Solute Exclusion Technique Research Articles

How much water can wood cell walls hold? A triangulation approach to determine the maximum cell wall moisture content.

Wood is a porous, hygroscopic material with engineering properties that depend significantly on the amount of water (moisture) in the material. Water in wood can be present in both cell walls and the porous void-structure of the material, but it is only water in cell walls that affects the engineering properties. An important characteristic of wood is therefore the capacity for water of its solid cell walls, i.e. the maximum cell wall moisture content. However, this quantity is not straight-forward to determine experimentally, and the measured value may depend on the experimental technique used. In this study, we used a triangulation approach to determine the maximum cell wall moisture content by using three experimental techniques based on different measurement principles: low-field nuclear magnetic resonance (LFNMR) relaxometry, differential scanning calorimetry (DSC), and the solute exclusion technique (SET). The LFNMR data were furthermore analysed by two varieties of exponential decay analysis. These techniques were used to determine the maximum cell wall moisture contents of nine different wood species, covering a wide range of densities. The results from statistical analysis showed that LFNMR yielded lower cell wall moisture contents than DSC and SET, which were fairly similar. Both of the latter methods include factors that could either under-estimate or over-estimate the measured cell wall moisture content. Because of this and the fact that the DSC and SET methods are based on different measurement principles, it is likely that they provide realistic values of the cell wall moisture content in the water-saturated state.

Equilibrium moisture content in wet pressing of paper

Equilibrium moisture is a limiting factor in achieving high solids in the later stages of pressing or pressing low basis weight grades. We have developed a model that relates equilibrium moisture directly to the pore size distribution of fibers as measured by the solute exclusion technique. The model shows that chemical pulping and refining increase equilibrium moisture by increasing pore volume at given pore sizes in fibers, which leads to lower pressed solids and greater energy expenditure in the dryer section. Means to increase equilibrium moisture without compromising pulp strength are briefly discussed.

Structural modification of pine and poplar wood by alkali pretreatment to improve ethanol production

The softwood pine and hardwood poplar were subjected to NaOH and Na2CO3 alkaline pretreatments for their conversion into ethanol. The resulting solids were characterized in detail for structural modifications. Both alkaline pretreatments enhanced ethanol yields, and the NaOH pretreatment at 93 °C resulted in the highest increase in ethanol yields of 297.5 and 249.5 % for pine and poplar woods, respectively, as compared to untreated woods. In all cases, the content of lignin and hemicellulose in pretreated solids was decreased by the pretreatments, while glucan contents, accessible surface area, and enzymatic hydrolysis yields were increased. New correlations between the accessible surface area, lignin removal, and the ethanol yields were obtained for both feedstocks, indicating that the accessibility of enzyme to the surface area is the most important factor affecting enzymatic hydrolysis. The results also showed that the solute exclusion technique is a simple approach that can be applied to predict ethanol yields from pine and poplar. Among the pretreatment conditions employed for soft- and hardwood, the NaOH pretreatment at 93 °C is the most efficacious. The glucose yields were 8.4 % and 13.1 % for untreated pine and poplar woods, respectively. However, pretreatment at 93 °C with NaOH increased the glucose yield to 46.5 % and 69.3 % for pine and poplar, respectively.

High consistency mechano-enzymatic pretreatment for kraft fibres: effect of treatment consistency on fibre properties

Fibre reactivity is essential for cellulose dissolution and derivatisation and a porous fibre structure is one key determinant for a highly reactive pulp. Mechanical and enzymatic treatments are known to improve fibre reactivity and more recently, the combination of mechano-enzymatic treatment has been shown to synergistically enhance the beneficial effect. The aim of this work was to do a systematic study on the effect of dry matter content during enzymatic modification of fibres and define the conditions that optimally improve fibre porosity. The combined mechano-enzymatic treatments at 15–25 w% consistency had the most pronounced effect on fibre porosity and morphology analysed by solute exclusion technique, nitrogen sorption and scanning electron microscopy. Light microscopy imaging confirmed that the combined mechano-enzymatic treatment at high consistency (> 10 w%) resulted in extensive fibrillation of the treated fibres which was not observed after sole mechanical or enzymatic treatments.

Determining pore size distribution in wet earlywood cell wall by solute exclusion using total organic carbon technique (TOC)

Solute exclusion is a probe technique useful to determine the pore volume and pore size distribution of a cell wall network; however, accurate measuring of molecular probe concentration is critical. In this study, the concentration of polyethylene glycol (PEG) and glucose was measured by detection of their organic carbon using a total organic carbon (TOC) equipment. PEG and glucose concentration by TOC is based on the quantitative oxidation of the organic matter contained in the solution, and then, produced CO2 is stoichiometrically correlated with the TOC value. In this work on Pinus radiata earlywood cell wall, the fiber saturation point was 0.72 mL/g and it was reached by a molecular probe size with a radius of gyration R g = 5.96 A. Moreover, the mean pore size present in the wood samples has a size equivalent to a molecular probe with a radius of gyration of 3.77 A. Since TOC is based on the quantitative oxidation of the organic matter contained in the solution, it is essential to wash and rinse the wood samples to prevent that other carbon sources as extractives modify PEG and glucose concentration. Finally, the accuracy of TOC to determine the pore size distribution in the wet wood cell wall is subject to limitations of the solute exclusion technique.

