them, the non-derivatizing N-methylmorpholine-N-oxide (NMMO) process [24,25] has been successful on the industrial scale, even if the recovery of the NMMO solvent is complicated, energy demanding, and costly. These are reasons enough to believe that this system does not meet all the requirements for a complete replacement of the viscose technology. Ionic liquids (ILs) have been extensively studied in recent years since these nonvolatile organic solvents are capable of dissolving large amounts of cellulose and other carbohydrates [26]. However, there are some important limiting factors for large-scale applications of ILs such as their high cost of production, high viscosity, sensitivity to moisture content and poorly developed appropriate purification processes [27,28].
3. Cellulose dissolution: on the mechanisms Regarding the mechanism of dissolution, we already mentioned above that the widely accepted picture considers, almost exclusively, that cellulose dissolution results from the solvent ability to eliminate the inter- and intramolecular hydrogen bonds among biopolymer molecules. On the other hand, cellulose can be regarded as an amphiphilic molecule and thus research aiming in creating a basis for the development of new solvents should focus, not only in eliminating hydrogen bonding but, more importantly, on eliminating hydrophobic interactions. There are several arguments to believe in an amphiphilicity of
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cellulose and the highlighted role of hydrophobic interactions and this has been recently discussed [4–6,29]. Let us now discuss the suggested mechanisms in literature for some of the classical solvents used for cellulose dissolution. In the aqueous complexing systems, such as the cuprammonium hydroxide, “cuam,” the suggested mechanism assumes that the metal complexes dissolve cellulose via initial deprotonation of the hydroxyls followed by a coordinative binding to the hydroxyl groups in the C2 and C3 positions of each anhydroglucose unit [30]. In the case of the cold alkali, one of the leading opinions is that NaOH forms hydrates with water capable to break the inter- and intramolecular hydrogen bonds between cellulose molecules [31]. These hydrates would have the capacity to bond with one or two hydroxyl groups of each AGU. On the other hand, our vision is that the charging effect, in solvent systems such as the alkali, should not be neglected [32]; when a neutral molecule, such as cellulose, is charged up, its solubility is expected to increase and thus cellulose tends to be more soluble/be more penetrated at either high or low pH. This is because the dissociated counterions strongly contribute to the translational entropy of mixing. We note that this system is limited to moderately low DPs and actually this has been used to explain the difficulties faced in disruption of the long-range order inherent to the higher DPs. Another particularity of the cold alkali track, is that the concentration range where NaOH works as a direct solvent is very narrow. This has been discussed on the basis of a concentration dependent size of the above mentioned NaOH/water hydrates [33]; at lower alkali concentrations, these hydrates are speculated to have hydrodynamic diameters too large to penetrate and diffuse into the very densely packed crystalline regions of cellulose. The performance of these alkali systems can be increased when certain additives, such as zinc oxide (ZnO), urea and thiourea are present even if their role is still unclear. ZnO has been suggested to form strong hydrogen bonds between cellulose and Zn(OH)2− 4 , a stable zinc hydroxide species in solution [34]. Regarding urea or thiourea, some authors argue that urea hydrates possibly self-assemble at the surface of the NaOH hydrogen-bonded cellulose to form an inclusion complex (IC), which would lead to full dissolution [35–39]. Zhou et al. state that the role of urea and thiourea is to function as hydrogen bond donors and acceptors to prevent the reassociation of cellulose molecules, therefore conferring stability to the solution [40]. Molecular dynamic (MD) simulations indicate that, in contrast to water molecules, the cellulose chain prefers to interact with urea molecules [41]. A stronger and more stable interaction of cellulose with urea would decrease the self-interaction of cellulose chains and promote dissolution of cellulose in urea-containing solvent mixtures. Kunze and Fink [42] argued that NaOH and urea hydrates work in synergy to separate the cellulose chains from each other. They are also supposed to prevent cellulose from regenerating its intermolecular hydrogen bonding by forming a stable “hydrate coat” on their surface. The same conclusion is shared by Isobe et al. [43], which have recently reanalyzed the role of urea in NaOH solvent system. These authors have concluded that urea has no direct interaction with cellulose but helps the alkali to penetrate into cellulose crystalline regions, stabilizing the swollen cellulose molecules. In addition, the authors propose that the stabilizing effect of urea derives from the fact that urea hinders the hydrophobic association of cellulose [43]. These ideas follow the established role of urea in protein denaturation [44]. Recent theoretical work, based on molecular modeling [45] and Kirkwood–Buff solution theory [46], suggests a preferential association of urea to the hydrophobic sites of proteins. This alternative idea highlights the amphiphilic properties of cellulose, and therefore, it is believed that urea may weaken the entropic effect by accumulating in the proximity of the hydrophobic surface of cellulose leading to a higher thermal stability of cellulose in aqueous alkali solvent [43]. These suggestions clearly support our recent discussions, which emphasize the role of hydrophobic interactions in cellulose insolubility [4–6,29]. Another important variable in NaOH-based solvents is the relatively low temperature needed to efficiently dissolve cellulose in opposition to
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the standard picture of solubility, which implies that the entropic driving force is strengthened with increasing temperature. Typically, sub-zero temperatures are required for dissolution in soda [47]. This phenomenon has been analyzed thermodynamically by the total enthalpy of cellulose dissolution [48]. It was suggested that the only endothermic term is associated with breaking the hydrogen bonds in the crystalline regions. All other terms relate to interactions between cellulose hydroxyl groups and the solvent system and are exothermic. Therefore, the conclusion is that the overall process of cellulose dissolution is exothermic and is thus favored by a lower temperature. Additionally, the network of solvent hydrates (i.e., Na+ and OH− ions surrounded by a “cage” of water molecules) is suggested to be temperature dependent probably due to the increasing hydrogen bonding strength [49]. As mentioned above, some authors believe that this network of solvent hydrates disrupts mainly the closely chain packed cellulose through the formation of new hydrogen bonds between cellulose and NaOH hydrates. Thus, when the temperature is raised, the hydrogen bonds are assumed to be weaker and the network of hydrates is gradually destroyed [49]. Regarding the temperature dependence, our description of the phenomenon is based on temperature-dependent conformational changes in the cellulose chain, which would make the polymer less polar at