Morphology of dry lignins and size and shape of dissolved kraft lignin particles by X-ray scattering

Lignin is a highly branched polymer consisting of phenylpropane units, and it is one of the ingredients of the supporting matrix in plant cell walls. The morphology of several lignins extracted from plant cell walls using different methods was studied by small-angle and ultra-small-angle X-ray scattering. A power-law type intensity was observed for the dry lignins, but on the basis of the power-law exponent the fractal approach often applied to lignins is not fully justified. However, the intensity of kraft lignin did show a power law with surface fractal dimension D(s) = 2.7 +/- 0.1. The specific surface area of the lignins ranged from about 0.5 to 60 m(2)/g with 20% relative accuracy. The radius of gyration was determined from small-angle X-ray scattering data for aqueous solutions of kraft lignin. The shape of the particles in NaCl and NaOH solutions was found to be elongated. The particles were about 1-3 nm thick, while the length (5-9 nm) depended on the solvent and on the lignin concentration. The size of these primary particles was approximately the same as the size of the pores in the fractal aggregates of the dry kraft lignin. Their size was determined to be about 3.5 nm.     

The influence of lignin on the self-assembly behaviour of xylan rich fractions from birch (Betula pendula)

A crude xylan isolate obtained by prehydrolysis and mild alkaline extraction from birch wood chips (Betula pendula), and a carefully delignified xylan fraction from the same source, were examined by dynamic light scattering (DLS) and cryogenic transmission electron microscopy (cryo-TEM) with regard to their propensity to self-assemble in water into insoluble aggregates. The delignification involved the extraction with chloroform of a crude xylan solution in a pyridine/acetic acid/water mixture. It resulted in a purified xylan fraction in a yield of 23% in which 75 and 90% of the lignin had been removed as indicated by Klason and UV-determination, respectively. It was found that both xylan fractions formed agglomerates by self-assembly in water. However, DLS and cryo-TEM indicated that the aggregates were larger in size (90 vs. 40 nm) and greater in mass when more lignin was present. The addition of an alkaline solution of isolated lignin (obtained by steam explosion) to increasing concentrations of a delignified xylan revealed increasing turbidity. Our conclusion is that lignin induces agglomeration of xylan in aqueous solutions, but xylan concentration plays an active role in the aggregation phenomena. An agglomeration mechanism for lignin rich xylan fractions is proposed.     

Application of Differential UV Spectroscopy to the Determination of Lignin Substances in Polluted Water

The absorption spectra of Bjorkman lignin, lignin sulfate, lignosulfonates, and humic substances in neutral and alkaline media are considered. It was found that the difference spectra of lignins (an alkaline solution with reference to a neutral solution) exhibited several intense bands, whereas humic substances exhibited equal shifts of the spectra over the entire spectral range. This distinctive property of the difference spectra of lignin substances, as compared to the spectra of humic substances, can be used for determining lignins in polluted water.     

Surfactant properties of kraft lignins

Formation of the adsorption layer at the solution-air interface is characterized by data on the surface tension in alkaline solutions of kraft lignins extracted from deresined spruce and birch wood. The effects of the lignin nature and solution concentration on intermolecular interaction in the surface layer are studied.     

A neutron scattering study of the structure and water partitioning of selectively deuterated copolymer micelles

We present a scattering study of a selectively deuterated micelle-forming diblock copolymer. The copolymer comprises a partially deuterated polystyrene (d,h-PS) block and an imidazolium-functionalized PS (IL) block. In toluene solutions, the copolymers assemble into elongated micelles where the IL block forms the micelle core. Through dynamic light scattering (DLS) measurements, we obtain the overall size of the micelles. In our small-angle neutron scattering (SANS) studies, we use contrast matching to characterize the IL core and the PS shell of the micelles independently. The PS block forming the micelle shell exhibits either a starlike or brushlike conformation depending upon the size of the core to which it is tethered. We find the IL block to be in an extended conformation, driving the formation of slightly elongated and relatively stiff micelle cores. The elongated micelle core cross-sectional radius and length depend linearly on the length of the IL block. We find that the micelles can sequester a few water molecules for each IL repeat unit; the addition of water slightly increases the cross section of the elongated micelles.     

Micelles of imidazolium-functionalized polystyrene diblock copolymers investigated with neutron and light scattering

We synthesize a series of block copolymers comprising a polystyrene (PS) block and an imidazolium-functionalized PS (IL) block and characterize their assembly properties. We use small-angle neutron scattering and dynamic light scattering to determine the micelle size and shape in dilute solutions and to assess the micelle interactions in concentrated solutions. By studying a series of copolymers with fixed PS block length, we find that the length of the IL block governs the micelle dimensions. Our data suggest that these copolymers form elongated micelle structures where the IL block is extended in the micelle core. We find that these micelles can sequester water and that interactions between the micelles lead to structure factor peaks at elevated concentrations.     

