Molecular morphology of cellulose

Native celluloses of various biological origins, as well as regenerated celluloses were examined by electron microscopy after suitable dispersion. In all cases the specimens were found to be composed of a common filamentary unit which is rectangular in cross section and has the approximate dimensions 35 × 20 Å. It is suggested that these are the basic morphological units of cellulose; they are therefore called protofibrils. For protofibrils of regenerated cellulose it is shown that: (1) the molecular contour length greatly exceeds the protofibril length, (2) the mass of the protofibril corresponds to that of a single molecule, and (3) the protofibril length increases with molecular weight. Additionally, high resolution electron micrographs of native and regenerated protofibrils show an apparent axial texture with a periodicity of about 40 Å. From these observations and the knowledge that the molecular chain axis is aligned parallel to the protofibril axis, a model of the protofibril is deduced. The model consists of a ribbon which is pleated on itself so as to form a planar zigzag structure of rectangular cross section. This supersedes a previously proposed model of circular cross section. The structure is composed of a single folded, chain, arranged so that the short extended segments between the folds are parallel to the protofibril axis. The protofibril is thus regarded as the morphological expression of the cellulose molecule. Microfibrils and protofibrils often exhibit kinks, the angle between the kinked portions being 120°. This phenomenon is satisfactorily explained by the protofibril model and in fact provides good support for it. Finally, various properties of cellulose are considered in relation to the model. By contrast with the earlier crystalline–amorphous concepts of cellulose fine structure, it is suggested that protofibrils are completely crystalline structures, and that the properties of cellulose may be understood by considering processes that occur at the level of the protofibril as a unit.     

Availability and state of order of hydroxyl groups on the surfaces of microstructural units of crystalline cotton cellulose

The relative accessibilities of the hydroxyl groups of the D-glucopyranosyl units of hydrocellulose have been studied by means of the reaction of N,N-diethylaziridinium chloride, which produces 2-(diethylamino)ethyl cellulose. The deviation in the distribution of substituents among the 2-O-, 3-O-, and 6-O-positions of the D-glucopyranosyl residues in a hydrocellulose from that in a disordered cellulose in which the three types of hydroxyl groups are equally accessible is the basis for estimating the selective accessibilities of the hydroxyl groups in the crystalline cellulose. A particular hydrocellulose, lying within the range of leveling-off degree of polymerization, was studied in detail; this hydrocellulose, designated EHC (“Exemplar Hydrocellulose”), was formed from fibrous cotton by hydrolysis for 0.67 hr in 2.5N hydrochloric acid at reflux. EHC exhibited higher selective accessibility (larger deviation from equal accessibility) of the hydroxyl groups at C-2, C-3, and C-6, than samples of hydrocellulose formed in shorter or longer periods of hydrolysis. This selective accessibility is discussed in terms of intra- and intermolecular hydrogen bonding on the surfaces of crystalline microstructural units in EHC.     

Partial crystalline transformation of solvated cellulose IIII crystals, reproduced by theoretical calculations

In hot-water molecular dynamics simulation at 370 K, four cellulose III_I crystal models, with different lattice planes and dimensions, exhibited partial crystalline transformations of (1 ?1 0) chain sheets, in which hydroxymethyl groups were irreversibly rotated from gt into tg conformations, accompanied by hydrogen-bond exchange from the original O3–O6 to cellulose-I-like O2–O6 bonds. The final hydrogen-bond exchange ratio was about 95 % for some of the crystal models after 50 ns simulation. The corrugated (1 ?1 0) chain sheet was converted to a cellulose-I-like flat chain sheet with a slightly right-handed twist. The 3D structures of the three types of isolated chain sheet models were optimized using density functional theory calculations to compare their stabilities without crystal packing forces. The cellulose Iβ (1 0 0) models were more stable than the cellulose III_I (1 ?1 0) models. The optimized structure of cellulose III_I (1 0 0) models deviated largely from the initial sheet form. It was proposed to the crystalline transformation from cellulose III_I to Iβ that conversion of the chain sheet structure first take place, followed by sliding of the chain sheet along the fiber axis.     