higher temperatures, thus reducing the attractive interactions with the polar solvent and vice-versa [5]. Pinkert et al. [50] offers a different view based on the idea that the inability of NaOH-based solvent systems to dissolve cellulose at increased temperatures strongly suggests that an ordered solvent state of some kind is required to achieve dissolution. In the case of the NMMO solvent, the active moiety is the NO group with its strongly dipolar character. It is believed that the oxygen in this group is able to form one or two hydrogen bonds with AGU of cellulose. The proposed mechanism assumes the cleavage of intermolecular hydrogen bonds of cellulose and the formation of a soluble complex of stronger hydrogen bonds between the cellulosic hydroxyls and the NO group of NMMO [18]. For mixed inorganic/organic systems such as lithium chloride/ dimethylacetamide (LiCl/DMAc), several dissolution mechanisms have been suggested [51,52]. Generally, the dissolving mechanism is believed to go via an intermediate involving the interaction of Cl− (due to its basicity) with cellulose in addition to an exchange of DMAc in the lithium co-ordination sphere by cellulose hydroxyl groups (Fig. 3). Accumulation of C1− along the cellulose chain produces a negatively charged polymer with the macrocation, [Li-DMAc], as the counterion. This process can be regarded as a polyelectrolyte effect, where polymer molecules are forced apart due to charge repulsion. Moreover, the high osmotic pressure drives a continuous influx of solvent resulting in further disruption of the cellulose binding forces until the polymer is totally solvated. A similar polyelectrolyte effect is suggested for the tetrabutylammonium fluoride/dimethyl sulfoxide (TBAF/DMSO)
system where the solubilization of the biopolymer is expected to be enhanced by the electrostatic repulsion between the negatively-charged cellulose chains (due to the condensation of F−) [53]. Regarding ionic liquids, although great experimental and computational progress has been made, there is no clear understanding of the role of individual ionic species in dissolution. Up to date, two main views of the interactions between ILs and cellulose prevail: (1) the dissolution process is governed by the interactions between the anion and the biopolymer, with no specific role for the cation; and (2) the major driving force for cellulose dissolution comes from the H-bond interactions of the cellulose hydroxyls with the anion and the cation of ILs. In literature, we find several examples where either changing the cation or the anion produces remarkable effects on dissolution efficiency. Interestingly, some of the good hydrogen-bond-accepting anions that have been found to dissolve cellulose can be rendered ineffective through pairing with certain cations [54]. One of the structural features that makes ILs so interesting is their strong asymmetry. Typically, the cations are bulky species with amphiphilic properties. Such amphiphilicity is normally not considered when discussing the mechanism of dissolution of cellulose. This is particularly relevant since crystalline cellulose has an amphipathic-like structure (Fig. 1b): hydrophobic surfaces consisting of pyranose-ring hydrogens and hydrophilic regions arising from the hydroxyl groups directed toward the sides of the ring. The non-polar surfaces can be organized into hydrophobic sheets paired against one another, rather than structuring large amounts of water in solution. (Note that in this type of interaction, the driving force for association is not simply van der Waals' interactions but rather hydrophobic association driven by the liberation of structured water molecules [55].) We believe that the dissolution of an amphiphilic polymer, such as cellulose, would be facilitated in amphiphilic solvents. Therefore, the amphiphilic properties of all cations in ILs fit in our suggestion. In fact, this also follows from the earlier discussion on the effect of additives such as PEG, urea, and thiourea on dissolution in alkali solutions.
4. Cellulose regeneration Typically, the regeneration of cellulose occurs when contacting the cellulose solution with a coagulation bath. The polymer profile at the point of precipitation exhibits a very high interfacial concentration, thus favoring the formation of a dense polymer “skin.” The bulk of the sample is at near the initial concentration and is in a fluid state. Thus, a rapid inflow of the coagulant can take place through the weak points at the skin interface. Rapid growth of finger-like voids in the fluid region is expected to occur due to the moving interface created by the coagulant (less viscous) and the solution (more viscous). The kinetics of regeneration is mainly controlled by the relative velocities of the counter-diffusion process—diffusion of the solvent from the solution
+
Fig. 3. Example of a hydrogen-bond breaking mechanism for the cellulose dissolution in the lithium chloride/dimethylacetamide (LiCl/DMAc) solvent system. Cellulose hydroxyl groups interact with an intermediate (Li+-DMAC macrocation) via hydrogen bonding bridged by the chloride anion. The lithium cation interacts with the carbonyl oxygen via an ion–dipole interaction.
Please cite this article as: Medronho B, Lindman B, Brief overview on cellulose dissolution/regeneration interactions and mechanisms, Adv Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cis.2014.05.004
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into the coagulation bath and the non-solvent from the bath into the solution [56]. The exchange of solvent with non-solvent leads to a desolvation of the cellulose molecules and to the supposed reformation of the intraand inter-molecular hydrogen bonds [57]. The regeneration of cellulose from, for instance, NMMO solutions using water as the coagulation system results from a phase separation process. Typically, two mechanisms of phase separation can take place during liquid-liquid demixing of polymer solutions; either nucleation and growth (i.e., the nuclei of one phase grow in the mixture) or spinodal decomposition (i.e., a periodic variation of concentration leads to the final phase separation). In the NMMO-water system, if there is a clear difference between these two mechanisms in their first stages (nucleation and growth shows isolated entities while spinodal decomposition has a 3D network-like morphology), typically both tend to the same morphology at the end due to surface tension effects [58]. The mechanical and surface chemical properties of regenerated cellulose are known to depend strongly on the type of cellulose solvent and coagulant. For a bibliography, the reader is directed to the work of Isobe et al. [59] and references therein. For instance, in molten salts systems, the regenerated morphologies can vary significantly depending on the salt used; in thiocyanate melts, fiber-like samples (similar to the raw cellulose) have been obtained while lamellar-like samples and layered structures were regenerated from LiClO4 · 3H2O and chlorides, respectively [60]. In IL systems, the presence of water affects many of the solvent properties. To date, experimental studies on cellulose/IL/non-solvent ternary systems have not been conclusive, and a mechanistic understanding of the interactions between water, IL, and cellulose remains unknown. For instance, MD results are taken to indicate that the ordered cationanion polar interaction network is disrupted by water that forms a network of interactions with the anion and with