IONIC SELF-ASSEMBLY AND HUMIDITY SENSITIVITY OF POLYELECTROLYTE MULTILAYERS

Multilayer thin films of alternately adsorbed layers of polyelectrolytes PDDA and PS-119 were formed on bothplanar silica substrates and optical fibers through the ionic self-assembly technique. Intrinsic Fabry-Perot cavities werefabricated by stepwise assembling the polyelectrolytes onto the ends of optical fibers for the purposes of fiber optical deviceand sensor development. Ionically assembled polyelectrolyte multilayer thin films, in which there are hydrophilic side groupswith strong affinity towards water molecules, are a category of humidity-sensitive functional materials. The polyelectrolytemultilayer thin film Fabry-Perot cavity-type fiber optical humidity sensor can work over a wide range from about 0% RH toabout 100% RH with a response time less than 1s.     

Effects of fiber physical and chemical characteristics on the interaction between endoglucanase and eucalypt fibers

Roles played by fiber physical and chemical characteristics in enzymatic hydrolysis of cellulosic materials were investigated by analyzing the interaction between an endoglucanase complex and eucalypt kraft fibers. PFI refining was employed to create the difference of fiber size distribution and morphology. Oxygen delignification and bleaching were employed to prepare fibers with different lignin and pentosan contents. The enzyme accessibility was monitored by adsorption at 4 °C and during hydrolysis at 40 °C. Molecular weight changes and reducing sugar released were monitored for digestibility of the samples. Greater maximum adsorption capacities of the enzymes were shown for the pulps with shorter and wider fibers and more fine fractions after refining. Highest amount of enzyme was adsorbed onto fibers with the least lignin contents at 4 °C. Fewer desorbed from fibers with higher lignin contents during hydrolysis at 40 °C. For unrefined fibers, less molecular weight reductions were observed for fibers with higher lignin contents. However, extensive fibrillation by refining negated the effects of lignin on the action of endoglucanase, similar molecular weight reductions were observed for fibers with three different lignin contents. Refining could be able to expose more reaction sites on the fiber surface, hence the impacts of lignin and pentosan diminished during hydrolysis for refined fibers.     

Nanocomposite-based lignocellulosic fibers 1. Thermal stability of modified fibers with clay-polyelectrolyte multilayers

The layer-by-layer (LbL) assembly process of creating highly structured thin films derived from layers of polyelectrolytes and nanoparticles was adopted in this study to modify the surface of lignocellulosic fibers. Aqueous dispersions of clay nanoplatelets were created with ultrasonication and characterized with dynamic light scattering and atomic force microscopy in which confirmed the presence of individual clay nanoplatelets. Film thickness of never-dried clay and poly(diallyldimethylammonium chloride) (PDDA) multilayers was studied with a quartz crystal microbalance with dissipation monitoring (QCM-D). Using identical LbL deposition parameters, a slurry of steam-exploded wood fibers was modified by alternate adsorption of PDDA and clay with multiple rinsing steps after each adsorption cycle. Zeta potential measurements were used to characterize the fiber surface charges after each adsorption step while SEM images revealed that the LbL film masked the cellulose microfibril structure. Using a thermogravimetric analyzer, LbL modified steam-exploded wood fibers were observed to attain increased thermal stability relative to the unmodified material tested in both air and nitrogen atmospheres. Significant char for the LbL clay coated steam-exploded wood suggests the multilayer film serves as a barrier creating an insulating layer to prevent further decomposition of the material. This nanotechnology may have a positive impact on the processing of lignocellulosic fibers in thermoplastic matrices, designing of paper-based overlays for building products, and modification of cellulosic fibers for textiles.     

