Structure of Guar in Solutions of H2O and D2O: An Ultra-Small-Angle Light-Scattering Study

Scaling Laws in Polysaccharide Rheology: Comparative Analysis of Water and Ionic Liquid Systems

This study investigates the rheological behavior of two plant-based polysaccharides, with different degrees of hydrophilicity, agar (highly hydrophilic) and guar gum (hydrophilic), in water and 1-ethyl-3-methylimidazolium acetate (EMImAc). The rheological response of these polymers is highly dependent on the solvent's ability to disrupt intermolecular associations. In water, agar forms hydrogels, while guar gum behaves as a viscoelastic liquid with slow modes. The plateau modulus (GN0) scales with polymer concentration (c) as GN0 ∼ c3, consistent with other natural polymers. In EMImAc, both polysaccharides form viscoelastic liquids, exhibiting GN0 ∼ c2.3, as expected for semiflexible polymer solutions. However, the terminal relaxation time, τD, and the specific viscosity, ηsp, scale as τD ∼ c5.3 and ηsp ∼ c7.6, indicative of intermolecular chain-chain associations. Despite the solvent or polysaccharide, the fractional viscosity overshoot and the shear strain at the maximum stress show a terminal Weissenberg number dependence similar to other synthetic polymers.

The nature of fatty acid interaction with a polyelectrolyte-surfactant pair revealed by NMR spectroscopy

The interaction mechanisms of an oppositely charged polyelectrolyte-surfactant pair and dodecanoic (lauric) acid (LA) were experimentally investigated using a combination of nuclear magnetic resonance (NMR) techniques. It is observed that LA significantly affects the interaction between the anionic surfactant sodium dodecylethersulfate (SDES) and the cationic polymer guar modified with grafted hydroxypropyl trimethylammonium chloride (Jaguar C13 BF). Typically, oppositely charged polymers and surfactants interact electrostatically at a certain surfactant concentration known as the critical aggregation concentration (CAC). Once the polymer is neutralized by the surfactant, an insoluble complex (precipitate) is observed (phase separation), and, at concentrations beyond the surfactant critical micellar concentration (CMC'), the system returns to a one phase entity. In a system in which a mixture of SDES-LA is added to the polymer, NMR data show that below the neutralization onset, some of the polymer interacts with SDES, while some of the polymer is adsorbed at the surface of LA solid aggregates present in the system. Furthermore, SDES is found to aggregate in a lamellar-like structure at the polymer side chain prior to the SDES CMC'. Above the SDES (CMC'), LA is solubilized and incorporated at the palisade region of SDES micelles. Analysis of (1)H resonances provided estimated concentrations of all species in the system phases at all stages of interaction.

Guar gum: processing, properties and food applications-A Review

Guar gum is a novel agrochemical processed from endosperm of cluster bean. It is largely used in the form of guar gum powder as an additive in food, pharmaceuticals, paper, textile, explosive, oil well drilling and cosmetics industry. Industrial applications of guar gum are possible because of its ability to form hydrogen bonding with water molecule. Thus, it is chiefly used as thickener and stabilizer. It is also beneficial in the control of many health problems like diabetes, bowel movements, heart disease and colon cancer. This article focuses on production, processing, composition, properties, food applications and health benefits of guar gum.

Not all anionic polyelectrolytes complex with DTAB

The influence of hydroxypropyl guar (HPG), with and without boric acid, on dodecyltrimethyl ammonium bromide (DTAB) micellization was characterized by surface tension measurements, isothermal titration calorimetry, and small-angle neutron scattering. Although HPG is a nonionic water-soluble polymer, borate ions form weak bonds with HPG, transforming it into an anionic polyelectrolyte, HPG-borate. Surprisingly, the three independent measurements showed that HPG-borate does not promote DTAB micellization or phase separation normally seen when mixing oppositely charged polyelectrolytes and surfactants. However, the neutron scattering results suggested that HPG-borate binds to and flocculates existing DTAB micelles. The unusual behavior of HPG-borate with DTAB was underscored by showing that carboxymethyl guar (CMG) formed precipitates with DTAB.