the cation [61]. More specifically, the suggested mechanism assumes that while water diffuses inside the first solvation shell of cellulose, the number of hydrogen bonds among water molecules and the polymer increases and the number of H-bonds between the anion and the sugar decreases. Meanwhile, the cations are considered to be maintained in the second solvation shell of cellulose due to their strong electrostatic interactions with the anions present. The formation of this network displaces the cation out of the first solvation shell leading to cellulose precipitation. In this model, phase separation of the cellulose as a function of water results from competition between the water–anion, cellulose–anion, cellulose–water, cellulose–cation, and anion–cation interactions [61]. The regeneration mechanism for alkali systems suggests that the inclusion complex associated with cellulose, NaOH, and, in some cases, urea or thiourea hydrates is disrupted by adding a non-solvent such as water, leading to the self-association of cellulose. The regenerated cellulose film is said to be formed through a rearrangement of the hydrogen bonds [62,63]. Moreover, in acidic non-solvents, the H+ assumes a key role to trigger cellulose regeneration by neutralizing the alkaline content. Additionally, as in the Lenzing viscose production process, the addition of a strong electrolyte to H2SO4, such as Na2SO4 and ZnSO4, can reduce the H+ concentration in the coagulant, leading to a counterdiffusion rate and slower acid–alkali neutralization process than by just H2SO4 alone. This will considerably influence the final properties of the regenerated cellulose material [10]. Recently, a very interesting study has been presented by Isobe et al. [59]. The regeneration of cellulose, either using a coagulant or upon heating, was followed in an aqueous alkali-urea solvent by monitoring the process by time-resolved synchrotron X-ray scattering. The authors suggested that when the medium surrounding the cellulose molecules becomes energetically unfavorable for molecular dispersion, regeneration starts, and the initial process would consist in stacking the hydrophobic glucopyranoside rings (driven by hydrophobic interactions) to form monomolecular sheets, which then would line up by hydrogen
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bonding to form Na–cellulose type IV crystallites, a hydrate form of cellulose II (Fig. 4). This constitutes the first experimental evidence of the development of hydrophobically stacked monomolecular sheets which was firstly hypothesized by Hermans [64] and later by Hayashi et al. [65]. These authors found that structural disorders lie mainly in the hydrogenbonded intermolecular region and assumed the primary structure as a monomolecular sheet formed by stacking of glucopyranoside planes by van der Waals forces. Recently, the theoretical work of Miyamoto et al. [9] simulated the regeneration of cellulose by MD, reproducing the hypothesis of Hermans and Hayashi. While these studies gave a consistent picture of the primary structure formation of regenerated cellulose, they were either inferred from the resulting structure or based on computer simulations. Therefore, the work of Isobe et al. [59] not only shines light into the regeneration mechanism of cellulose but also constitutes an alternative vision to the typical regeneration mechanism found in literature which, as we have seen, essentially focuses exclusively on the reformation of the broken inter- and intramolecular hydrogen bonds among cellulose molecules. 5. Final remarks From this brief review on cellulose dissolution and regeneration, it is clear that quite contrasting starting points have been chosen through the years for the analysis of the mechanisms. As can be inferred, the nature of the solvents that can efficiently dissolve cellulose covers an enormous range making it difficult to provide a single consistent mechanism. While we recognize that there may be different mechanisms for different cases, it seems important to make a classification based on some simple views on solubility, in general, and polymer solubility, in particular. Solubility is governed by entropy, notably translational and configurational entropy, the enthalpy in general, being unfavorable. Regarding translational entropy, this will be very much higher for ionic polymers with dissociating counterions than for non-ionic polymers. This is amply demonstrated for cellulose where protonation or deprotonation, due to addition of simple acids and bases, offers efficient ways of swelling and dissolution of cellulose. An effective ionization can be obtained not only from changes in the state of protonation but also from association/adsorption of an ionic species. Thus, it is well known that the solubility of non-ionic polymers, such as poly(ethylene glycol) and cellulose derivatives, can be strongly increased by association of ions. This is particularly efficient for ionic surfactants but also ions with a high polarizability can act as pseudo-surfactants. Actually, this is the basis for the lyotropic series, often referred to as the Hofmeister series. Thus, it is found that large anions, such as iodide, can increase the solubility of non-ionic macromolecules, whereas small ions with a high charge density, have the opposite effect. The large polarizable ions are effectively hydrophobic and tend to have a low solubility in a polar solvent and/ or being enriched at a hydrophobic surface. Generally speaking, we can thus expect that a highly unsymmetrical electrolyte would provide good solubility conditions for a non-ionic polymer such as cellulose, since the ions would be non-uniformly distributed in the solution. This is the case for molten salt systems with small, strong polarizing cations (i.e., Li+ and Zn2+) and large polarizable anions but, from the broad overview above, looks like a rather general trend since many of the efficient solvents of cellulose follow this pattern, including many of the ionic liquids. As we have previously argued, cellulose manifests strong hydrophobic properties and is thus amphiphilic [4,5]. As arguments for amphiphilic properties we can mention the ability of other polyglucoses, such as cyclodextrin and amylose, to solubilize non-polar and amphiphilic compounds in aqueous solution. It is also strongly supported by simulations [66]. Another clear support for amphiphilic properties can be found from studies of surfactant binding to cellulose derivatives. Thus, surfactants bind to cellulose derivatives, soluble due to
Please cite this article as: Medronho B, Lindman B, Brief overview on cellulose dissolution/regeneration interactions and mechanisms, Adv Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cis.2014.05.004
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Time
Intensity (a.u.)
Complete regeneration
control
q (Å-1)
Fig. 4. On top, the time evolution of synchrotron x-ray diffraction profiles of a cellulose solution under regeneration in a 5 wt% Na2SO4 aqueous solution is shown. In the control (10 wt.% cellulose solution without coagulant) no Bragg reflections are observed. After a certain time, a first Bragg reflection (110) is observed while the second one (020) is only visible after complete regeneration. On bottom, a schematic illustration of cellulose regeneration deduced from the X-ray data, where first the hydrophobic stacking of cellulose molecules occurs and then is followed by their binding via hydrogen bonding (adapted from reference [55]).
