Dissolution mechanism of cellulose in quaternary ammonium hydroxide: Revisiting through molecular interactions

Preparation of graphene/carbon nanotube-cellulose composites assisted by ionic liquids: A review

As is well known, cellulose, as a natural polymer material with abundant reserves, plays an irreplaceable role as the major raw material in energy and chemical-related fields. With the continuous advancement of technology, native single-component cellulose is often unsatisfactory for practical applications, constructing composites is an effective means of expanding the applications. When compounded with other ingredients to prepare composites, cellulose usually needs to be dissolved and regenerated to obtain good dispersion. Current studies have revealed that cellulose is insoluble in conventional solvents, and the limited types of solvent systems that can dissolve cellulose tend to degrade the cellulose during the dissolution process, altering the cellulose properties. Ionic liquids (ILs) are a class of solvents that are capable of dissolving cellulose without adversely affecting the cellulose during the dissolution process, such as degradation. Graphene and carbon nanotubes (CNTs) are poorly dispersed and easily agglomerated by π-π stacking in general solvents, whereas ILs can effectively shield them from π-π stacking, resulting in a favorable and steady dispersion. Thus, the cellulose composites of graphene/CNTs can be prepared with the assistance of ILs. In this paper, the solubilization of cellulose by ILs and the solubilization mechanism to the preparation of cellulose composites with graphene/CNTs are reviewed, the interactions between graphene, CNTs and cellulose in the composites are elucidated, and the preparation of cellulose composites with graphene/CNTs is introduced in terms of their structure, properties and application potential.

Tailoring Hydronium ion Driven Dissociation-Chemical Cross-Linking for Superfast One-Pot Cellulose Dissolution and Derivatization to Build Robust Cellulose Films

Concepts of sustainability must be developed to overcome the increasing environmental hazards caused by fossil resources. Cellulose derivatives with excellent properties are promising biobased alternatives for petroleum-derived materials. However, a one-pot route to achieve cellulose dissolution and derivatization is very challenging, requiring harsh conditions, high energy consumption, and complex solubilizing. Herein, we design a one-pot tailoring hydronium ion driven dissociation-chemical cross-linking strategy to achieve superfast cellulose dissolution and derivatization for orderly robust cellulose films. In this strategy, there is a powerful driving force from organic acid with a pKa below 3.75 to dissociate H+ and trigger the dissolution and derivatization of cellulose under the addition of H2SO4. Nevertheless, the driving force can only trigger a partial swelling of cellulose but without dissolution when the pKa of organic acid is above 4.26 for the dissociation of H+ is inhibited by the addition of inorganic acid. The cellulose film has high transmittance (up to ∼90%), excellent tensile strength (∼122 MPa), and is superior to commercial PE film. Moreover, the tensile strength is increased by 400% compared to cellulose film prepared by the ZnCl2 solvent. This work provides an efficient solvent, which is of great significance for emerging cellulose materials from renewable materials.

2D Nanomaterials and Their Drug Conjugates for Phototherapy and Magnetic Hyperthermia Therapy of Cancer and Infections

Photothermal therapy (PTT) and magnetic hyperthermia therapy (MHT) using 2D nanomaterials (2DnMat) have recently emerged as promising alternative treatments for cancer and bacterial infections, both important global health challenges. The present review intends to provide not only a comprehensive overview, but also an integrative approach of the state-of-the-art knowledge on 2DnMat for PTT and MHT of cancer and infections. High surface area, high extinction coefficient in near-infra-red (NIR) region, responsiveness to external stimuli like magnetic fields, and the endless possibilities of surface functionalization, make 2DnMat ideal platforms for PTT and MHT. Most of these materials are biocompatible with mammalian cells, presenting some cytotoxicity against bacteria. However, each material must be comprehensively characterized physiochemically and biologically, since small variations can have significant biological impact. Highly efficient and selective in vitro and in vivo PTTs for the treatment of cancer and infections are reported, using a wide range of 2DnMat concentrations and incubation times. MHT is described to be more effective against bacterial infections than against cancer therapy. Despite the promising results attained, some challenges remain, such as improving 2DnMat conjugation with drugs, understanding their in vivo biodegradation, and refining the evaluation criteria to measure PTT or MHT effects.

Cellulose Dissolution, Modification, and the Derived Hydrogel: A Review

The cellulose-based hydrogel has occupied a pivotal position in almost all walks of life. However, the native cellulose can not be directly used for preparing hydrogel due to the complex non-covalent interactions. Some literature has discussed the dissolution and modification of cellulose but has yet to address the influence of the pretreatment on the as-prepared hydrogels. Firstly, the "touching" of cellulose by derived and non-derived solvents was introduced, namely, the dissolution of cellulose. Secondly, the "conversion" of functional groups on the cellulose surface by special routes, which is the modification of cellulose. The above-mentioned two parts were intended to explain the changes in physicochemical properties of cellulose by these routes and their influences on the subsequent hydrogel preparation. Finally, the "reinforcement" of cellulose-based hydrogels by physical and chemical techniques was summarized, viz., improving the mechanical properties of cellulose-based hydrogels and the changes in the multi-level structure of the interior of cellulose-based hydrogels.