Small-angle X-ray scattering of reduced ribonuclease A: effects of solution conditions and comparisons with a computational model of unfolded proteins

The disulfide-reduced form of bovine ribonuclease A, with the Cys thiols irreversibly blocked, was characterized by small-angle x-ray scattering. To help resolve the conflicting results and interpretations from previous studies of this model unfolded protein, we measured scattering profiles using a range of solution conditions and compared them with the profiles predicted by a computational model for a random-coil polypeptide. Analysis of the simulated and experimental profiles reveals that scattering intensities at intermediate angles, corresponding to interatomic distances in the range of 5-20 A, are particularly sensitive to changes in solvation and can be used to assess the internal scaling behavior of the polypeptide chain, expressed as a mass fractal dimension, D(m). This region of the scattering curve is also much less sensitive to experimental artifacts than is the very small angle regime (the Guinier region) that has been more typically used to characterize unfolded proteins. The experimental small-angle x-ray scattering profiles closely matched those predicted by the computational model assuming relatively small solvation energies. The scaling behavior of the polypeptide approaches that of a well-solvated polymer under conditions where it has a large net charge and at high urea concentrations. At lower urea concentrations and neutral pH, the behavior of the chain approaches that expected for theta-conditions, where the effects of slightly unfavorable interactions with solvent balance those of excluded volume, leading to scaling behavior comparable to that of an idealized random walk chain. Though detectable, the shift toward more compact conformations at lower urea concentrations does not correspond to a transition to a globule state and is associated with little or no reduction in conformational entropy. This type of collapse, therefore, is unlikely to greatly reduce the conformational search for the native state.

Role of background ions in guar gum adsorption on oxide minerals and kaolinite

Adsorption of guar gum onto alumina, titania (rutile), hematite, quartz, and kaolinite was investigated as a function of pH, ionic strength (from distilled water to saturated NaCl and KCl), and the type of background electrolyte (0.01 mol/L LiCl, NaCl, KCl, and CsCl). It was demonstrated that the adsorption density of the polymer does not depend on pH for any of the tested minerals, so only hydrogen bonding was identified as the dominant adsorption mechanism. The minerals could, however, be divided into two groups depending on the effect of the salt type on polymer adsorption. Guar gum adsorption onto quartz and kaolinite significantly increased in the presence of even a small amount of KCl, while NaCl equally enhanced guar gum adsorption on these two minerals only at concentrations approaching saturation. In contrast, no significant differences between the effects of KCl and NaCl on polysaccharide adsorption were observed on titania, alumina, and hematite. The results were correlated with the chaotropic (KCl) and kosmotropic (NaCl) properties of the background salts, and-based on a review of the available literature data-with the presence (quartz) or absence (titania, alumina, hematite) of an extensive hydration layer on the oxide surfaces. It was concluded that the main role of background ions in the studied systems was to control the stability of the interfacial water layer on oxide particles whose presence serves as a barrier to guar gum adsorption.

A fractal analysis approach to viscoelasticity of physically cross-linked barley β-glucan gel networks

The structure and gelation kinetics of mixed linkage barley beta-glucans of varying Mw have been investigated. The fractal concept has been applied to describe the structure development of barley beta-glucan gels using a scaling model and dynamic rheometry data. The model supports that the gel structure consists of fractal clusters that upon aggregation lead to a three-dimensional network. The analysis showed that with increasing Mw a denser (more packed) network is formed as indicated by the corresponding fractal dimension (df) values. The microelastic parameter of the model, alpha, showed that all gel structures were in the transition regime implying structural reordering upon ageing. The description of the microstructure as a fractal network seems to be able to explain syneresis and other observations from large deformation testing of such systems. The molecular treatment of the gelation kinetics suggests that the gelling behavior is governed by the probability of collision of chain fragments with consecutive cellotriosyl units. This is greater for small chains due to their higher diffusion rates, for chains having lower amounts of cellulose like fragments and finally for those showing smaller degree of intrachain interactions. As a result, the faster gelling systems exhibit lower fractal dimensionality (more disordered systems) something that is in accordance with current kinetic theories.

Physicochemical Characterization of Konjac Glucomannan

Four commercial konjac glucomannan (KGM) samples and a glucomannan derived from yeast were characterized by aqueous gel permeation chromatography coupled with multi angle laser light scattering (GPC-MALLS). Disaggregation of aqueous glucomannan solutions through controlled use of a microwave bomb facilitated reproducible molar mass distribution determination alleviating the need for derivatization of the polymer or the use of aggressive solvents. Further characterization was undertaken by use of capillary viscometry and photon correlation spectroscopy (PCS). The weight average molecular masses (M(w)) determined were in the region of 9.0 +/- 1.0 x 10(5) g mol(-1) for KGM samples and 1.3 +/- 0.4 x 10(5) g mol(-1) for the yeast glucomannan. The values determined for KGM in aqueous solution are in agreement with those reported for KGM in aqueous cadoxen. The degradation of samples observed upon autoclaving has been quantified by GPC-MALLS and intrinsic viscosity determination, allowing comparison with reported Mark-Houwink parameters. Shear flow experiments were undertaken for a range of KGM solutions of concentration 0.05 to 2.0% using a combination of controlled stress and controlled strain rheometers. The concentration dependence of the zero shear specific viscosity was determined by analysis of the data using the Ellis model. The dependence of the zero shear specific viscosity on the coil overlap parameter was defined and interpretation discussed in terms of the Martin and Tuinier equations.