hydrophilic groups, which do not associate with surfactants. An illustrative example is that of cationic hydroxyethyl cellulose, which binds anionic surfactant in excess of charge stoichiometry; obviously, this can only be due to hydrophobic interactions between the cellulose backbone and the alkyl chain of the surfactant [67]. In view of this, eliminating the hydrophobic interactions between cellulose molecules must be very important for the aqueous solubility of cellulose. We note that urea can facilitate cellulose dissolution. Urea has a much lower polarity than water and is well known to eliminate hydrophobic association in water. As alluded to above, it acts as protein denaturant by reducing intramolecular hydrophobic association. Furthermore, it inhibits hydrophobic association of surfactants as can be seen from the increase in critical micelle concentration on urea addition. Other substances of intermediate polarity, such as urea derivatives and poly(ethylene glycol), have similar effects and are well-known enhancers of the aqueous solubility of cellulose. The alternative notion to hydrophobic interactions as the limiting factor of dissolution of cellulose in water is hydrogen-bonding. However, this would require that the carbohydratecarbohydrate hydrogen-bonding would be much stronger than watercarbohydrate and water-water hydrogen bonding, which is not the case; regarding a solo hydrogen–bonding mechanism, it is striking that all these interactions are very similar in magnitude; ca. 5 kcal/mol. Furthermore, a hydrogen-bonding mechanism is inconsistent with a high aqueous solubility of polysaccharides, which have as high hydrogenbonding capacity as cellulose.
Acknowledgments The authors acknowledge support from the Portuguese Foundation for Science and Technology (FCT, project PTDC/AGR-TEC/4049/2012 and a postdoc grant assigned to B. Medronho, SFRH/BPD/74540/2010). Financial support was also received from Södra Skogsägarnas Stiftelse, Stiftelsen Nils och Dorthi Troëdssons forskningsfond and the CelluNova consortium.
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Contents lists available at ScienceDirect
Advances in Colloid and Interface Science journal homepage: www.elsevier.com/locate/cis
Brief overview on cellulose dissolution/regeneration interactions and mechanisms Bruno Medronho a,⁎, Björn Lindman b,c a b c
IBB-CGB, Faculty of Sciences and Technology, Ed. 8, University of Algarve, Campus de Gambelas, Faro 8005-139, Portugal Division of Physical Chemistry, Department of Chemistry, Center for Chemistry and Chemical Engineering, Lund University, Lund SE-221 00, Sweden Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
a r t i c l e
i n f o
Available online xxxx Keywords: Cellulose dissolution Cellulose regeneration Hydrophobic interactions Hydrogen bonding Solvents
a b s t r a c t The development of cellulose dissolution/regeneration strategies constitutes an increasingly active research field. These are fundamental aspects of many production processes and applications. A wide variety of suitable solvents for cellulose is already available. Nevertheless, most solvent systems have important limitations, and there is an intense activity in both industrial and academic research aiming to optimize existing solvents and develop new ones. Cellulose solvents are of highly different nature giving great challenges in the understanding of the subtle balance between the different interactions. Here, we briefly review the cellulose dissolution and regeneration mechanisms for some selected solvents. Insolubility is often attributed to strong intermolecular hydrogen bonding between cellulose molecules. However, recent work rather emphasizes the role of cellulose charge and the concomitant ion entropy effects, as well as hydrophobic interactions. © 2014 Published by Elsevier B.V.
Contents 1. Cellulose generalities . . . . . . . . . . 2. Solvents used in cellulose dissolution . . 3. Cellulose dissolution: on the mechanisms 4. Cellulose regeneration . . . . . . . . . 5. Final remarks . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . .
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1. Cellulose generalities Cellulose is a readily available and renewable biopolymer abundantly found in nature, typically combined with lignin and hemicelluloses in the cell wall of upper parts of plants [1]. This homopolysaccharide is formed by linearly connecting D-glucose units condensed through β(1–4) glycosidic bonds (Fig. 1). The degree of polymerization (DP) can vary considerably depending on the source (i.e., DP from 100 up to 20000) [2]. From the single anhydroglucopyranose unit, AGU, up to the micro and macro fibrils, cellulose organizes in a rather complex fashion where an extended intra- and intermolecular network of hydrogen bonds is indicated as the basis of cohesion between cellulose molecules (Fig. 1). It is believed that intramolecular hydrogen bonds provide ⁎ Corresponding author. Tel.: +351 289800910; fax: +351 289818419. E-mail address: [email protected] (B. Medronho).
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chain stiffness, while, on the other hand, intermolecular hydrogen bonds allow the linear polymer molecules to assemble in sheet-like structures [3]. A more recent perspective highlights the amphiphilic nature of cellulose (Fig. 2) [4–6]; the equatorial direction of a glucopyranose ring has a hydrophilic character because all three hydroxyl groups are located on the equatorial positions of the ring. On the other hand, the axial direction of the ring is hydrophobic since the hydrogen atoms of C–H bonds are located on the axial positions of the ring. Thus, cellulose molecules have an intrinsic structural anisotropy where the rather flat ribbons present sides with clear differences in polarity [7–9]. Such structural anisotropy is expected to considerably influence both the microscopic and macroscopic properties of cellulose but has largely been neglected in most discussions. Cellulose can be considered as a semi-crystalline polymer with amorphous regions, of low order, coexisting with higher order crystalline domains [10]. The origin and pretreatments of a cellulose sample
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Please cite this article as: Medronho B, Lindman B, Brief overview on cellulose dissolution/regeneration interactions and mechanisms, Adv Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cis.2014.05.004
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Fig. 1. Molecular structure of cellulose where the extended network of intra and inter-hydrogen bonding is represented. The anhydrocellobiose unit (i.e., disaccharide of two glucose molecules) is highlighted.