Molecular aggregates of partially fluorinated quaternary ammonium salt gemini surfactants

The size and shape of novel partially fluorinated gemini surfactant 1,2-bis[dimethyl-(3-perfluoroalkyl-2-hydroxypropyl)ammonium]ethane bromide (CnFC3-2-C3CnF, where n=4, 6, and 8) were investigated in aqueous solution by means of light scattering and transmission electron microscopy (TEM). The sizes of these molecular aggregates changed with increasing carbon number of the alkyl chain and concentration. For example, the apparent hydrodynamic radius by dynamic light scattering was 18 nm at a concentration of cmcx5 for n=4, 115 nm at the cmcx15 for n=6, and 62 nm at the cmcx30 for n=8, at 298.2 K. The shapes of CnFC3-2-C3CnF aggregates drastically changed with the alkyl chain length; the aggregates were mainly in the form of large or irregular small aggregates (n=4), string-like aggregates (n=6), and vesicles (n=8). The bromide-ion activity was measured using a bromide-ion-selective electrode to determine the degree of counterion binding to the aggregates. The degree of counterion binding to aggregate was very small compared with that in the typical hydrogenated gemini surfactants. These results indicated that the small curvature of large aggregates was not influenced by an electrostatic repulsion between the cationic head groups in the case of the bulky molecular volume of fluorinated gemini surfactants.     

Molecular characterization of starch polysaccharides

Light scattering and rheologic measurements have been carried out with native starch in DMSO and with degraded amylopectin of a starch hydrolysate in water at different temperatures. The measurements of dilute solutions in DMSO were made to get a reference for the molecularly dispersed state. In water association take place which under certain conditions leads to gelation. An aqueous solution of about 11.5% (w/w) degraded amylopectin was investigated in detail. Structural change was observed during cooling the solution below 20°C with both techniques. A fractal dimensions of about 2.5 was found at temperatures above 20°C indicating non-swollen random aggregates. Light scattering proved to be the more sensitive method than rheology, but in both experiments a structural collapse was detectable. A fractal dimension of about 1 was now obtained which is characteristic for rigid, elongated supra-structures. A certain time after this collapse an increase of loss and storage module and particle growth in light scattering were obtained. The crossing of the G' and G” was taken as point of gelation and the same gelling time was estimated from light scattering. Hence, the combination of oscillatory rheology with light scattering is an appropriate procedure for a thorough investigation of such complex, structure forming systems.     

Easy synthesis of beta-O-4 type lignin related polymers

The beta-O-4 structure is the most abundant substructure in lignin. Lignin related polymers composed of only the beta-O-4 structure were prepared using simple aromatic compounds as starting materials. Acetophenone derivatives were brominated, polymerized in the presence of K2CO3 and reduced with NaBH4 to give the lignin related polymers. These are linear polymers which resemble natural lignins in their structures, although they do not have a gamma-hydroxymethyl group. The number average degree of polymerization (DPn) was determined with peracetate of the polymers by gel permeation chromatography. The DPn of guaiacyl type polymers ranged from 15.2-21.4, where the value for the syringyl type was 11.3 and for the p-hydroxyphenyl type 16.9. The Guaiacyl type polymer was very soluble in usual lignin solvents such as 1, 4-dioxane-water (96 : 4, v/v) and DMSO, but only slightly soluble in acetone-water (9 : 1, v/v).     

Intense fluorescence from light-driven self-assembled aggregates of nonionic azobenzene derivative

We describe here the highly fluorescent self-assembled spherical aggregates of an azobenzene molecule without a specific ionic component in organic solution under UV light illumination. The first stage of trans-to-cis photoisomerization by UV light at 365 nm was followed by a significant enhancement, up to about 1000 times, of the emission from an azobenzene molecule (CN2Azo) with a long alkyl chain, which is due to the spontaneous formation of spherical organic aggregates. Fluorescence emission was further enhanced in the dark, and the quantum yield increased to about 0.3. We also report the significant size and structural changes of the aggregates, from nanometer-scale micelle-like aggregates to micrometer-scale vesicular aggregates, obtained only from the variation in the concentration of an azobenzene derivative. The light-driven azobenzene aggregates show the size and structure dependences of emission wavelength from violet-blue to green-yellow.     

