J.-F. Revol

Effect of microcrystallite preparation conditions on the formation of colloid crystals of cellulose

Stable colloidal suspensions of cellulose microcrystallites may be prepared from filter paper by sulfuric acid hydrolysis. Above a critical concentration, the suspensions form a chiral nematic ordered phase, or ‘colloid crystal’. The preparation conditions govern the properties of the individual cellulose microcrystallites, and hence the liquid crystalline phase separation of the cellulose suspensions. The particle properties and the phase separation of the suspensions were strongly dependent on the hydrolysis temperature and time, and on the intensity of the ultrasonic irradiation used to disperse the particles. The particle size of the microcrystallites was characterized with transmission electron microscopy and photon correlation spectroscopy. The surface charge was determined by conductometric titration. It was possible to fractionate the microcrystallites by size using the partitioning between isotropic and liquid crystalline phases; the longer microcrystallites migrate to the liquid crystalline phase

Helicoidal self-ordering of cellulose microfibrils in aqueous suspension

In many skeletal support systems of plants and animals, cellulose, chitin, and collagen occur in the form of microfibrils ordered in a chiral nematic fashion (helicoids). However, these structures remain poorly understood due to the many constituents present in biological tissues. Here we report an in vitro system that attracts by its simplicity. Only one chemical component, cellulose, is present in the form of fibrillar fragments dispersed in water. Above a critical concentration the colloidal dispersion separates spontaneously into a chiral nematic liquid crystalline phase. On drying this phase solidifies into regularly twisted fibrillar layers that mimic the structural organization of helicoids in nature.

Chiral nematic suspensions of cellulose crystallites; phase separation and magnetic field orientation

Abstract Suspensions of rod-like cellulose crystallites of axial ratio ≈ 20–40, prepared by acid hydrolysis of natural cellulose fibres with sulphuric acid, give stable ordered fluids that display well-formed textures and disclinations characteristic of chiral nematic liquid crystalline phases. The critical volume fraction for phase separation of salt-free suspensions is typically 0.03, with a relatively narrow biphasic region. Because of the negative diamagnetic susceptibility of cellulose, the ordered phase becomes oriented in a magnetic field with its chiral nematic axis parallel to the applied field.

In vitro chiral nematic ordering of chitin crystallites.

Microfibrillar fragments of purified crab and shrimp chitin were prepared by hydrolysis in 3 M HCl at its boiling point (104 degrees C). After removal of the acid by centrifugal washing and dialysis, an ultrasound treatment converts the residual product to a colloidal suspension stabilized by NH3+ charges. When dewatered to a critical concentration, spontaneous formation of a two-phase equilibrium system system occurs. The upper phase (lower concentration) is isotropic and the lower phase is anisotropic. The latter displays chiral nematic order and dries to a solid film which mimics the helicoid organization characteristic of the chitin microfibrils in the cuticle of arthropods.

Atomic force microscopy of cellulose microfibrils: comparison with transmission electron microscopy

Abstract Atomic force microscopy (AFM) and transmission electron microscopy (TEM) have been used to image well-characterized algal cellulose microfibrils. Cross-sections of the microfibrils observed by TEM are square, whereas the AFM topography of these microfibril surfaces shows a rounded profile due to convolution with the shape of the AFM tip. Height and base width measurements taken from cross-sections of these AFM micrographs also show a marked dependence on the scan rate of the AFM tip. AFM images of the surface of the highly crystalline cellulose microfibrils were obtained at atomic resolution under ambient conditions; the images showed periodicities along the microfibril axis of 1.07 and 0.53 nm that may correspond to the fibre and glucose unit repeat distances, respectively.

Effect of degree of deacetylation of chitin on the properties of chitin crystallites

In the present article, chitin from crab shell was systematically deacetylated using a NaOH treatment with control of the reaction time. The degree of deacetylation, monitored using solid-state NMR, revealed that the reaction was pseudo-first order. Based on this, swollen and NaOH-saturated particles are proposed as the reaction system. The weight loss of the partially saponified and neutralized samples after HCl hydrolysis increased linearly with the degree of deacetylation. The crystallinity of the samples was found to increase after acid hydrolysis. According to conductimetric titration, the surface charge density of the crystallites, after acid hydrolysis, was found to increase with base treatment time. The effect of surface charge on the formation of a chiral nematic phase, due to the rodlike nature of the crystallites, was explored. These results show that because the contribution of charged particles to the ionic strength was significant the double layer compression was affected, especially since the surface charge density was close to the Manning limit.