On the indirect polyelectrolyte titration of cellulosic fibers. Conditions for charge stoichiometry and comparison with ESCA

The effect of electrolyte (NaHCO3) concentration on the adsorption of poly-DADMAC (poly-diallyldimethylammonium chloride) onto cellulosic fibers with different charge profiles was investigated. Surface carboxymethylated fibers were obtained by grafting carboxymethyl cellulose (CMC) onto the fiber surface and bulk carboxymethylated fibers were obtained by reacting the fibers with monochloroacetic acid. It was shown that nonionic interactions do not exist between cellulose and poly-DADMAC, rather electrostatic interactions govern the adsorption. Charge stoichiometry prevails under electrolyte-free conditions, whereas surface charge overcompensation occurs at higher electrolyte concentrations. It was shown that charge stoichiometry prevails if the thickness of the electric double layer kappa(-1) was larger than the mean distance between the charges on the fiber surface, as predicted by polyelectrolyte adsorption theories, taking lateral correlation effects into account. In a second set of experiments the ESCA technique served to independently calibrate the polyelectrolyte titrations for determining the surface charge of cellulosic fibers. Various molecular masses of poly-DADMAC were adsorbed to carboxymethylated fibers having different charge profiles. The adsorption of low M(w) poly-DADMAC (7.0 x 10(3)), analyzed by polyelectrolyte titration, was about 10 times higher than that of the high M(w) poly-DADMAC (9.2 x 10(5)). Despite the difference in accessibility of these two polyelectrolytes to the fiber cell wall, ESCA surface analysis showed, as expected, only slight differences between the two polyelectrolytes. This gives strong credibility to the idea that surface charge content of cellulosic fibers can be analyzed by means of adsorption of a high-molecular-mass cationic polymer, i.e., by polyelectrolyte titration.     

STUDIES ON RELATION BETWEEN INTRINSIC VISCOSITY OF POLYELECTROLYTES IN SOLUTIONS USED FOR LAYER-BY-LAYER SELF-ASSEMBLY AND THEIR CORRESPONDING ADSORPTION AMOUNTS IN THE RESULTANT MULTILAYER MEMBRANES

The adsorption amount of poly(styrene sulfonate)and poly(dimethyldiallyl ammonium chloride)(PSS/PDDA) self-assembled multilayer membranes in designed dipping solvents were measured by UV-Vis-spectroscopy and quartz crystal microbalance(QCM).Intrinsic viscosities of PSS and PDDA in corresponding dipping solvents were determined by an Ubbelohde viscometer.It is found that the adsorption amount of PSS/PDDA self-assembled multilayer membranes built up in different dipping solutions,added salt concentration,p...     

Effect of polyethyleneimine ion on the sorption of a reactive dye onto Leacril fabric: electrokinetic properties and surface free energy of the system

Data are presented on the kinetics, electrokinetics, and surface free energy in the process of adsorption of polyethyleneimine (PEI) as a pretreatment of Leacril, later dyed with the reactive dye Remazol Brilliant Blue R (RBBR). The electrokinetic potential of Leacril is negative, due probably to the presence of sulfonate and sulfate end-group onto Leacril fibers. The zeta potential of Leacril decreases in absolute value as a function of NaCl concentration in solution, probably because of compression of the electrical double layer. The zeta potential of Leacril as a function of the concentration of PEI in solution increases because of the adsorption of PEI ions through chemical reaction between the sulfonate end-groups of Leacril and the amine groups of PEI. The adsorption kinetics shows that an increase in the concentration of PEI, brings about an increase in the amount of RBBR adsorbed onto the fiber. This may be an indication of the chemical reaction between the reactive groups of the polyelectrolyte and dye molecules. The behavior of the surface free energy of the systems involved confirms these conclusions.     

Simulated Gastric and Intestinal Fluid Electrolyte Solutions as an Environment for the Adsorption of Apple Polyphenols onto β-Glucan