Comparing microcrystalline with spherical nanocrystalline cellulose from waste cotton fabrics

Microcrystalline cellulose (MCC) and spherical nanocrystalline cellulose (SNCC) were successfully prepared from waste cotton fabrics through acid hydrolysis. The comparative analysis of the morphology and structure between the obtained MCC and SNCC was carried out. The SNCC suspension exhibited higher stability than the MCC suspension. Transmission electron microscopy in combination with atomic force microscopy showed that the cellulose nanospheres with average size of 35?nm were achieved, while the average particle size of MCC was 49?μm. The MCC and SNCC had similar functional groups and crystalline structure as confirmed by Fourier transform infrared spectroscopy and X-ray diffraction analysis, respectively. Viscometric average molecular weight measurement and thermo gravimetric analysis indicated that the degree of polymerization and thermal stability of SNCC was lower than that of MCC. These results should improve understanding of the characteristics of MCC and SNCC derived from waste cotton fabrics and lead to many new applications.     

Quantitative determination of cellulose accessibility to cellulase based on adsorption of a nonhydrolytic fusion protein containing CBM and GFP with its applications

Heterogeneous cellulose accessibility is an important substrate characteristic, but all methods for determining cellulose accessibility to the large-size cellulase molecule have some limitations. Characterization of cellulose accessibility to cellulase (CAC) is vital for better understanding of the enzymatic cellulose hydrolysis mechanism (Zhang and Lynd, Biotechnol. Bioeng. 2004, 88, 797-824; 2006, 94, 888-898). Quantitative determination of cellulose accessibility to cellulase (m2/g of cellulose) was established based on the Langmuir adsorption of the fusion protein containing a cellulose-binding module (CBM) and a green fluorescent protein (GFP). One molecule of the recombinant fusion protein occupied 21.2 cellobiose lattices on the 110 face of bacterial cellulose nanofibers. The CAC values of several cellulosic materials -- regenerated amorphous cellulose (RAC), bacterial microcrystalline cellulose (BMCC), Whatman No. 1 filter paper, fibrous cellulose powder (CF1), and microcrystalline cellulose (Avicel) -- were 41.9, 33.5, 9.76, 4.53, and 2.38 m2/g, respectively. The CAC value of amorphous cellulose made from Avicel was 17.6-fold larger than that of crystalline cellulose - Avicel. Avicel enzymatic hydrolysis proceeded with a transition from substrate excess to substrate limited. The declining hydrolysis rates over conversion are mainly attributed to a combination of substrate consumption and a decrease in substrate reactivity. Declining heterogeneous cellulose reactivity is significantly attributed to a loss of CAC where the easily hydrolyzed cellulose fraction is digested first.     

A morphological interpretation of water chemical exchange and mobility in cellulose materials derived from proton NMR T2 relaxation

Proton T₂ relaxation times of water in cellulosic fibres have been interpreted using a 3-term average model. Motional and chemical exchange contributions to relaxation show opposing temperature behaviour, enabling the use of Arrhenius analysis to determine proton exchange rates and water rotational correlation times. Both parameters vary dramatically with extent of hydration, with chemical exchange dominating relaxation at saturated water contents. Interpretations are based on a morphological model with two types of accessible cellulose, at void surfaces and internally within the cellulose phase. In native cellulose fibres, the presence of crystalline fibrils with low internal accessibility leads to rapid proton exchange at low moisture contents. Regenerated cellulose fibres typically have lower crystallinity and higher internal accessibility, which results in slower exchange as result of migration of water between void and internal environments. Exchange behaviour in regenerated fibres is highly dependent on structural organisation, which depends on the manufacturing process.     

Near-infrared spectroscopic analysis of aging degradation in antique washi paper using a deuterium exchange method

The diffusion process of deuterated water (D₂O) in washi (Japanese traditional paper) was investigated by means of a deuterium exchange method and Fourier-transform near-infrared (FT-NIR) transmission spectroscopy. The samples were the modern (AD 2003) hand-made washi and those from an archival collection of cultural artifacts (AD 1791 and 1615). Four absorption bands were identified in the NIR spectral range from 7200 to 6000 cm⁻¹ which are due to OH groups in the amorphous, semi-crystalline and two types of crystalline regions of cellulose. The accessibility of D₂O increased with decreasing state of order of cellulose, and the saturation accessibility increased with the age of the samples. It was suggested that during aging hemicellulose, which forms a composite with cellulose in paper, was progressively hydrolyzed, resulting in the expansion of inter-molecular distance between cellulose chains. The oldest sample showed a low diffusion rate compared with the others. SEM observation of the textile structures indicated that the oldest sample had two layers due to beating. It was estimated that the tight surface layer blocked the diffusant in the initial stage of the diffusion process.     