Atomic force microscopy and transmission electron microscopy of cellulose from Micrasterias denticulata; evidence for a chiral helical microfibril twist

Atomic force microscopy (AFM), tapping mode atomic force microscopy (TM-AFM) and transmission electron microscopy (TEM) have been used to image the cell wall, ultrathin sections of whole cells and cellulose microfibrils prepared from the green alga Micrasterias denticulata. Measurements of the microfibril dimensions are in agreement with earlier observations carried out by electron microscopy. Images at the molecular level of the surface of the microfibrils were obtained with AFM and show regular periodicities along the microfibril axis that correspond to the fibre and glucose repeat distances of cellulose. Twisted regions visible at intervals along the microfibrils dried down onto substrates were noted to be right-handed in over 100 observations by TEM, AFM and TM-AFM.

EFFECT OF ELECTROSTATIC INTERACTION ON PHASE SEPARATION BEHAVIOUR OF CHITIN CRYSTALLITE SUSPENSIONS

Optical and electron microscopy were used to observe the dynamics of the phase separation in aqueous chitin suspensions prepared by HCl hydrolysis of crab chitin. Freeze-fracture transmission electron microscopy reveals that chitin crystallites are partially aggregated in the suspension and have an average length of 200 nm and an average width of 8 nm. They exhibit a positive surface charge of approximately 0.5 e/nm2 when fully protonated. The liquid crystal-forming aqueous suspensions of such crystallites are investigated through phase diagrams and Zeta potential measurements for different ionic strength. Exposure of the suspension to a low concentration of univalent electrolyte has a negligible effect on phase separation because of the contribution of the charged crystallites themselves to the ionic strength. The thickness of the effective repulsive layer is estimated both from the phase diagrams according to Onsagers theory and from the computed interaction energy derived from the Poisson-Boltzmann equation using the experimental Zeta potential as surface potential. When the contribution of crystallites to the ionic strength is taken into account and an hypothetical linear charge density close to the Manning limit is assumed, there is good agreement with the Debye length.

Characterization and supramolecular architecture of the cellulose‐protein fibrils in the tunic of the sea peach (Halocynthia papillosa, Ascidiacea, Urochordata)

Summary— The cellulose‐protein fibrils, which constitute by far the bulk of the fibrous fraction of the sea peach tunic (Halocynthia papillosa), were structurally and chemically characterized, either in situ or after extraction procedures, with the use of classical electron microscoy combined with diffraction contrast imaging and electron diffraction, histochemistry, affinity cytochemistry and chemical analysis. These fibrils exhibit a cross‐sectional shape close to a parallelogram. The cyrstallites forming their core, with lateral dimensions ranging from roughly 5 to 20 nm, are composed of native cellulose of higher crystallinity than that of plant cellulose. They are associated with acid mucopolysaccharidés (amps) and proteins which form a coating material appearing as a continuous sheath enveloping the axial crystallite in the cuticular layer or as patches more‐or‐less periodically distributed around and along the fibre axis in the fundamental layer. Tunicin, the alkali‐insoluble fibrous fraction, is not pure cellulose, yielding only 22–60% of its dry weight as glucose equivalents, depending on the tunical layer. It is suggested that in addition to the high degree of crystallinity of the tunical cellulose, the presence of a significant amount of coating material composed of amino acids and proteoglycans firmly linked to cellulose molecules contributes to tunicins high resistance to hydrolysis.

In situ synthesis of ferrites in ionic and neutral cellulose gels

Abstract The filaments of a tyrecord rayon were modified by two methods to enhance the degree of swelling of the material: (1) carboxyl and sulfonic acid substituents were introduced into the rayon and (2) the filaments were swollen in sodium hydroxide. The water-swollen filaments were rendered magnetic by in situ synthesis of ferrites and the resulting magnetic filaments were characterized by transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). Two other non-ionic, highly swollen cellulose gels were used as matrices for in situ synthesis of ferrites: a never-dried, wet-spun model cellulose filament and a never-dried bacterial cellulose membrane. TEM micrographs of thin cross-sections of the magnetic gels showed that the nanometre-sized ferrites were uniformly distributed whereas the treated rayon filaments had ferrites predominantly at the filament surface. All the materials were superparamagnetic as determined by VSM. However, a ferrimagnetic component was detected after several reaction cycles for the bacterial cellulose membrane by Mossbauer spectroscopy.