Effect of deacetylation rate on gelation kinetics of konjac glucomannan

Effect of deacetylation rate on the gelation behaviors on addition of sodium carbonate for native and acetylated konjac glucomannan (KGM) samples with a degree of acetylation (DA) range of 1.38-10.1 wt.% synthesized using acetic anhydride in the presence of pyridine as catalyst was studied by dynamic viscoelastic measurements. At a fixed alkaline concentration (C(Na)), both the critical gelation times (t(cr)) and the plateau values of storage moduli (G'(sat)) of the KGM gels increased with increasing DA. While at a fixed ratio of alkaline concentrations to values of DA (C(Na)/DA), the similar t(cr) and (G'(sat)) values independent of DA were observed. On the whole, increasing KGM concentration or temperature shortened the gelation time and enhanced the elastic modulus for KGM gel. The effect of deacetylation rate related to the C(Na)/DA on the gelation kinetics of the KGM samples were discussed.

Flocculation in mixtures of cationic polyelectrolytes and anionic surfactants

The phase behavior of mixtures of a cationic polyelectrolyte ('Jaguar') and an anionic surfactant (either sodium dodecyl sulfate or sodium lauryl ether sulfate) has been studied. For a given polyelectrolyte (and added NaCl) concentration, with increasing surfactant concentration, three phase regions were identified. The first region is a single homogeneous phase. Within this region, at some surfactant concentration, above the critical aggregation concentration, stable open-network 'particles' form, typically approximately 100 nm in size, which are net positively charged. However, as the surfactant concentration is increased further, these particles aggregate and form a two-phase system, i.e. a separated gel phase, containing a high percentage of water, co-existing with an aqueous surfactant phase. At some higher surfactant concentration still, the particles become sufficiently negatively charged that they re-stabilize. Beyond this surfactant concentration, therefore, the system reverts to being a single, homogeneous phase. Within the two-phase (aggregated particle) region an iso-electric point for the particles has been observed at a certain surfactant concentration, by electrophoresis. Furthermore, in this aggregated, gel-phase region, there appear to be typically approximately 2-4 surfactant molecules associated with each cationic site of the polymer chains. It is postulated that association of the anionic surfactant molecules occurs within the polyelectrolyte chains, binding them together, to form the particles referred to. These associated surfactant structures have been referred to here as 'internal' micelles. A crude estimate has been made, based on turbidity/time measurements, that there may be up to approximately 1000 polymer chains in each primary particle, bound together by the internal surfactant micelles. Small-angle light scattering studies of the aggregating particles indicate a fractal dimension for the aggregates, which would correspond to a diffusion-limited aggregation process.

Characterization of Association and Gelation of Pectin in Methanol−Water Mixtures

Turbidity, swelling, and rheological features of semidilute systems of pectin in methanol-water media of different composition have been investigated. By increasing the percentage of methanol in the mixture, the thermodynamic properties of the pectin/methanol/water system become poorer, as shown by increasing turbidity and decreasing swelling. Effects of oscillatory and steady shear flows on intermolecular associations and gelation of pectin in methanol/water mixtures are reported. The effects of methanol concentration on the growth and structure of shear-induced gels, stabilized through hydrogen bonds, are analyzed. Steady shear measurements on these systems reveal shear thickening at low shear rates and disruption of intermolecular associative junctions at high shear rates.

Preparation and characterization of molecular weight fractions of guar galactomannans using acid and enzymatic hydrolysis

A procedure is described for the preparation of large amounts of guar galactomannan by acid hydrolysis that yields samples of various molecular weights (MW) with uniform polydispersity. This contrasts with preparation by enzymatic degradation that yields samples with a marked increase in polydispersity and a much broader molecular weight distribution (MWD). Acid hydrolyzed guar samples had a Mark-Houwink-Sakurada (MHS) relationship of [eta]=3.04x10(-4) M(w)(0.747) dl/g and a characteristic ratio of 11.87 as determined by gel permeation chromatography (GPC) and dilute solution viscometry. The Huggins coefficient for degraded guars is much smaller (approximately 0.4) than that of the native guar (approximately 0.79), suggesting a weakening of intermolecular association in guar prepared by acid hydrolysis.

Scattering from supramacromolecular structures

We study theoretically the scattering imprint of a number of branched supramacromolecular architectures, namely, polydisperse stars and dendrimeric, hyperbranched structures. We show that polydispersity and nature of branching highly influence the intermediate wave vector region of the scattering structure factor, thus providing insight into the morphology of different aggregates formed in polymer solutions.