Early stages of formation of branched host-guest supramolecular polymers

A structural characterization of host-guest supramolecular copolymers, formed by an adamantane dimer and two beta-cyclodextrin trimers in aqueous solution, has been carried out by combining small angle X-ray scattering and light scattering experiments. A shape-reconstruction method was applied to the SAXS data to obtain relatively high-resolution conformation information, and a correlation with the experimental dynamic light scattering results was performed, by estimating the hydrodynamic radii of the reconstructed shape through a shell model method. When applied on the solutions of the trimers, the analysis provides a globular reconstructed shape with a hydrodynamic radius in agreement with the experimental one. For the polymers, elongated structures were inferred which grow both in length and in cross section by increasing the concentration. Depending on the beta-cyclodextrin trimer employed in the polymer preparation, polymerization degrees ranging between roughly 7 and 14 or 9 and 22 were obtained in the concentration range 4.00-10.0 or 3.10-6.60 mM of the trimer (6.00-15.0 or 4.65-9.90 mM of the dimer). Aggregation schemes were proposed accounting for the formation of hyperbranched, linear, and network like polymers. The experimental results are not far from those expected on the basis of the aggregation in hyperbranched structure, for which the growth of elongated aggregates can be predicted in the early stages of the polymerization. However, the coexistence of the other structures, in particular of the linear one, cannot be ruled out.     

Structure of Alkali Lignins Fractionated from Ricinus communis and Bagasse. 1. Chemical Constituents

Fractionation of alkali lignins of the soda and sulfate pulping processes of Ricinus communis and bagasse was carried out by using successive equal concentrations of the alkaline reagent. Soda lignins were soluble in organic solvents, while the sulfate ones were sparingly soluble. Thus, two fractions of the sulfate lignins, soluble and insoluble, could be obtained from acetone. The different alkali lignin fractions were subjected to elemental and functional group analyses. For both Ricinus communis and bagasse, the carbon content of the fractions of the various types of lignin is in the order: soluble sulfate > soda > insoluble sulfate, while the methoxy is in the order: soda > soluble sulfate > insoluble sulfate. The phenolic OH content, as well as OH/C₉ of soda lignins of bagasse, are lower than those of soluble sulfate lignins. For Ricinus communis, the phenolic OH content and OH/C₉ is higher for some of the fractions (first three stages of cooking) of soda lignin than the corresponding fractions of kraft (sulfate) lignin, while the reverse takes place for the other fractions. For the same type of lignin, the fractions showed changes in their carbon, methoxyl, and phenolic OH contents. The change may be regular, i.e., increase or decrease with the order of stage of cooking, or irregular. Molecular weights of the different alkali lignins which are soluble in organic solvents ranged between 750 and 840.     

Structure and size of spontaneously formed aggregates in Aerosol OT/PEG mixtures: effects of polymer size and composition

Dynamic light scattering and Cryo-TEM measurements have allowed us to obtain the size and structure of spontaneous aggregates formed by mixtures of Aerosol OT, AOT, and ethylene glycol polymers of different molecular mass. The results presented in this work show that small unilamellar vesicles predominate in pure Aerosol OT solutions and in dilute polymer solutions mixed with AOT. In the latter case, elongated micelles coexist with unilamellar vesicles. When polymer concentration increases above a certain concentration, the small vesicles disappear and the size of the elongated micelles decreases to a radius compatible with spherical micelles. For PEG concentrations above the overlapping ones, spherical micelles coexist with very large aggregates probably formed by large rod like micelles or by superstructures of elongated micelles embedded in a polymer network. This behavior is consistent with theoretical models based in molecular mean-field theory [M. Rovira-Bru, D.H. Thompson, I. Szleifer, Biophys. J. 83 (2002) 2419]. The properties of the different types of aggregates are obtained by fluorescence spectroscopy and electrophoretic mobility measurements.     

Evidence of Shear Rate Dependence on Restructuring and Breakup of Latex Aggregates

Small-angle static light scattering has been used to probe the evolution of aggregate size and structure in the shear-induced aggregation of latex particles. The size of aggregates obtained from the particle-sizing instrument (Coulter LS230) was compared with the size of those obtained with another approach utilizing the Guinier equation on the scattering data. Comparison of the two methods for studying the effects of mixing on the evolution of the aggregate size with time revealed similar trends. The aggregate structures were quantified in terms of their fractal dimensions on the grounds of the validity of Rayleigh-Gans-Debye scattering theory for the fractal aggregates. Analysis of the scattering patterns of aggregates verified that restructuring of the aggregates occurred as the aggregates were exposed to certain shear environments, resulting in a scale-dependent structure that could not be quantified by a fractal dimension. The effect of restructuring on aggregate size was particularly noticeable when the aggregates were exposed to average shear rates of 40 to 80 s(-1), whereas no significant restructuring occurred at lower shear rates. At 100 s(-1), the fragmentation of aggregates appeared to be more significant than aggregate compac-tion. Copyright 2001 Academic Press.     