The effects of lignin removal and drying on the porosity and enzymatic hydrolysis of sugarcane bagasse

The porosity of lignocellulosic materials is a key feature for the enzymatic hydrolysis of the constituent polysaccharides, being affected by its drying and lignin content. Here we evaluated the influence of both parameters in the porosity of sugarcane bagasse, using raw and chlorite-delignified samples. A fraction of these samples was air dried at room temperature and the remainder one was kept wet. All the samples were subjected to porometry (solute exclusion technique), determination of water retention value and assessment of enzymatic saccharification of polysaccharides. Samples with higher lignin contents had lower porosities and exhibited worse enzymatic conversions of polysaccharides. Mild drying collapsed only the smaller pores, which already were inaccessible to enzymes, and therefore did not affect the efficiencies of saccharification. Our results show that the lignin content plays an important role in the porosity of the material and that its removal improves the enzymatic saccharification of the constituent polysaccharides.

Physicochemical design of the morphology and ultrastructure of cellulose beads

Cellulose was dissolved in NaOH–urea–water and beads were prepared by coagulation into nitric acid as well as saline solution. Morphology and ultrastructure of the beads were modified by controlling the molarity of the acid (0–10M) and temperature (5–50°C) of the coagulation media and the cellulose concentration (3–7%). The beads were characterized by optical image analysis (shape, volume, and size distribution) and weight (total porosity). Cross-sections of CO2 critical point dried beads were studied by field emission scanning electron microscopy (FE-SEM) and specific surface areas of 336–470m2g−1 were determined from nitrogen adsorption isoterms. Pore size distribution was analyzed using solute exclusion technique. Our results demonstrate that the ultrastructure can be controlled by alteration of the coagulation conditions. Changes in size, shape and surface area were substential. Also generation of micro- (⩽2Å), meso-, or macropores (⩾50Å) can be favored.

Quantitative Analysis of Cationic Poly(vinyl alcohol) Diffusion into the Hairy Structure of Cellulose Fiber Pores: Charge Density Effect

The diffusion of charged polymers into the pores of cellulose fibers has not yet been fully understood due to the complexity of the interaction between polymers and fibers. In this paper, the diffusion of cationic-modified poly(vinyl alcohol) (CPVA) with tailored charge densities and a relatively high molecular weight into the pores of bleached aspen high-yield pulp (via a chemi-thermomechanical pulping process) was quantitatively investigated via an adsorption analysis, charge density analysis, and solute exclusion technique (SET). The results showed that the adsorption of the low-charged CPVA was substantially higher than that of the high-charged CPVA on fibers. The surface charge density analysis confirmed that approximately 17 mg/g of the high-charged CPVA adsorbed on the outer surface and on the macropores of fibers and the remaining (23 mg/g) diffused into the pores. The SET analysis confirmed that the pore size of fibers was more significantly reduced by applying the low-charged CPVA than the high-charged one. The influencing factors for the diffusion of CPVA into the large and small pores were related to the repulsion force developed between the adsorbed polymers and approaching polymers, entropy increase, and the polymer flexibility. The Brunauer-Emmett-Teller surface area analysis showed an increase in the surface area of fibers upon CPVA adsorption. It was proposed that the diffused CPVA prevented complete fiber pore collapse during drying, which eventually increased the surface area of fibers.

Characterization of high-yield pulp (HYP) by the solute exclusion technique

By following the solute exclusion technique, we determined the pore characteristics of 3 hardwood high-yield pulps (aspen, birch, maple). The fiber saturation point (FSP) was 1.40, 1.36 and 1.19 g water/g pulp, for aspen, birch, maple, respectively, which is lower than that of chemical pulps. Different fractions obtained from the Bauer–McNett classifier showed that the HYP fines have much more pore volume than their long fiber counterparts. The effects of beating, drying and re-wetting on the pore characteristics of HYP were also studied. Beating led to increased total pore volumes. Upon drying and re-wetting, much of the small pores from the HYP underwent permanent closure while the big pores were only slightly affected. Finally, the relationship between the water retention value (WRV) and FSP for HYP was examined.