Interactions with dietary fibers in the gastrointestinal tract might affect the potential bioactivities of phenolic compounds. In this study, the interactions between apple phenolic compounds and β-glucan (a dietary fiber) were studied by studying the adsorption process in simulated gastric and intestinal fluid electrolyte solutions. Phenolic compounds were extracted from apples, adsorbed onto β-glucan (2 h, 37 °C, in gastric or intestinal fluid electrolyte solutions), and determined using high performance liquid chromatography. Phenolic compounds (flavan-3-ols, flavonols, phenolic acids, and dihydrochalcone) were stable in the gastric fluid (pH 3). In the intestinal fluid (pH 7), flavan-3-ols were not found and chlorogenic acid isomerized. Polyphenols from the apple peel (up to 182 and 897 mg g⁻¹) and flesh (up to 28 and 7 mg g⁻¹) were adsorbed onto β-glucan in the gastric and intestinal fluids, respectively. The adsorption was affected by the initial concentration of the polyphenols and β-glucan and by the environment (either gastric or intestinal fluid electrolyte solution). By increasing the initial polyphenol amount, the quantity of adsorbed polyphenols increased. Increasing the amount of β-glucan decreased the amount adsorbed. The results can be helpful in explaining the fate of phenolic compounds in the gastrointestinal tract.     

Buildup of polyelectrolyte multilayers of polyethyleneimine and microfibrillated cellulose studied by in situ dual-polarization interferometry and quartz crystal microbalance with dissipation

Polyethyleneimine (PEI) and Microfibrillated cellulose (MFC) have been used to buildup polyelectrolyte multilayers (PEM) on silicone oxide and silicone oxynitride surfaces at different pH values and with different electrolyte and polyelectrolyte/colloid concentrations of the components. Consecutive adsorption on these surfaces was studied by in situ dual-polarization interferometry (DPI) and quartz crystal microbalance measurements. The adsorption data obtained from both the techniques showed a steady buildup of multilayers. High pH and electrolyte concentration of the PEI solution was found to be beneficial for achieving a high adsorbed amount of PEI, and hence of MFC, during the buildup of the multilayer. On the other hand, an increase in the electrolyte concentration of the MFC dispersion was found to inhibit the adsorption of MFC onto PEI. The adsorbed amount of MFC was independent of the bulk MFC concentration in the investigated concentration range (15-250 mg/L). Atomic force microscopy measurements were used to image a MFC-treated silicone oxynitride chip from DPI measurements. The surface was found to be almost fully covered by randomly oriented microfibrils after the adsorption of only one bilayer of PEI/MFC. The surface roughness expressed as the rms-roughness over 1 microm2 was calculated to be 4.6 nm (1 bilayer). The adsorbed amount of PEI and MFC and the amount of water entrapped by the individual layers in the multilayer structures were estimated by combining results from the two analytical techniques using the de Feijter formula. These results indicate a total water content of ca. 41% in the PEM.     

Investigations of polyelectrolyte adsorption at the solid/liquid interface by sum frequency spectroscopy: evidence for long-range macromolecular alignment at highly charged quartz/water interfaces

IR-visible sum frequency spectroscopy (SFS) was employed to investigate the molecular level details of the adsorption of the positively charged polyelectrolyte, polydiallyldimethylammonium chloride (PDDA), at the quartz/water interface. Below pH 9.0, signal from the interfacial water structure was visible, but none from the adsorbed polymer could be detected. This indicated that the PDDA was not well enough aligned at the interface under these conditions to elicit a sum frequency response. At more basic pH values (>or=9.6), however, adsorbed PDDA molecules became well-ordered as indicated by the presence of CH stretch peaks from methylene and methyl groups. The intensities of the CH stretch modes were independent of the adsorbed amount of PDDA at pH 12.3 but decreased as the pH of the bulk solution was lowered. The conditions for polymer alignment fell outside the parameters where layer-by-layer growth of oppositely charged polyelectrolytes was possible because the net charge on the surface under high pH conditions remained negative.     