Structural features and thermal degradation properties of various lignin macromolecules obtained from poplar wood (Populus albaglandulosa)

Four different lignins obtained from poplar wood (milled wood lignin: ML, organosolv lignin: OL, ionic liquid lignin: IL and Klason lignin: KL) were subjected to several types of chemical/thermal analyses to compare their structural features and thermal decomposition properties. The ML, OL, IL and KL yield from poplar wood was 5.5, 3.9, 5.8, 19.5 wt%, respectively. Functional group analysis revealed that during the OL and KL extraction processes, the condensation reaction involved with phenolic hydroxyl groups of lignins significantly prevailed, which led to a highly condensed OL and KL structure. Thermogravimetric analysis (TGA) results showed that OL and KL thermal stability was much higher than that of ML and IL. The derivatization followed by reductive cleavage (DFRC) data showed that the thermal stability was highly associated with the frequency of arylglycerol-β-aryl ether (β-O-4) linkages in the lignin polymers. Pyrolysis-GC/MS (Py-GC/MS) analysis confirmed that acetic acid and several types of phenolic compounds were the main lignin pyrolysis products. The maximum sum of ML (13.8 wt%), OL (9.9 wt%) and IL (11.8 wt%) pyrolysis products was obtained at the pyrolysis temperature of 600 °C, whereas KL (1.6 wt%) was significantly lower due to its high thermal stability and condensation degree. The S- and G-type pyrolysis products (S/G) ratio varied from 1.61 to 1.93 for ML, 2.28 to 5.28 for OL, 2.06 to 2.86 for IL and 1.40 to 2.20 for KL, depending on the pyrolysis temperature, which ranged between 400 °C and 700 °C.     

Spontaneous vesicle formation of single chain and double chain cationic surfactant mixtures

The concentration vs composition diagram of aggregate formation of the dodecyltrimethylammonium bromide (DTAB) and didodecyldimethylammonium bromide (DDAB) mixture in aqueous solution at rather dilute region was constructed by analyzing the surface tension, turbidity, and electrical conductivity data and inspected by cryo-TEM images and dynamic light scattering data. Although the aqueous solution of DTAB forms only micelles, the transition from monomer to small aggregates and then to vesicle was found at 0.1 < X2 相似文献   

Regioselectivity of enzymatic and photochemical single electron transfer promoted carbon-carbon bond fragmentation reactions of tetrameric lignin model compounds

New types of tetrameric lignin model compounds, which contain the common β-O-4 and β-1 structural subunits found in natural lignins, have been prepared and carbon-carbon bond fragmentation reactions of their cation radicals, formed by photochemical (9,10-dicyanoanthracene) and enzymatic (lignin peroxidase) SET-promoted methods, have been explored. The results show that cation radical intermediates generated from the tetrameric model compounds undergo highly regioselective C-C bond cleavage in their β-1 subunits. The outcomes of these processes suggest that, independent of positive charge and odd-electron distributions, cation radicals of lignins formed by SET to excited states of sensitizers or heme-iron centers in enzymes degrade selectively through bond cleavage reactions in β-1 vs β-O-4 moieties. In addition, the findings made in the enzymatic studies demonstrate that the sterically large tetrameric lignin model compounds undergo lignin peroxidase-catalyzed cleavage via a mechanism involving preliminary formation of an enzyme-substrate complex.     

Supramolecular p-n-heterojunctions by co-self-organization of oligo(p-phenylene vinylene) and perylene bisimide dyes

Comparative studies on hydrogen-bonded versus covalently linked donor-acceptor-donor dye arrays obtained from oligo(p-phenylene vinylene)s (OPVs) as donor and bay-substituted perylene bisimides (PERYs) as acceptor dyes are presented. Both systems form well-ordered J-type aggregates in methylcyclohexane, but only hydrogen-bonded arrays afford hierarchically assembled chiral OPV-PERY dye superstructures consisting of left-handed helical pi-pi co-aggregates (CD spectroscopy) of the two dyes that further assemble into right-handed nanometer-scale supercoils in the solid state (AFM study). In the case of hydrogen-bonded arrays, the stability of the aggregates in solution increases with increasing conjugation length of the OPV unit. The well-defined co-aggregated dyes presented here exhibit photoinduced electron transfer on subpicosecond time scale, and thus, these supramolecular entities might serve as valuable nanoscopic functional units.