determines the degree of crystallinity of cellulose, which typically is found between 40% and 60%. Several suggestions on how these crystalline and non-crystalline regions are intermixed have been developed over the years, such as single crystals or uniform elementary fibrils, but nowadays, the so-called “fringed fibrillar model” is widely accepted [11–16]. In this model, the cellulose nanofibril is not regarded a single crystal but rather as a less structured arrangement of non-uniform crystalline segments accompanied by amorphous parts, both longitudinally and laterally displaced [12]. Apart from being used in unmodified forms, such as wood or cotton, cellulose can be extracted from its natural sources and either used in the paper industry or, in a smaller scale, in some specific applications such as regenerated fibers (e.g., Lyocell or viscose). In fact, it is believed that forest-based raw materials, such as cellulose, can play a major role in replacing fossil oil-based fibers and cotton by new ecological man-made fibers in both woven and non-woven end applications. The success of this replacement is intimately dependent on the develop of new solvents and strategies for dissolution and regeneration. Cellulose may also be modified by chemical, enzymatic or microbiological methods to obtain new valuable derivatives and materials. Large-scale production of cellulose derivatives (mainly ethers and esters) and regenerated materials (i.e., fibers, films, food casing, membranes, and sponges, among others) find applications in several important commercial areas such as the membrane, polymer and paint industries [3]. A challenging issue is the fact that many important applications of cellulose involve its dissolution. Due to the complexity of such a biopolymeric network, the partially crystalline structure and the extended non-covalent interactions among molecules, chemical processing of
cellulose is rather difficult. Cellulose does not melt nor is it soluble in common aqueous and organic solvents [2,17]. However, this polysaccharide is soluble in more exotic media with no apparent common properties [18]. The more consensual vision among leaders in the field has been that the key to dissolve cellulose resides in the solvent capacity to break the above mentioned intra- and intermolecular hydrogen bond network [19]. Other interactions among cellulose molecules have been mostly ignored. Recently, we have reanalyzed this problem and argued against this accepted picture [4–6]. Instead, we have concluded that cellulose has clear amphiphilic properties and a careful examination of the interactions involved suggests that hydrophobic interactions play a significant role in governing cellulose solubility. 2. Solvents used in cellulose dissolution Why is cellulose dissolution so important? The following reasons can be enumerated: preparation of regenerated and innovative materials such as fibers (e.g., textile applications) and films (e.g., packaging applications), production of valuable cellulose derivatives in a homogenous environment (note that typical solvents cannot penetrate inside crystalline regions of cellulose and heterogeneous modification is restricted only to the surface of the crystallites), and finally, to degrade cellulose more efficiently (e.g., important for, for instance, biorefinery purposes). Typically, cellulose dissolution is preceded by polymer swelling, which is defined as a process where the solvent molecules penetrate and labilize the cellulose structure to a certain extent, leaving the volume and physical properties of the biopolymer significantly altered
Fig. 2. Hydrophilic and hydrophobic parts of the cellulose molecule: left, top view of the glucopyranose ring plane highlighting hypothetic hydrogen bonding between the hydroxyl groups located on the equatorial positions of the intermediate ring; right, side view of the glucopyranose ring plane showing the hydrogen atoms of C–H bonds on the axial positions of the ring.
Please cite this article as: Medronho B, Lindman B, Brief overview on cellulose dissolution/regeneration interactions and mechanisms, Adv Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cis.2014.05.004
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while the solid, or semi-solid fractions, remain practically unchanged. On the other hand, complete dissolution is expected to fully destroy the supramolecular structure, resulting in a solution where the polymer is, at best, molecularly dispersed. Interestingly, the same solvent may act as a dissolving agent or merely as a swelling agent, depending on the conditions used in the experiment (i.e., concentrations, temperature, etc.) [6]. The dissolution of a polymer, such as cellulose, in a solvent is, of course, governed by the free energy of mixing [20]. The mixing process will occur spontaneously when the free energy change on mixing is negative. Otherwise, phase separation may result from the mixing process. The polymer molecular weight is a key parameter in dissolution; the higher the molecular weight, the weaker is the entropic driving force contribution for dissolution [21]. Under these conditions, the enthalpy term is crucial in determining the sign of the Gibbs free energy change. One should mention that polymer dissolution is often controlled by kinetics rather than by thermodynamics and cellulose is a very clear example of this. From a thermodynamic point of view, a reasonable solvent for cellulose dissolution must be able to overcome the low entropy gain by favorable solvent/polymer interactions. In practice, finding a non-derivatizing solvent that does not reduce the DP nor does react with the cellulose is one the most challenging parts in cellulose dissolution. From the traditional viscose route, which is generally a slow process and environmentally hostile (e.g., discharge of toxic gases), up to state-of-art ionic liquid systems, several derivatizing and non-derivatizing (aqueous and non- aqueous) solvents, with strikingly unrelated properties, have been developed [6]. The list is vast and includes quite unusual combinations and experimental conditions: simple or multicomponent mixtures, aqueous and organic media, inorganic and organic salts, high and low temperatures, high and low pHs, etc. A few classical examples of non-derivatizing solvents can be mentioned, such as aqueous inorganic complexes (e.g., cuprammonium hydroxide), concentrated salt solutions (e.g., zinc chloride, ammonium, calcium and sodium thiocyanate solutions), salts dissolved in organic solvents (e.g., lithium chloride/N,N-dimethylacetamide, ammonia/ ammonium salt, tetrabutylammonium fluoride/dimethyl sulfoxide), aqueous alkali (lithium hydroxide or sodium hydroxide solutions), etc. [22]. A more detailed and updated list can be find elsewhere [1,6,23]. What most of these solvents have in common, for different reasons, is the fact that they are not easily scaled up. Recently, developed processes for producing regenerated cellulose fibers are emerging as possible alternatives to the viscose process. Among them, the non-derivatizing N-methylmorpholine-N-oxide (NMMO) process [24,25] has been successful on the industrial scale, even if the recovery of the NMMO solvent is complicated, energy demanding, and costly. These are reasons enough to believe that this system does not meet all the requirements for a complete replacement of the viscose technology. Ionic liquids (ILs) have been extensively studied in recent years since these nonvolatile organic solvents are capable of dissolving large amounts of cellulose and other carbohydrates [26]. However, there are some important limiting factors for large-scale applications of ILs such as their high cost of production, high viscosity, sensitivity to moisture content and poorly developed appropriate purification processes [27,28].