Kinetics of simultaneous decrystallization and crystallization induced in high-modulus rayon by bromine water

Studies of the effect of aqueous Br₂ solution on the fine structure of cellulose have shown that the accessibility of the cellulose is a complex function of the duration of treatment. Following immersion of rayon-cellulose in the Br₂ solution, its accessibility is observed to increase until a peak is reached, after which the accessibility decreases with time. For higher temperatures and greater concentrations of Br₂, a sharper peak and faster rates of increase and decrease of accessibility are obtained. In order to explain this behavior, a mathematical model of the kinetics is postulated involving two opposing first-order processes occurring simultaneously. One process describes the decrystallization of crystalline regions within the cellulose by opening intermediate linkages due to Van-der-Waals forces involved. The second process relates to the crystallization of the less-ordered regions. The rate constants for these processes are determined by fitting experimental curves to the postulated analytical expression with the aid of a digital computer program. Experiments carried out on high-modulus rayon showed that for this type of cellulose, the effect described above was very pronounced. From the calculated rate constants, the activation energies of the crystallization and decrystallization reactions were determined. The plots of the rate constants versus 1/T showed that a break occurs at 25°C, corresponding to a second-order transition point of cellulose. The energies of activation of the crystallization process are in the range of 7–9 and 10–13 kcal/mol in the temperature ranges 10–25 and 25–40°C, respectively. The activation energies of the decrystallization process are 13 and 18–24 kcal/mol in these temperature ranges.     

Effects of pretreatments on crystalline properties and morphology of cellulose nanocrystals

Cellulose nanocrystals (CNCs) have drawn tremendous attention because of their extraordinary physical and chemical properties as well as renewability and sustainability. In this work, after a range of pretreatments, such as freeze-drying, ball-milling, mercerization, N-methylmorpholine-N-oxide dissolution and ionic liquid dissolution, various CNCs with different crystalline properties and morphologies were obtained by hydrolysis or oxidation. XRD and AFM were used to determine the influences of pretreatments on the crystalline properties and morphologies of CNCs. New methods, i.e., specific pretreatments followed by sulfuric acid hydrolysis or 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) oxidation, were developed to obtain sphere-like CNCs. It was found that sphere-like CNCs were more likely to be obtained from cellulose materials possessing high accessibility. Pretreatments produced cellulose with various crystallinities and polymorphs, and therefore changed the yields of CNCs and influenced their morphology. CNCs prepared by TEMPO oxidation generally had smaller size than the corresponding products obtained by sulfuric acid hydrolysis. In addition, for the dissolved/regenerated cellulose, TEMPO oxidation was a better method to yield sphere-like CNCs than sulfuric acid hydrolysis.     

Supramolecular Structure - A Key Parameter for Cellulose Biodegradation

Summary: Three different cellulosic substrata, like microcrystalline cellulose, cotton cellulose and spruce dissolving pulp, were chosen for biodegradation. The kinetics of the enzymatic hydrolysis of these celluloses by Trichoderma reesei, has been investigated. The experiments proved the fact that both the morphological structure and the crystalline one are crucial to the process and the ratio of the reactions. In addition, in order to obtain the most accessible cellulose substratum it was studied the biodegradation of cellulose allomorphs of spruce dissolving pulp. The insoluble cellulose fraction remaining after enzymatic hydrolysis was examined by X-ray diffraction method and it was established the degree of crystallinity and the average crystallite size. The enzymatic degradation is also proved by the decrease in the degree of polymerization of hydrolyzed samples.     

Effect of a nonionic surfactant on Trichoderma cellulase treatments of regenerated cellulose and cotton yarns

It has been shown that some surfactants affect the hydrolysis of cellulose by cellulase. In this study, the effect of the surfactant Tween 20 on the hydrolysis of different cellulosic fibers was investigated and related to the cellulose fiber structure. It was found that this non-ionic surfactant enhanced the enzymatic saccharification of highly crystalline cellulosics such as Avicel, Tencel and cotton but not of cuprammonium rayon. The enhanced saccha-rification effected by the surfactant is attributed to inhibition of non-productive sorption of the endoglucanase of the cellulose surface which gives greater access to the cellulose chain ends by the exoglucanase. Although all three fibers lost tensile strength as a result of the enzymatic treatment, no further decrease was effected by the presence of the surfactant.     