Surface characterization of stepwise oxidized spruce thermomechanical pulp samples by different analytical methods

An investigation of the fiber samples with various amounts of lignin, hemicelluloses and cellulose was conducted by means of dye adsorption, spectrometric and chemical analyses. Five fiber samples were prepared by the oxidation of unbleached Norway spruce thermo-mechanical pulp with acidic potassium permanganate solution. Each sample was oxidized with defined amount of KMnO₄ and the degree of oxidation was determined as kappa number. Fibers were additionally characterized by conductometric titration, the determination of hemicelluloses and infrared spectrometry. Sorption of various types of dye, namely Methylene Blue (MB), Crystal Violet (CV) and Astra Blue (AB) on the fiber surface was studied and compared to the kappa number and other fiber characteristics. The adsorption of MB and CV is found to follow the amount of anionic (carboxylic) groups, which at first increase and later decrease toward the final stages of oxidation. The behavior of AB is nearly the opposite, its adsorption increases with the depletion of lignin and carboxylate groups, pointing to a different mechanism of AB binding on fibers.     

Equilibrium aspects of polycation adsorption on silica surface: how the adsorbed layer responds to changes in bulk solution

Adsorption of cationic high molecular weight polyacrylamides (CPAM) (M(w) is about 800 kDa) with different fractions of cationic units tau = 0.09 and tau = 0.018 onto silica surface was studied over a wide range of pH (4-9) and KCl concentration (c(s) = 10(-3)-10(-1) M) by in-situ null ellipsometry. We discuss how the adsorbed layer depends on the bulk conditions as well as kinetically responds to changes in solution conditions. The adsorbed amount Gamma of CPAM increases with pH for all studied electrolyte concentrations until a plateau Gamma is reached at pH > 6. At low pH we observed an increase in adsorbed amount with electrolyte concentration. At high pH there is no remarkable influence of added salt on the values of the adsorbed amount. The thickness of adsorbed polymer layers, obtained by ellipsometry, increases with electrolyte concentration and decreases with pH. At low c(s) and high pH the polyelectrolyte adsorbs in a flat conformation. An overcompensation of the surface charge (charge reversal) by the adsorbed polyelectrolyte is observed at high c(s) and low pH. To reveal the reversibility of the polyelectrolyte adsorption with respect to the adsorbed amount and layer thickness, parameters such as polyelectrolyte concentration (c(p)), c(s), and pH were changed during the experiment. Generally, similar adsorbed layer properties were obtained independent of whether adsorption was obtained directly to initially bare surface or by changing pH, c(s), or the concentration of polyelectrolyte solution in the presence of a preadsorbed layer, provided that the coverage of the preadsorbed layer was low. Once a steady state of the measured parameters (Gamma, d) was reached, experimental conditions were restored to the original values and corresponding changes in Gamma and adsorbed layer thickness were recorded. For initially low surface coverage it was impossible to restore the layer properties, and in this case we always ended up with higher coverage than the initial values. For initial high surface coverage it was usually possible to restore the initial layer properties. Thus, we concluded that polyelectrolyte appears only partially reversible to changes in the solution conditions due the slow rearrangement process within the adsorbed layer.     

Competitive Adsorption of Chloroform and Iron Ion onto Activated Carbon Fiber

Chloroform in tap water has been a significant problem because it may be a carcinogenic substituent. Iron ion exists in tap water because of dissolution from iron water pipes. Iron ions in tap water cause discoloration and a bad odor. The isotherms of chloroform and iron ion adsorption onto activated carbon fibers in a single solution (chloroform or iron ion) and in a binary mixture solution (chloroform and iron ion) were investigated to estimate the competitiveness between chloroform and iron ions. The amount of adsorbed iron ions increased with increasing pore volume of the activated carbon fibers, while that of chloroform decreased. The amount of chloroform adsorbed onto the activated carbon fibers in the binary mixture solution was greater than that in the single solution. These results indicate that the adsorption of chloroform and iron ion onto activated carbon fibers could be competitive.     

Reduction of Iron(III) Ion by Activated Carbon Fiber

The mechanisms of adsorption of iron(II) ion, iron(III) ion, and reduced iron(III) ion onto an activated carbon fiber and the ability of carbon fibers to reduce iron(III) ion were investigated on the basis of the amounts of iron ion adsorbed. The amount of iron(II) ion adsorbed onto the activated carbon fiber increased with increasing adsorption temperature. Iron(II) ion was more easily removed by the activated carbon fiber than iron(III) ion. Iron(III) ion was adsorbed onto the activated carbon fiber after being reduced to iron(II) ion. The reduction ability of A-20 was stronger than that of A-10 because the hydrophilic groups of A-20 were larger than those of A-10. It is concluded that the activated carbon fiber has a reduction effect on iron(III) ion and that the reduction effect of the activated carbon fiber depended on the number of hydrophilic groups on the activated carbon fiber. Copyright 2000 Academic Press.     