3. Cellulose dissolution: on the mechanisms Regarding the mechanism of dissolution, we already mentioned above that the widely accepted picture considers, almost exclusively, that cellulose dissolution results from the solvent ability to eliminate the inter- and intramolecular hydrogen bonds among biopolymer molecules. On the other hand, cellulose can be regarded as an amphiphilic molecule and thus research aiming in creating a basis for the development of new solvents should focus, not only in eliminating hydrogen bonding but, more importantly, on eliminating hydrophobic interactions. There are several arguments to believe in an amphiphilicity of
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cellulose and the highlighted role of hydrophobic interactions and this has been recently discussed [4–6,29]. Let us now discuss the suggested mechanisms in literature for some of the classical solvents used for cellulose dissolution. In the aqueous complexing systems, such as the cuprammonium hydroxide, “cuam,” the suggested mechanism assumes that the metal complexes dissolve cellulose via initial deprotonation of the hydroxyls followed by a coordinative binding to the hydroxyl groups in the C2 and C3 positions of each anhydroglucose unit [30]. In the case of the cold alkali, one of the leading opinions is that NaOH forms hydrates with water capable to break the inter- and intramolecular hydrogen bonds between cellulose molecules [31]. These hydrates would have the capacity to bond with one or two hydroxyl groups of each AGU. On the other hand, our vision is that the charging effect, in solvent systems such as the alkali, should not be neglected [32]; when a neutral molecule, such as cellulose, is charged up, its solubility is expected to increase and thus cellulose tends to be more soluble/be more penetrated at either high or low pH. This is because the dissociated counterions strongly contribute to the translational entropy of mixing. We note that this system is limited to moderately low DPs and actually this has been used to explain the difficulties faced in disruption of the long-range order inherent to the higher DPs. Another particularity of the cold alkali track, is that the concentration range where NaOH works as a direct solvent is very narrow. This has been discussed on the basis of a concentration dependent size of the above mentioned NaOH/water hydrates [33]; at lower alkali concentrations, these hydrates are speculated to have hydrodynamic diameters too large to penetrate and diffuse into the very densely packed crystalline regions of cellulose. The performance of these alkali systems can be increased when certain additives, such as zinc oxide (ZnO), urea and thiourea are present even if their role is still unclear. ZnO has been suggested to form strong hydrogen bonds between cellulose and Zn(OH)2− 4 , a stable zinc hydroxide species in solution [34]. Regarding urea or thiourea, some authors argue that urea hydrates possibly self-assemble at the surface of the NaOH hydrogen-bonded cellulose to form an inclusion complex (IC), which would lead to full dissolution [35–39]. Zhou et al. state that the role of urea and thiourea is to function as hydrogen bond donors and acceptors to prevent the reassociation of cellulose molecules, therefore conferring stability to the solution [40]. Molecular dynamic (MD) simulations indicate that, in contrast to water molecules, the cellulose chain prefers to interact with urea molecules [41]. A stronger and more stable interaction of cellulose with urea would decrease the self-interaction of cellulose chains and promote dissolution of cellulose in urea-containing solvent mixtures. Kunze and Fink [42] argued that NaOH and urea hydrates work in synergy to separate the cellulose chains from each other. They are also supposed to prevent cellulose from regenerating its intermolecular hydrogen bonding by forming a stable “hydrate coat” on their surface. The same conclusion is shared by Isobe et al. [43], which have recently reanalyzed the role of urea in NaOH solvent system. These authors have concluded that urea has no direct interaction with cellulose but helps the alkali to penetrate into cellulose crystalline regions, stabilizing the swollen cellulose molecules. In addition, the authors propose that the stabilizing effect of urea derives from the fact that urea hinders the hydrophobic association of cellulose [43]. These ideas follow the established role of urea in protein denaturation [44]. Recent theoretical work, based on molecular modeling [45] and Kirkwood–Buff solution theory [46], suggests a preferential association of urea to the hydrophobic sites of proteins. This alternative idea highlights the amphiphilic properties of cellulose, and therefore, it is believed that urea may weaken the entropic effect by accumulating in the proximity of the hydrophobic surface of cellulose leading to a higher thermal stability of cellulose in aqueous alkali solvent [43]. These suggestions clearly support our recent discussions, which emphasize the role of hydrophobic interactions in cellulose insolubility [4–6,29]. Another important variable in NaOH-based solvents is the relatively low temperature needed to efficiently dissolve cellulose in opposition to
Please cite this article as: Medronho B, Lindman B, Brief overview on cellulose dissolution/regeneration interactions and mechanisms, Adv Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cis.2014.05.004
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the standard picture of solubility, which implies that the entropic driving force is strengthened with increasing temperature. Typically, sub-zero temperatures are required for dissolution in soda [47]. This phenomenon has been analyzed thermodynamically by the total enthalpy of cellulose dissolution [48]. It was suggested that the only endothermic term is associated with breaking the hydrogen bonds in the crystalline regions. All other terms relate to interactions between cellulose hydroxyl groups and the solvent system and are exothermic. Therefore, the conclusion is that the overall process of cellulose dissolution is exothermic and is thus favored by a lower temperature. Additionally, the network of solvent hydrates (i.e., Na+ and OH− ions surrounded by a “cage” of water molecules) is suggested to be temperature dependent probably due to the increasing hydrogen bonding strength [49]. As mentioned above, some authors believe that this network of solvent hydrates disrupts mainly the closely chain packed cellulose through the formation of new hydrogen bonds between cellulose and NaOH hydrates. Thus, when the temperature is raised, the hydrogen bonds are assumed to be weaker and the network of hydrates is gradually destroyed [49]. Regarding the temperature dependence, our description of the phenomenon is based on temperature-dependent conformational changes in the cellulose chain, which would make the polymer less polar at higher temperatures, thus reducing the attractive interactions with the polar solvent and vice-versa [5]. Pinkert et al. [50] offers a different view based on the idea that the inability of NaOH-based solvent systems to dissolve cellulose at increased temperatures strongly suggests that an ordered solvent state of some kind is required to achieve dissolution. In the case of the NMMO solvent, the active moiety is the NO group with its strongly dipolar character. It is believed that the oxygen in this group is able to form one or two hydrogen bonds with AGU of cellulose. The proposed mechanism assumes the cleavage of intermolecular hydrogen bonds of cellulose and the formation of a soluble complex of stronger hydrogen bonds between the cellulosic hydroxyls and the NO group of NMMO [18]. For mixed inorganic/organic systems such as lithium chloride/ dimethylacetamide (LiCl/DMAc), several dissolution mechanisms have been suggested [51,52]. Generally, the dissolving mechanism is believed to go via an intermediate involving the interaction of Cl− (due to its basicity) with cellulose in addition to an exchange of DMAc in the lithium co-ordination sphere by cellulose hydroxyl groups (Fig. 3). Accumulation of C1− along the cellulose chain produces a negatively charged polymer with the macrocation, [Li-DMAc], as the counterion. This process can be regarded as a polyelectrolyte effect, where polymer molecules are forced apart due to charge repulsion. Moreover, the high osmotic pressure drives a continuous influx of solvent resulting in further disruption of the cellulose binding forces until the polymer is totally solvated. A similar polyelectrolyte effect is suggested for the tetrabutylammonium fluoride/dimethyl sulfoxide (TBAF/DMSO)
system where the solubilization of the biopolymer is expected to be enhanced by the electrostatic repulsion between the negatively-charged cellulose chains (due to the condensation of F−) [53]. Regarding ionic liquids, although great experimental and computational progress has been made, there is no clear understanding of the role of individual ionic species in dissolution. Up to date, two main views of the interactions between ILs and cellulose prevail: (1) the dissolution process is governed by the interactions between the anion and the biopolymer, with no specific role for the cation; and (2) the major driving force for cellulose dissolution comes from the H-bond interactions of the cellulose hydroxyls with the anion and the cation of ILs. In literature, we find several examples where either changing the cation or the anion produces remarkable effects on dissolution efficiency. Interestingly, some of the good hydrogen-bond-accepting anions that have been found to dissolve cellulose can be rendered ineffective through pairing with certain cations [54]. One of the structural features that makes ILs so interesting is their strong asymmetry. Typically, the cations are bulky species with amphiphilic properties. Such amphiphilicity is normally not considered when discussing the mechanism of dissolution of cellulose. This is particularly relevant since crystalline cellulose has an amphipathic-like structure (Fig. 1b): hydrophobic surfaces consisting of pyranose-ring hydrogens and hydrophilic regions arising from the hydroxyl groups directed toward the sides of the ring. The non-polar surfaces can be organized into hydrophobic sheets paired against one another, rather than structuring large amounts of water in solution. (Note that in this type of interaction, the driving force for association is not simply van der Waals' interactions but rather hydrophobic association driven by the liberation of structured water molecules [55].) We believe that the dissolution of an amphiphilic polymer, such as cellulose, would be facilitated in amphiphilic solvents. Therefore, the amphiphilic properties of all cations in ILs fit in our suggestion. In fact, this also follows from the earlier discussion on the effect of additives such as PEG, urea, and thiourea on dissolution in alkali solutions.