Well defined thermostable cellulose nanocrystals via two-step ionic liquid swelling-hydrolysis extraction

Cellulose nanocrystals were prepared from cotton fibers by a two-stage method involving ionic liquid swelling treatment followed by hydrolysis under mild acid conditions. Controlled swelling of cellulosic fibers was achieved in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) at 80 °C, while avoiding extensive dissolution of crystalline regions. Since the accessibility of the substrate was considerably enhanced, the hydrolysis occurred even under mild conditions, using up to 60 times less sulfuric acid than the traditional extraction methods based on concentrated sulfuric acid. The effects of process parameters on nanoparticle morphology, composition and stability were investigated. The individual rod-like nanocrystals, observed under field emission gun scanning electron microscopy, exhibited an average diameter of around 20 nm and a length ranging from 150 to 350 nm. According to X-ray photoelectron spectroscopy and thermogravimetric analysis, the surface of the so-extracted nanoparticles proved to be deprived of contaminating sulfate groups leading to significantly higher thermal stability with respect to cellulose nanocrystals extracted by traditional method in concentrated sulfuric acid.     

Hydrolysis of Egyptian medicated cotton by HCL–FeCl3 reagent

The initial rate of hydrolysis of Egyptian medicated cotton with HCl or HCL-FeCl₃ as reagent, as well as the total carbonyl content of the hydrolysis products, were determined. The results obtained showed that although cellulose fibers are hydrolyzed more rapidly in an acid-FeCl₃ medium than in acid alone, the fibers resisted oxidation in a Fe³⁺ solution. Therefore the observed rate increase in Fe³⁺ solution cannot be explained on the basis of increased oxidation of hydroxyl groups. Consequently it has been suggested that the increase in the rate of hydrolysis is due to complexing between Fe³⁺ and the carbonium/oxonium ion or the leaving group. A comparison between the fine structure of the hydrolysis products of cotton cellulose by the use of HCl and HCl-FeCl₃ reagent is also given.     

Cellulose solvent-based pretreatment for corn stover and avicel: concentrated phosphoric acid versus ionic liquid [BMIM]Cl

Since cellulose accessibility has become more recognized as the major substrate characteristic limiting hydrolysis rates and glucan digestibilities, cellulose solvent-based lignocellulose pretreatments have gained attention. In this study, we employed cellulose solvent- and organic solvent-based lignocellulose fractionation using two cellulose solvents: concentrated phosphoric acid [~85?% (w/w) H₃PO₄] and an ionic liquid Butyl-3-methylimidazolium chloride ([BMIM]Cl). Enzymatic glucan digestibilities of concentrated phosphoric acid- and [BMIM]Cl-pretreated corn stover were 96 and 55?% after 72?h at five filter paper units of cellulase per gram of glucan, respectively. Regenerated amorphous cellulose by concentrated phosphoric acid and [BMIM]Cl had digestibilities of 100 and 92?%, respectively. Our results suggested that differences in enzymatic glucan digestibilities of concentrated phosphoric acid- and [BMIM]Cl-pretreated corn stover were attributed to combinatory factors. These results provide insights into mechanisms of cellulose solvent-based pretreatment and effects of residual cellulose solvents and lignin on enzymatic cellulose hydrolysis.     

Accessibility of hydroxyl groups in birch kraft pulps quantified by deuterium exchange in D2O vapor

Deuterium exchange in a deuterium oxide (D₂O) atmosphere (95 % relative humidity), quantified by a dynamic vapor sorption (DVS) apparatus, was applied for assessing the accessibility of hydroxyl groups in birch kraft pulps. Achieving the maximum deuteration level exhibited slower kinetics than was earlier reported for experiments with ground wood and bacterial cellulose. The deuterium exchange process followed two parallel phenomena. Applying multiple drying and rewetting cycles gave kinetic information also on the hornification phenomenon occurring during these cycles. Dry birch pulps treated with sodium hydroxide solution of varying alkalinities at elevated temperatures were assessed for their accessible hydroxyl groups by DVS with deuterium exchange. This method was evaluated against deuteration combined with Fourier transform infra-red spectroscopy and water retention value (WRV). DVS measurements were in correlation with WRV and both the methods indicated that an alkaline treatment of dry birch pulp improves cellulose accessibility. The level of irreversible deuteration also decreased as the alkalinity was increased. DVS was shown to provide quantitative information on the accessibility but to be a time-consuming method for the pulp samples. A potential means to decrease the duration of the measurement is increased D₂O exposure by excluding the drying phases.     