Kinetics of adsorption of polyvinylamine on cellulose fibers. II. Adsorption from electrolyte solutions

Adsorption from electrolyte solutions of fully hydrolyzed polyvinylamine on cellulose fibers was investigated by supplying the polymer to the fibers at controlled rate. This was implemented by employing a reactor only open to the fluid in which the fiber dispersion were confined and homogenized. The adsorbed layers may be defined as diffuse or dense layers. Diffuse layers are characterized by a surface coverage limited to 0.65 mg/g cellulose in salt-free solutions. Addition of NaCl or CaCl(2) to the fiber dispersion and the polymer solution promotes the adsorption rate and increases the amount of adsorption to 1.5 mg/g cellulose. For dense polymer layers, for which the coverage amounts to values close to 10 mg/g cellulose in salt-free systems, addition of electrolyte does not change the kinetic and adsorption characteristics. Insofar as the variation of the molecular areas of the polymer within the diffuse layers as a function of the ionic strength parallels the variation of the molecular characteristics of solute molecules, the formation of diffuse layers is expected to proceed by random deposition of solute molecules which later individually sustain strong reconformation. Adsorption isotherms show a limited influence of the ionic strength. Obviously, the passage from dense layers of high surface coverage to low adsorption values at equilibrium requires extended reconformation of adsorbed macromolecules and desorption of a great part of the molecules already adsorbed.     

Adsorption of a cationic surfactant onto cellulosic fibers I. Surface charge effects

The adsorption of four cationic surfactants with different alkyl chain lengths on cellulose substrates was investigated. Cellulose fibers were used as model substrates, and primary alcohol groups of cellulose glycosyl units were oxidized into carboxylic groups to obtain substrates with different surface charges. The amount of surfactant adsorbed on the fiber surface, the fiber zeta-potential, and the amount of surfactant counterions (Cl(-)) released into solution were measured as a function of the surfactant bulk concentration, its molecular structure, the substrate surface charge, and the ionic strength. The contribution of each of these parameters to the shape of the adsorption isotherms was used to verify if surfactant adsorption and self-assembly models usually used to describe the behavior of surfactant/oxide systems can be applied, and with which limitations, to describe cationic surfactant adsorption onto oppositely charged cellulose substrates.     

Copper adsorption and Si, Al, Ca, Mg, and Na release from clinoptilolite

Copper adsorption onto clinoptilolite (natural zeolite), Al/Si dissolution, and Mg, Ca, and Na release from the substrate were the subjects of the investigation described here. Experimental variables were Cu and electrolyte concentrations and solution pH. Copper adsorption was found to increase with increased pH and with decreased electrolyte concentration. Large amounts of K were also adsorbed from electrolyte. Since solution pH was assumed as a variable, the effects of [H(+)] differentiation on Cu adsorption and on Al/Si dissolution were also examined. Al dissolution was affected mainly by electrolyte concentration, whereas Si dissolution was affected mainly by adsorbed Cu amount. It was assumed that the release of Mg, Ca, and Na occurs through ion-exchange reactions with solution K(+), because their release is affected more by electrolyte concentration than by adsorbed Cu. From the study of FTIR spectra for various samples used in the present investigation, we observed that the removal of framework Si/Al shifts the band which was attributed to O-T-O stretching vibration toward higher frequency. Significant changes were observed for the bands assigned to Si-OH-Al bridges and to monomeric and polymeric hydrogen bonds at the region between 3650 and 3200 cm(-1). It is proposed that the Cu species caused the destruction of H-bonded structures, whereas K adsorbed species were located at exchangeable sites after an ion-exchange process between K and Ca, Mg, and Na from the zeolite's surface. An expansion of the zeolite framework was detected from XRD patterns under acid conditions.     