Qualitative characterization of the diffusion of cationic-modified PVA into the cellulose fiber pores

In this work, the diffusion of cationic poly (vinyl alcohol) (CPVA) with well-defined structures into the pores of bleached kraft fibers was investigated at equilibrium level of adsorption by means of adsorption, solute exclusion technique (SET), and nuclear magnetic resonance (NMR) relaxation method. The results showed that the interaction between CPVA and dextran polymers was weak, which allowed us to use the SET method to investigate the variation in the pore size of fibers upon diffusing CPVAs. Both adsorption and SET methods confirmed the diffusion of CPVA polymers into the fiber pores. However, the NMR technique was unable to reflect the pore size changes. The main reason for such results was probably the error that occurred in removing water between fibers via applying the water retention value (WRV) test prior to the NMR analysis. In fact, not all of the water on the fiber surface, especially on the surface of CPVA-modified fibers, could be removed by the WRV, introducing some error in the NMR analysis.

Poly[( N-isopropylacrylamide- co-acrylamide- co-(hydroxyethylmethacrylate))] thermoresponsive microspheres: An accurate method based on solute exclusion technique to determine the volume phase transition temperature

Poly[( N-isopropylacrylamide- co-acrylamide- co-(hydroxyethylmethacrylate))] [poly(NIPAAm- co-AAm- co-HEMA)] copolymer was synthesized as a new thermoresponsive material possessing a lower critical solution temperature (LCST) around 37 °C in phosphate buffer, pH 7.4, at a solution concentration of 1%, w/v. The influence of polymer concentration on LCST was determined by cloud point measurements and by microcalorimetric analysis. The copolymer was transformed in hydrogel microspheres by suspension reticulation of OH groups with glutaraldehyde. The volume phase transition temperature (VPTT) of microspheres was determined by a new approach, which involves measurement of the increase in concentration of a blue dextran (BD) solution at different temperatures in the presence of dry microspheres. The minimum BD concentration that gives reliable and reproducible results was determined to be 1 mg/ml. However, the higher is the concentration of BD in solution the smaller is the error. Contrary to solution of the linear polymer which displays a sharp phase transition temperature, the dependence of water regain of the hydrogel with temperature lasts from 4 °C to 50 °C.

Development of Super Water-absorbent from Cellulosic Materials. (4). Porous Structure of Water Absorbent.

In this paper, porous structures of the swollen hydrogels of the cross-linked CM-LBKP were examined by solute exclusion technique. Water absorbents prepared by cross-linking of CM-LBKP with different types of polyethylene glycol diglycidyl ethers (PEGDGEs) had very porous structures. The total pore volume of cross-linked CM-LBKP up to 560 Å in width was about 100 times of that of untreated pulps. Increased amount of cross-linking agent bound to CM-LBKP not only decreased the total pore volume, but also decreased the pore size with increased contribution of small pores up to 560 Å in width. Re-cross-linking of the water soluble fraction from the first cross-linking of CM-LBKP seemed to proceed more homogeneously and eventually to give more homogeneous distribution of cross-linkage. Thereby re-cross-linking products had higher water absorbency and lower water solubility.

Microcrystalline cellulose-water interaction--a novel approach using thermoporosimetry.

To study the physical state of water in microcrystalline cellulose (MCC) and in silicified microcrystalline cellulose wet masses and the effect of granulation on different water fractions. Thermoporosimetry, together with the solute exclusion technique, was used to measure different water fractions and pore size distributions of wet granules. To understand the effect of granulation on the physical state of water, both ungranulated and granulated wet masses were studied. In addition, dynamic and isothermal step melting procedures were compared. Four distinct fractions of water (nonfreezing, freezing bound, free, and bulk water) could be detected in MCC wet masses. Granulation decreased the volume of bulk water and increased the volume of freezing bound and free water. Consequently, granulated wet masses were able to hold more water inside the particles compared to ungranulated wet masses. Thus, granulation had a similar effect on MCC as beating has on cellulose fibers in the papermaking proces Thermoporosimetry and solute exclusion increased the understanding of MCC-water interaction and showed how the physical state of water in MCC wet masses changes during granulation.

Probing red maple pit membrane pore size at fiber saturation point and oven dry using polystyrene macromolecules

A modified solute exclusion technique was used to pressure impregnate a polystyrene molecular weight (MW) series dissolved in styrene into red maple samples at approximately the fiber saturation point (FSP) and oven dry (OD). Radial penetration was less than tangential and FSP less than OD. There was a marked change in radial OD penetration at 900,000 MW. There was no marked penetration change with MW in the tangential direction, although there appeared to be a slight decrease in FSP penetration at the higher MW tested.

Synthesis of macroporous hydrogels with rapid swelling and deswelling properties for delivery of macromolecules

Thermally reversible macroporous poly(N-isopropylacrylamide) (polyNIPAAm) hydrogel has been synthesized in aqueous solution at a temperature above the lower critical solution temperature (LCST). Rapid swelling and deswelling kinetics of this macroporous hydrogel are characterized. Pore size measurement by a solute exclusion technique reveals that this macroporous hydrogel has a larger pore size distribution than the hydrogel synthesized at a temperature below the LCST. This kind of thermally reversible macroporous hydrogel will be very useful in delivery of macromolecules.