Paracrystalline lattice disorder in cellulose. I. Reappraisal of the application of the two-phase hypothesis to the analysis of powder x-ray diffractograms of native and hydrolyzed cellulosic materials

An examination of powder x-ray diffractograms of native and hydrolyzed cellulosic materials obtained from widely different sources revealed the presence of materials having a higher degree of molecular order than ramie hydrolyzate, the conventional crystalline standard for cellulose. With the use of these materials as new crystalline standards, a critical reappraisal has been made of the validity of the application of the two-phase (i.e., fringed-micelle) hypothesis to the fine structure of cotton and related cellulosic materials. It is concluded that the lattice structure of cotton and related celluloses of plant or bacterial origin is liquid-like or paracrystalline.     

Reactive and non-reactive counterion surfactants in water and aqueous acetonitrile

Rates of hydrolysis of hydroxamic acids with perfluorooctanoic acid as a reactive counterion surfactant and also with sodium sulfonate surfactants plus HCl have been determined in both water and aqueous acetonitrile solvents. The pseudophase ion exchange model satisfactorily explains the micellar effects for both the reactive and non-reactive counterion surfactants in both solvent systems.     

Availability and disposition of hydroxyl groups on surfaces of crystalline cellulose II

The relative availabilities of hydroxyl groups at C(2), C(3), and C(6) on the D -glucopyranosyl units of a particular, highly ordered hydrocellulose II have been studied by means of the reaction of N,N-diethylaziridinium chloride, which introduces 2-(diethyl-amino)ethyl substituents. The hydrocellulose II was formed by hydrolysis of fibrous cotton cellulose II during 40 min reflux in 2.5M hydrochloric acid and is designated EHC-II (exemplar hydrocellulose II) because it represents the most highly ordered (crystalline) particles in a series of hydrocelluloses. The deviation in the distribution of substituents among the 2-O-, 3-O-, and 6-O- positions of the D -glucopyranosyl units in EHC-II from that in a disordered cellulose, in which the three types of hydroxyl groups are equally available, is the basis for estimated availabilities of the three types of hydroxyl groups on the surfaces of elementary fibrils in EHC-II. The experimental values of relative availabilities of O(2)H:O(3)H:O(6)H for EHC-II were 1.0:0.26:0.72 compared to estimated values of 1.0:0.0:.075 for surfaces of elementary fibrils of an idealized, perfectly ordered cellulose II, a model that is based on intensities of x-ray diffraction peaks.     

Preparation and characterization of cellulose nanofibers from partly mercerized cotton by mixed acid hydrolysis

Cellulose nanofibers with a diameter of 70 nm and lengths of approximately 400 nm were fabricated from partly mercerized cotton fibers by acid hydrolysis. Morphological evolution of the hydrolyzed cotton fibers was investigated by powder X-ray diffraction, Fourier transform infrared analysis and field emission scanning electron microscopy. The XRD results show that the cellulose I was partially transformed into cellulose II by treatment with 15 % NaOH at 150° for 3 h. The crystallinity of this partially mercerized sample was lower than the samples that were converted completely to cellulose II by higher concentrations of NaOH. The intensities of all of the diffraction peaks were noticeably increased with increased hydrolysis time. Fourier transform infrared results revealed that the chemical composition of the remaining nanofibers of cellulose I and II had no observable change after acidic hydrolysis, and there was no difference between the hydrolysis rates for cellulose I or II. The formation of cellulose nanofibers involves three stages: net-like microfibril formation, then short microfibrils and finally nanofibers.     

Enhancing enzymatic hydrolysis of crystalline cellulose and lignocellulose by adding long-chain fatty alcohols

The effects of long-chain fatty alcohols (LFAs) on the enzymatic hydrolysis of crystalline cellulose by two commercial Trichoderma reesei cellulase cocktails (CTec2 and Celluclast 1.5L) were studied. It was found that n-butanol inhibited the enzymatic hydrolysis, but n-octanol, n-decanol and n-dodecanol had strong enhancement on enzymatic hydrolysis of crystalline cellulose in the buffer pH range from 4.0 to 6.0. LFAs can increase the hydrolysis efficiency of crystalline cellulose from 37 to 57 % at Celluclast 1.5L loading of ten filter paper units (FPU)/g glucan. LFAs have similar enhancement on the enzymatic hydrolysis of crystalline cellulose mixed with lignin or xylan. The enhancement of LFAs increased with the decrease of the crystallinity index. LFAs not only enhanced the high-solid enzymatic hydrolysis of lignocellulose, but also improved the rheological properties of high-solid lignocellulosic slurries by decreasing the yield stress and complex viscosity. Meanwhile, LFAs can improve the enzymatic hydrolysis of cellobiose to glucose, especially at low cellulase loading.