4. Cellulose regeneration Typically, the regeneration of cellulose occurs when contacting the cellulose solution with a coagulation bath. The polymer profile at the point of precipitation exhibits a very high interfacial concentration, thus favoring the formation of a dense polymer “skin.” The bulk of the sample is at near the initial concentration and is in a fluid state. Thus, a rapid inflow of the coagulant can take place through the weak points at the skin interface. Rapid growth of finger-like voids in the fluid region is expected to occur due to the moving interface created by the coagulant (less viscous) and the solution (more viscous). The kinetics of regeneration is mainly controlled by the relative velocities of the counter-diffusion process—diffusion of the solvent from the solution
+
Fig. 3. Example of a hydrogen-bond breaking mechanism for the cellulose dissolution in the lithium chloride/dimethylacetamide (LiCl/DMAc) solvent system. Cellulose hydroxyl groups interact with an intermediate (Li+-DMAC macrocation) via hydrogen bonding bridged by the chloride anion. The lithium cation interacts with the carbonyl oxygen via an ion–dipole interaction.
Please cite this article as: Medronho B, Lindman B, Brief overview on cellulose dissolution/regeneration interactions and mechanisms, Adv Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cis.2014.05.004
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into the coagulation bath and the non-solvent from the bath into the solution [56]. The exchange of solvent with non-solvent leads to a desolvation of the cellulose molecules and to the supposed reformation of the intraand inter-molecular hydrogen bonds [57]. The regeneration of cellulose from, for instance, NMMO solutions using water as the coagulation system results from a phase separation process. Typically, two mechanisms of phase separation can take place during liquid-liquid demixing of polymer solutions; either nucleation and growth (i.e., the nuclei of one phase grow in the mixture) or spinodal decomposition (i.e., a periodic variation of concentration leads to the final phase separation). In the NMMO-water system, if there is a clear difference between these two mechanisms in their first stages (nucleation and growth shows isolated entities while spinodal decomposition has a 3D network-like morphology), typically both tend to the same morphology at the end due to surface tension effects [58]. The mechanical and surface chemical properties of regenerated cellulose are known to depend strongly on the type of cellulose solvent and coagulant. For a bibliography, the reader is directed to the work of Isobe et al. [59] and references therein. For instance, in molten salts systems, the regenerated morphologies can vary significantly depending on the salt used; in thiocyanate melts, fiber-like samples (similar to the raw cellulose) have been obtained while lamellar-like samples and layered structures were regenerated from LiClO4 · 3H2O and chlorides, respectively [60]. In IL systems, the presence of water affects many of the solvent properties. To date, experimental studies on cellulose/IL/non-solvent ternary systems have not been conclusive, and a mechanistic understanding of the interactions between water, IL, and cellulose remains unknown. For instance, MD results are taken to indicate that the ordered cationanion polar interaction network is disrupted by water that forms a network of interactions with the anion and with the cation [61]. More specifically, the suggested mechanism assumes that while water diffuses inside the first solvation shell of cellulose, the number of hydrogen bonds among water molecules and the polymer increases and the number of H-bonds between the anion and the sugar decreases. Meanwhile, the cations are considered to be maintained in the second solvation shell of cellulose due to their strong electrostatic interactions with the anions present. The formation of this network displaces the cation out of the first solvation shell leading to cellulose precipitation. In this model, phase separation of the cellulose as a function of water results from competition between the water–anion, cellulose–anion, cellulose–water, cellulose–cation, and anion–cation interactions [61]. The regeneration mechanism for alkali systems suggests that the inclusion complex associated with cellulose, NaOH, and, in some cases, urea or thiourea hydrates is disrupted by adding a non-solvent such as water, leading to the self-association of cellulose. The regenerated cellulose film is said to be formed through a rearrangement of the hydrogen bonds [62,63]. Moreover, in acidic non-solvents, the H+ assumes a key role to trigger cellulose regeneration by neutralizing the alkaline content. Additionally, as in the Lenzing viscose production process, the addition of a strong electrolyte to H2SO4, such as Na2SO4 and ZnSO4, can reduce the H+ concentration in the coagulant, leading to a counterdiffusion rate and slower acid–alkali neutralization process than by just H2SO4 alone. This will considerably influence the final properties of the regenerated cellulose material [10]. Recently, a very interesting study has been presented by Isobe et al. [59]. The regeneration of cellulose, either using a coagulant or upon heating, was followed in an aqueous alkali-urea solvent by monitoring the process by time-resolved synchrotron X-ray scattering. The authors suggested that when the medium surrounding the cellulose molecules becomes energetically unfavorable for molecular dispersion, regeneration starts, and the initial process would consist in stacking the hydrophobic glucopyranoside rings (driven by hydrophobic interactions) to form monomolecular sheets, which then would line up by hydrogen