Resistance of poly(ethylene oxide)-silane monolayers to the growth of polyelectrolyte multilayers

The ability of poly(ethylene oxide)-silane (PEO-silane) monolayers grafted onto silicon surfaces to resist the growth of polyelectrolyte multilayers under various pH conditions is assessed for different pairs of polyelectrolytes of varying molar mass. For acidic conditions (pH 3), the PEO-silane monolayers exhibit good polyelectrolyte repellency provided the polyelectrolytes bear no moieties that are able to form hydrogen bonds with the ether groups of the PEO chains. At basic pH, PEO-silane monolayers undergo substantial hydrolysis leading to the formation of negatively charged defects in the monolayers, which then play the role of adsorption sites for the polycation. Once the polycation is adsorbed, multilayer growth ensues. Because this is defect-driven growth, the multilayer is not continuous and is made of blobs or an open network of adsorbed strands. For such conditions, the molar mass of the polyelectrolyte plays a key role, with polyelectrolyte chains of larger molar mass adsorbing on a larger number of defects, resulting in stronger anchoring of the polyelectrolyte complex on the surfaces and faster subsequent growth of the multilayer. For polyelectrolytes of sufficiently low molar mass at pH 9, the growth of the multilayer can nevertheless be prevented for as much as five cycles of deposition.     

Study of copper adsorption on montmorillonites using thermal analysis methods

Copper was adsorbed onto Ca-exchanged montmorillonite (Cheto clay) under basic conditions. Differential thermogravimetric analysis (DTGA) combined with evolved gas mass spectroscopy (MS) was employed as the principal technology to study the distribution and structure of adsorbed copper species on the montmorillonite clay. The results showed that the original clay was easily rehydrated. After copper adsorption, a step-by-step replacement of hydrated calcium ions by copper-ammonia complex was observed through the gradual decrease of the first DTGA dehydration peak intensity with increasing copper loading. Compared with the original clay, copper loaded samples showed new DTGA peaks assigned to NH(3) and N(2)O. The presence of the N(2)O peak suggested that the loaded copper species were in agglomerated copper oxide form, which dispersed well over the edges and external surfaces of the clay layers.     

Adsorption behavior of cationic polymers on bentonite

The adsorption behavior of a series of cationic water-soluble polymers poly(diallyldimethylammonium chloride) with different molar masses onto raw bentonite was investigated through the elaboration of their adsorption isotherms and the quantification of the water content of the clay/polymer complexes formed. It was found that the type of adsorption isotherms obtained depends on the chain length of the polymer. The study showed a correlation between the amount of adsorbed polymer and the water content of the clay, after the adsorption experiments. The lower the molar mass of the polymer used, the larger was the reduction on water content of the complexes.     

Coadsorption of sodium dodecyl sulfate and a polyanion onto poly(ethylenimine) in multilayered thin films

Mixed surfactant-polyelectrolyte multilayer films were fabricated by both ionic self-assembly and spin assembly. A polycation [PEI = poly(ethylenimine)] was deposited from a dilute solution, while a polyanion (PAZO = poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) was deposited from a mixture containing a fixed concentration of polyanion and various concentrations of the anionic surfactant sodium dodecyl sulfate (SDS). Coadsorption of SDS and PAZO onto PEI layers was observed using both deposition methods and attributed to strong PEI-SDS interactions and entropic factors. Increasing the concentration of SDS resulted in films containing progressively less adsorbed PAZO. No further reduction in the amount of adsorbed PAZO was observed above the SDS critical micelle concentration. We attribute the film growth behavior to a fast adsorption of SDS onto PEI, followed by a slower adsorption of PAZO onto the remaining unoccupied binding sites. We observe that SDS interpenetrates throughout the PAZO and PEI layers, increasing the surface hydrophobicity of both. We observed similar behavior for both ionically self-assembled and spin-assembled systems.