Efficient Cellulose Dissolution and Film Formation Enabled by Superbase Amino Acid Ionic Liquids

Cellulose is a promising feedstock for the production of sustainable materials. To fully utilize its potential, exploring efficient cellulose solvents is a paramount prerequisite. In this study, ten superbase amino acid ionic liquids (SAAILs) are synthesized using 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with different amino acid anions via a simple neutralization method. The properties of these SAAILs, such as viscosity and glass transition temperature, varied with their cation and anion structures. The ability of the SAAILs to dissolve cellulose is related to their Kamlet-Taft parameters, particularly hydrogen bond basicity (β). The main driving force for cellulose dissolution in SAAILs is thought to be hydrogen bonding interactions between SAAILs and cellulose hydroxyl groups. Four SAAILs composed of DBN or DBU cations and proline, or aspartic acid anions are identified as promising solvents for preparing regenerated cellulose films (RCFs). The RCF prepared from [DBN]Proline(Pro) showed a favorable combination of high tensile strength (76.9 MPa), high Young's modulus (5201.2 MPa), good transparency (≈70% at 550 nm), and smooth surface morphology. These halogen- and metal-free SAAILs show the potential to provide a new avenue for cellulose processing.

Combination of adsorption/desorption and photocatalytic reduction processes for PFOA removal from water by using an aminated biosorbent and a UV/sulfite system

Per- and polyfluoroalkyl substances (PFAS) are stable organic chemicals, which have been used globally since the 1940s and have caused PFAS contamination around the world. This study explores perfluorooctanoic acid (PFOA) enrichment and destruction by a combined method of sorption/desorption and photocatalytic reduction. A novel biosorbent (PG-PB) was developed from raw pine bark by grafting amine groups and quaternary ammonium groups onto the surface of bark particles. The results of PFOA adsorption at low concentration suggest that PG-PB has excellent removal efficiency (94.8%-99.1%, PG-PB dosage: 0.4 g/L) to PFOA in the concentration range of 10 μg/L to 2 mg/L. The PG-PB exhibited high adsorption efficiency regarding PFOA, being 456.0 mg/g at pH 3.3 and 258.0 mg/g at pH 7 with an initial concentration of 200 mg/L. The groundwater treatment reduced the total concentration of 28 PFAS from 18 000 ng/L to 9900 ng/L with 0.8 g/L of PG-PB. Desorption experiments examined 18 types of desorption solutions, and the results showed that 0.05% NaOH and a mixture of 0.05% NaOH + 20% methanol were efficient for PFOA desorption from the spent PG-PB. More than 70% (>70 mg/L in 50 mL) and 85% (>85 mg/L in 50 mL) of PFOA were recovered from the first and second desorption processes, respectively. Since high pH promotes PFOA degradation, the desorption eluents with NaOH were directly treated with a UV/sulfite system without further adjustment. The final PFOA degradation and defluorination efficiency in the desorption eluents with 0.05% NaOH + 20% methanol reached 100% and 83.1% after 24 h reaction. This study proved that the combination of adsorption/desorption and a UV/sulfite system for PFAS removal is a feasible solution for environmental remediation.

Rapid and massive fractionation of hemicelluloses for purifying cellulose at room temperature by tetramethylammonium hydroxide

The fractionation of hemicelluloses is a promising method to improve the comprehensive utilization of lignocellulosic biomass. However, the effective fractionation of hemicelluloses is always limited by the structural complexity and easy degradability. In this study, tetramethylammonium hydroxide (TMAH) was developed to fractionate hemicelluloses from poplar holocellulose with high molecular weights and high yields at room temperature. Approximately 90% of hemicelluloses could be dissolved at room temperature in 1 h, and the yield was up to 81.9%. Compared with the fractionation using NaOH solution, the hemicelluloses isolated by TMAH solvent showed a more complete structure and higher purity. Meanwhile, the retention rate of cellulose after treatment with TMAH was up to 90.2%, and the crystal structure of cellulose in the residues was practically unchanged. Moreover, the TMAH solvent could be recycled to fractionate hemicelluloses. The work provides an elegant and significantly efficient method towards hemicelluloses fractionation and cellulose purification.