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bonding to form Na–cellulose type IV crystallites, a hydrate form of cellulose II (Fig. 4). This constitutes the first experimental evidence of the development of hydrophobically stacked monomolecular sheets which was firstly hypothesized by Hermans [64] and later by Hayashi et al. [65]. These authors found that structural disorders lie mainly in the hydrogenbonded intermolecular region and assumed the primary structure as a monomolecular sheet formed by stacking of glucopyranoside planes by van der Waals forces. Recently, the theoretical work of Miyamoto et al. [9] simulated the regeneration of cellulose by MD, reproducing the hypothesis of Hermans and Hayashi. While these studies gave a consistent picture of the primary structure formation of regenerated cellulose, they were either inferred from the resulting structure or based on computer simulations. Therefore, the work of Isobe et al. [59] not only shines light into the regeneration mechanism of cellulose but also constitutes an alternative vision to the typical regeneration mechanism found in literature which, as we have seen, essentially focuses exclusively on the reformation of the broken inter- and intramolecular hydrogen bonds among cellulose molecules. 5. Final remarks From this brief review on cellulose dissolution and regeneration, it is clear that quite contrasting starting points have been chosen through the years for the analysis of the mechanisms. As can be inferred, the nature of the solvents that can efficiently dissolve cellulose covers an enormous range making it difficult to provide a single consistent mechanism. While we recognize that there may be different mechanisms for different cases, it seems important to make a classification based on some simple views on solubility, in general, and polymer solubility, in particular. Solubility is governed by entropy, notably translational and configurational entropy, the enthalpy in general, being unfavorable. Regarding translational entropy, this will be very much higher for ionic polymers with dissociating counterions than for non-ionic polymers. This is amply demonstrated for cellulose where protonation or deprotonation, due to addition of simple acids and bases, offers efficient ways of swelling and dissolution of cellulose. An effective ionization can be obtained not only from changes in the state of protonation but also from association/adsorption of an ionic species. Thus, it is well known that the solubility of non-ionic polymers, such as poly(ethylene glycol) and cellulose derivatives, can be strongly increased by association of ions. This is particularly efficient for ionic surfactants but also ions with a high polarizability can act as pseudo-surfactants. Actually, this is the basis for the lyotropic series, often referred to as the Hofmeister series. Thus, it is found that large anions, such as iodide, can increase the solubility of non-ionic macromolecules, whereas small ions with a high charge density, have the opposite effect. The large polarizable ions are effectively hydrophobic and tend to have a low solubility in a polar solvent and/ or being enriched at a hydrophobic surface. Generally speaking, we can thus expect that a highly unsymmetrical electrolyte would provide good solubility conditions for a non-ionic polymer such as cellulose, since the ions would be non-uniformly distributed in the solution. This is the case for molten salt systems with small, strong polarizing cations (i.e., Li+ and Zn2+) and large polarizable anions but, from the broad overview above, looks like a rather general trend since many of the efficient solvents of cellulose follow this pattern, including many of the ionic liquids. As we have previously argued, cellulose manifests strong hydrophobic properties and is thus amphiphilic [4,5]. As arguments for amphiphilic properties we can mention the ability of other polyglucoses, such as cyclodextrin and amylose, to solubilize non-polar and amphiphilic compounds in aqueous solution. It is also strongly supported by simulations [66]. Another clear support for amphiphilic properties can be found from studies of surfactant binding to cellulose derivatives. Thus, surfactants bind to cellulose derivatives, soluble due to
Please cite this article as: Medronho B, Lindman B, Brief overview on cellulose dissolution/regeneration interactions and mechanisms, Adv Colloid Interface Sci (2014), http://dx.doi.org/10.1016/j.cis.2014.05.004
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B. Medronho, B. Lindman / Advances in Colloid and Interface Science xxx (2014) xxx–xxx
Time
Intensity (a.u.)
Complete regeneration
control
q (Å-1)
Fig. 4. On top, the time evolution of synchrotron x-ray diffraction profiles of a cellulose solution under regeneration in a 5 wt% Na2SO4 aqueous solution is shown. In the control (10 wt.% cellulose solution without coagulant) no Bragg reflections are observed. After a certain time, a first Bragg reflection (110) is observed while the second one (020) is only visible after complete regeneration. On bottom, a schematic illustration of cellulose regeneration deduced from the X-ray data, where first the hydrophobic stacking of cellulose molecules occurs and then is followed by their binding via hydrogen bonding (adapted from reference [55]).
hydrophilic groups, which do not associate with surfactants. An illustrative example is that of cationic hydroxyethyl cellulose, which binds anionic surfactant in excess of charge stoichiometry; obviously, this can only be due to hydrophobic interactions between the cellulose backbone and the alkyl chain of the surfactant [67]. In view of this, eliminating the hydrophobic interactions between cellulose molecules must be very important for the aqueous solubility of cellulose. We note that urea can facilitate cellulose dissolution. Urea has a much lower polarity than water and is well known to eliminate hydrophobic association in water. As alluded to above, it acts as protein denaturant by reducing intramolecular hydrophobic association. Furthermore, it inhibits hydrophobic association of surfactants as can be seen from the increase in critical micelle concentration on urea addition. Other substances of intermediate polarity, such as urea derivatives and poly(ethylene glycol), have similar effects and are well-known enhancers of the aqueous solubility of cellulose. The alternative notion to hydrophobic interactions as the limiting factor of dissolution of cellulose in water is hydrogen-bonding. However, this would require that the carbohydratecarbohydrate hydrogen-bonding would be much stronger than watercarbohydrate and water-water hydrogen bonding, which is not the case; regarding a solo hydrogen–bonding mechanism, it is striking that all these interactions are very similar in magnitude; ca. 5 kcal/mol. Furthermore, a hydrogen-bonding mechanism is inconsistent with a high aqueous solubility of polysaccharides, which have as high hydrogenbonding capacity as cellulose.
Acknowledgments The authors acknowledge support from the Portuguese Foundation for Science and Technology (FCT, project PTDC/AGR-TEC/4049/2012 and a postdoc grant assigned to B. Medronho, SFRH/BPD/74540/2010). Financial support was also received from Södra Skogsägarnas Stiftelse, Stiftelsen Nils och Dorthi Troëdssons forskningsfond and the CelluNova consortium.
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