Cellulose morphology and enzymatic reactivity: A modified solute exclusion technique

An expeditious and accurate simplification of Stone and Scallan's solute exclusion technique was developed, thereby avoiding several sources of experimental error coupled with the determination of cellulose pore volume. Using this method, it is shown that cellulolytic enzymes do not enter into the micropores of five studied celluloses. These results suggestes that hydrolysis occurs initially at the external surface of the fibers. This surface area was calculated with the help of adsorption isotherms of bovine serum albumin. The obtained values for the different samples agree with the microscopically observed cellulose morphology. The correlation obtained by several authors relating cellulose porosity and its digestibility is explained as a consequence of the lower crystallinity and easier fragmentation of the more porous celluloses during hydrolysis. (c) 1994 John Wiley & Sons, Inc.

Effect of Neutral Salt and pH on the Swelling of Cellulosic Super Water-absorbent.

To clarify the swelling behavior of a cellulose-partially hydrolyzed polyacrylamide graft copolymer as a super water-absorbent, the effects of swelling conditions such as the concentration of neutral salt and pH value on the swelling of the graft copolymer were studied. The absorbency of the copolymer, the porous structure of the hydrogel of the copolymer, and the dynamic viscoelasticity of the hydrogel were examined. The absorbency was decreased steeply by the addition of sodium chloride. It depended remarkably on the solution pH and had a maximum value at about pH 7. Examination using the solute exclusion technique revealed that the distribution of pore width in the graft copolymer swollen in water differed from those in aqueous sodium chloride solutions and in an acidic or alkaline solution. When the concentration of sodium chloride increased or the pH was reduced or increased from pH 7, the cumulative pore volume up to 560 A in pore width decreased. Their relative value, however, increased accompanying the increase in the values of dynamic viscoelasticity of the hydrogel.

Porous structure and rheological properties of hydrogels of highly water‐absorptive cellulose graft copolymers

AbstractThe porous structures of the highly swollen hydrogels of cellulose—acrylamide graft copolymers were studied by the solute exclusion technique. For these hydrogels, the cumulative volume of pores up to 560 Å in pore width was between 450 and 850 mL/g, and was between 20 and 30% of the total pore volume. With an increasing in the amount of crosslinker added in the grafting procedure, the cumulative pore volume up to 560 Å decreased. Furthermore, larger pores shrunk preferentially. In the range of concentration from 0.003 to 0.3%, the viscosity of the hydrogels increased remarkably with an increase in concentration. The concentration dependence of the viscosity showed a transition at about 0.03%, which corresponded to the reciprocal of the water retention values for each copolymer. At concentrations over 3%, fluidity of the hydrogels was lost and the water swollen copolymers became viscoelastic. This dynamic viscoelasticity remained essentially unchanged in the temperature range of 20 to 80°C. The dynamic viscoelasticity was dependent on the concentration, but the concentration dependence was smaller than that of the viscosity. The larger the amount of the crosslinker added, the larger the value of dynamic modulus became.

Surface area of pretreated lignocellulosics as a function of the extent of enzymatic hydrolysis

The distribution of surface area as a function of pore size in the pores of hardwood, pretreated by mild acid hydrolysis, was estimated from the pore volume distribution, as a function of the extent of enzymatic hydrolysis. Pore volume distributions were obtained using the solute exclusion technique. Throughout the reaction, the total surface area of the substrate was mostly in pores too small to be accessible to the enzyme, which has been estimated to be 51–90 A in diameter. For wood pretreated at 200°C, the surface area of pores large enough to be accessible to cellulase decreases rapidly as the enzymatic hydrolysis proceeds. The surface area in pores too small to be accessible to the enzyme decreases more slowly, presumably because the substrate containing these small pores reacts only at the external surface. In wood pretreated at 220°C, a more gradual decrease in the accessible pore surface area occurs. It is hypothesized that this occurs because of the increase of external surface as the size of the substrate particle decreases by more severe pretreatment. This external surface is thought to adsorb more of the enzyme, leaving less for the surface in the pores. For the more severely pretreated hardwood, more enzyme is adsorbed throughout the course of reaction.

Interrelations between the fibre structure and the reactive dyeing properties of NaOH-treated cotton fibres

Changes of fibre structure and the fixation of C.I. Reactive Blue 52 in cotton subjected to different kinds of NaOH-treatments have been studied. The fibre structure was characterized by measurements of the pore volume, pore surface and pore size distribution using a solute exclusion technique. The results revealed that for the reactive dye uptake the pore region with pore diameters of 20–50 Å may have a special importance.