Bleaching-free, lignin-tolerant, high-yield production of nanocrystalline cellulose from lignocellulosic biomass

Nanocrystalline cellulose (NCC) preparation in an integrated fractionation manner is expected to solve the problems of low yield and environmental impact in the traditional process. An integrated fractionation strategy for NCC production from wood was developed through catalytic biomass fractionation, the partial dissolution of cellulose-rich materials (CRMs) in aqueous tetrabutylphosphonium hydroxide, and short-term ultrasonication. The presented process could tolerate a high CRM lignin content of 21.2 wt % and provide a high NCC yield of 76.6 wt % (34.3 wt % of the original biomass). The increase in the CRM lignin content decreased the NCC yield, facilitated the crystal transition of NCC from cellulose I to cellulose II, and showed no apparent effects on the NCC morphology. A partial/selective dissolution mechanism is proposed for the presented strategy. This study provided a promising efficient fractionation-based method toward comprehensive and high-value utilization of lignocellulosic biomass through effective delignification and high-yield NCC production.

Progress on chemical modification of cellulose in “green” solvents

Chemical modification of cellulose in "green" solvents.

Multilayer photonic films based on interlocked chiral-nematic cellulose nanocrystals in starch/chitosan

In this study, a new approach to employ and control cellulose nanocrystal (CNC) chiral nematic structure as a biodegradable, intelligent material was investigated. Tuned CNC self-assembled films were interlocked between two layers of citric acid, cross-linked starch/chitosan (1:1) films through the solvent casting process. This method increased the mechanical properties of produced films and created a selective reflection band from UV to near-IR depending on the helical pitch of the chiral nematic CNC layer. The features of these intelligent films have potential for different applications, from UV protective packaging to biomedical uses. The water vapor permeability (WVP) of the produced films decreased considerably by adding a CNC layer into the cross-linked starch/chitosan structure. Also, the WVP was different for the different helical pitches of the CNC layer. The starch/chitosan (outer layer) also showed a remarkable antibacterial property against E. coli, P. fluorescens, S. Enteritidis, and S. aureus which could be useful for biomedical applications or antibacterial packaging.

A Fast Dissolution Pretreatment to Produce Strong Regenerated Cellulose Nanofibers via Mechanical Disintegration

This study investigates a fast dissolution and regeneration pretreatment to produce regenerated cellulose nanofibers (RCNFs) via mechanical disintegration. Two cellulose pulps, namely, birch and dissolving pulps, with degree of polymerizations of 1800 and 3600, respectively, were rapidly dissolved in dimethyl sulfoxide (DMSO) by using tetraethylammonium hydroxide (TEAOH) as aqueous electrolyte at room temperature. When TEAOH (35 wt % in water) was added to the pulp-DMSO dispersion (pulp:DMSO and TEAOH:DMSO weight ratios of 1:90 and 1:9, respectively), 95% of the dissolving pulp and 85% of the birch pulp fibers dissolved almost immediately. Addition of water caused the regeneration of cellulose without any chemical modification and only a minor decrease of DP, whereas the crystallinity structure of cellulose transformed from cellulose I to cellulose II. The regenerated cellulose could then be mechanically disintegrated into nanosized fibers with only a few passes through a microfluidizer, and RCNF showed fibrous structure. The specific tensile strength of the film produced from both RCNFs exceeded 100 kN·m/kg, and overall mechanical properties of RCNF produced from birch pulp were in line with reference CNF produced by using extensive mechanical disintegration. Although the thermal stability of RCNFs was slightly lower compared to their corresponding original cellulose pulp, the onset temperature of degradation of RCNFs was over 270 °C.

Designing cellulose hydrogels from non-woody biomass

Hydrogels are an attractive system for a myriad of applications. While most hydrogels are usually formed from synthetic materials, lignocellulosic biomass appears as a sustainable alternative for hydrogel development. The valorization of biomass, especially the non-woody biomass to meet the growing demand of the substitution of synthetics and to leverage its benefits for cellulose hydrogel fabrication is attractive. This review aims to present an overview of advances in hydrogel development from non-woody biomass, especially using native cellulose. The review will cover the overall process from cellulose depolymerization, dissolution to crosslinking reaction and the related mechanisms where known. Hydrogel design is heavily affected by the cellulose solubility, crosslinking method and the related processing conditions apart from biomass type and cellulose purity. Hence, the important parameters for rational designs of hydrogels with desired properties, particularly porosity, transparency and swelling characteristics will be discussed. Current challenges and future perspectives will also be highlighted.

Engineering of sustainable biomaterial composites from cellulose and silk fibroin: Fundamentals and applications

This review addresses composites prepared from cellulose (Cel) and silk fibroin (SF) to generate multifunctional, biocompatible, biodegradable materials such as fibers, films and scaffolds for tissue engineering. First, we discuss briefly the molecular structures of Cel and SF. Their structural features explain why certain solvents, e.g., ionic liquids, inorganic electrolyte solutions dissolve both biopolymers. We discuss the mechanisms of Cel dissolution because in many cases they also apply to (much less studied) SF dissolution. Subsequently, we discuss the fabrication and characterization of Cel/SF composite biomaterials. We show how the composition of these materials beneficially affects their mechanical properties, compared to those of the precursor biopolymers. We also show that Cel/SF materials are excellent and versatile candidates for biomedical applications because of the inherent biocompatibility of their components.

Probing Interactions in Combined Hydroxide Base Solvents for Improving Dissolution of Cellulose

To further understand cellulose-solvent interactions in aqueous hydroxide solutions, cellulose behavior in aqueous solutions of NaOH combined with tetramethylammonium hydroxide (TMAH) or benzyltrimethylammonium hydroxide (Triton B), as well as urea, was investigated. The rheological properties of the solutions were assessed through flow sweeps at different temperatures, and the intermolecular interactions were probed using solvatochromic dyes. The results showed that NaOH combined with TMAH had synergistic effects on cellulose dissolution and was a better solvent for cellulose than the combination of NaOH with Triton B, in spite of the superior dissolution ability of Triton B alone. This somewhat unexpected finding shows that the base pair needs to be selected with care. Interestingly, addition of urea had no significant effect on the solvatochromic parameters or dissolution capacity of solutions of Triton B but rendered improved stability of solutions containing NaOH and/or TMAH. It seems that both urea and Triton B weaken the hydrophobic assembly effect of these solutions, but urea is excluded from interacting with cellulose in the presence of Triton B. This study provides further insight into dissolution of cellulose and the possibility of utilizing combinations of hydroxide bases to achieve improved solution properties.

The role of urea in the solubility of cellulose in aqueous quaternary ammonium hydroxide

We examine the role of water and urea in cellulose solubility in tetrabutylammonium hydroxide (TBAH). Molecular dynamics simulations were performed for several different solvent compositions with a fixed cellulose fraction. For each composition, two simulations were carried out with cellulose fixed in each of the crystalline and the dissolved states. From the enthalpy and the entropy of the two states, the difference in Gibbs free energy (ΔG) and hence the spontaneity is determined. A comparison with solubility experiments showed a strong correlation between the calculated ΔG and the experimental measurements. A breakdown of the enthalpic and entropic contributions reveals the roles of water and urea in solubility. At high water concentration, a drop in solubility is attributed to both increased enthalpy and decreased entropy of dissolution. Water displaces strong IL–cellulose interactions for weaker water–cellulose interactions, resulting in an overall enthalpy increase. This is accompanied by a strong decrease in entropy, which is primarily attributed to both water and the entropy of mixing. Adding urea to TBAH(aq) increases solubility by an addition to the mixing term and by reducing losses in solvent entropy upon dissolution. In the absence of urea, the flexible [TBA]⁺ ions lose substantial degrees of freedom when they interact with cellulose. When urea is present, it partially replaces [TBA]⁺ and to a lesser extent OH⁻ near cellulose, losing less entropy because of its rigid structure. This suggests that one way to boost the dissolving power of an ionic liquid is to limit the number of degrees of freedom from the outset.

We examine the role of water and urea in cellulose solubility in tetrabutylammonium hydroxide (TBAH).

Cellulose in Ionic Liquids and Alkaline Solutions: Advances in the Mechanisms of Biopolymer Dissolution and Regeneration

This review is focused on assessment of solvents for cellulose dissolution and the mechanism of regeneration of the dissolved biopolymer. The solvents of interest are imidazole-based ionic liquids, quaternary ammonium electrolytes, salts of super-bases, and their binary mixtures with molecular solvents. We briefly discuss the mechanism of cellulose dissolution and address the strategies for assessing solvent efficiency, as inferred from its physico-chemical properties. In addition to the favorable effect of lower cellulose solution rheology, microscopic solvent/solution properties, including empirical polarity, Lewis acidity, Lewis basicity, and dipolarity/polarizability are determinants of cellulose dissolution. We discuss how these microscopic properties are calculated from the UV-Vis spectra of solvatochromic probes, and their use to explain the observed solvent efficiency order. We dwell briefly on use of other techniques, in particular NMR and theoretical calculations for the same purpose. Once dissolved, cellulose is either regenerated in different physical shapes, or derivatized under homogeneous conditions. We discuss the mechanism of, and the steps involved in cellulose regeneration, via formation of mini-sheets, association into “mini-crystals”, and convergence into larger crystalline and amorphous regions. We discuss the use of different techniques, including FTIR, X-ray diffraction, and theoretical calculations to probe the forces involved in cellulose regeneration.

Design of task-specific fluorinated ionic liquids: nanosegregation versus hydrogen-bonding ability in aqueous solutions

Fluorinated ionic liquids rich-nanosegregated behaviour reduces the impact of the addition of water upon the ionic liquids's H-bond acceptance ability.