Physical properties of xanthan,galactomannan and their mixtures in aqueous solutions

Some new information on the conformation of xanthan in aqueous solutions is given. A single helical chain conformation characterizes xanthan in the native state. When heated over the critical temperature for conformational change ( helixŕ coil), the xanthan is denatured and renatured when it is cooled down in a localy double helical structure. A galactomannan was also characterized and its persistence length was obtained (Lp ≈︁ 90A°). Then mixtures of the galactomannan and xanthan were investigated to propose a mechanism for the specific gelation. From the results of microcalorimetry and circular dichroism, it is concluded that a complex is formed between one disordered xanthan chain and one galactomannan chain and that an ordered conformation is stabilized at temperatures lower than 25°C when the galactomannan has a M/G ratio of ≈︁ 3. This temperature corresponds to the sol-gel transition. This is the first time that a structure of the crosslink points is demonstrated.     

Rheological properties of water-soluble cross-linked xanthan gum

Water-soluble crossslinked xanthan gum (CXG) was prepared from xanthan gum (XG) and epichlorohydrin under alkaline condition by ethanol solvent method. Rheological properties and heat resistance performance of different concentrations of aqueous XG and CXG solutions were investigated. The results showed that the apparent viscosity of 4 g · L^?1 CXG solution was 2.57 times that of 4 g · L^?1 XG solution. The storage modulus G′ and the loss modulus G″ of CXG solutions were greater than those of XG solutions, and viscoelastic and thixotropic properties were more significant in CXG solutions. At 80°C, these two solutions were sheared at 170 s^?1 for 90 minutes, the reserved viscosity was 32.30 and 62.15 mPa · s for XG and CXG solutions, respectively. The heat resistance performance of CXG solution was better than that of XG solution. Nonlinear co-rotational Jeffreys model could be applied to describe the flow curves of XG and CXG solutions correctly, and the calculated values were in good agreement with the experimental data.     

Evidence for a conformational transition in curdlan

Gelation occurred at a concentration of 0.3% curdlan solution at room temperature (25 °C). The curdlan showed a Newtonian behavior at 0.1%, but plastic behavior above 0.2% even at a temperature of 38 °C. The dynamic modulus remained at a constant value with an increase of temperature up to 40 °C., which was estimated to be a transition temperature, then it decreased rapidly with an increase of temperature until 55 °C; however, it increased gradually with a further increase of temperature. The dynamic viscoelasticity of curdlan solution remained at very low values on addition of urea (4 M) and decreased with increasing temperature. The optical rotation of curdlan (0.1%) increased with a decrease of temperature.Possible modes of intra- and intermolecular hydrogen bonding within and between curdlan molecules were proposed. A hydrophobic interaction might take place at high temperatures (>55 °C), the mode of which was also proposed.     

Investigation of Rheological Properties of Polyacrylamide/Chromium Triacetate Hydrogels Performed for Water Shutoff Treatment in Oil Reservoirs

In this research, polyacrylamide-based gels were made at 25°C and aged in an oven at 90°C for 24 hours. The bottle testing results indicated that gel composed of 15000 ppm partially hydrolyzed polyacrylamide (PHPA) and 2500 ppm chromium(III) acetate exhibited an acceptable strength. The strain sweep test results showed that elastic modulus is fixed on 6 Pa below strain of 100%. According to frequency test results, linear viscoelasticity region was observed at the frequency limits from 0.1 to 10 s^?1. Moreover, elastic modulus decreased from 8 Pa at 25°C to 4 Pa at 90°C. Furthermore, gelation time for gelant with the same composition was obtained (14, 13, and 20 minutes) in sequence.     

Evidence for Conformational Transitions in Amylose1

Abstract

The flow behavior, dynamic viscoelasticity, and optical rotation of an aqueous solution of amylose were measured using a rheogoniometer and a polarimeter, respectively. The amylose solutions showed shear-thinning behavior at a concentration of 1.2%, but plastic behavior above 1.4% at 25 °C. With increasing amylose concentrations the viscosity decreased rapidly with increasing temperature from 20 to 25, 30, and 35 °C. These latter temperatures are estimated to be first transition temperatures at the respective concentrations. Viscosities were scarcely changed until temperatures reached 70, 90, and 90 °C, which were estimated to be second transition temperatures, for 1.2, 1.4, and 1.6% solutions, respectively. Gelation occurred at a concentration of 1.2% at room temperature (2.5 °C). The dynamic modulus of amylose increased gradually with increasing temperature from 20 to 30 °C and kept a constant value until the temperature reached 65, 75, and 80 °C for 1.0, 1.2 and 1.4% solutions, respectively, which were estimated to be transition temperatures, then dynamic modulus decreased rapidly. The dynamic modulus of amylose stayed at a very low value with addition of urea (4.0 M). The optical rotation of amylose solution (1.0%) increased a little with deceasing temperature up to 25 °C, then it increased rapidly with further decrease of the temperature. Possible mode of intra- and intermolecular hydrogen bonding within and between amylose molecules were proposed.     

Effects of poly(N‐isopropylacrylamide) on the solution properties of hydrophobically modified acrylamide copolymer

The hydrophobic interaction between hydrophobically modified acrylamide copolymer (HMPAM) and poly(N‐isopropylacrylamide) (PNIPAM) in aqueous solutions was investigated. The results show that the solution properties of HMPAM are significantly influenced by the addition of PNIPAM. In dilute regime, the intrinsic viscosity of HMPAM in 0.025 wt % PNIPAM/0.1 M NaCl mixed solution is 17.52 dL g^?1, about 2 times 8.66 dL g^?1, that in 0.1 M NaCl solution, which is due to the attractive interaction between the hydrophobic parts of PNIPAM and HMPAM molecules. In semidilute regime, below the saturation concentration, the addition of PNIPAM can lead to both the apparent viscosity and the modulus of HMPAM solutions increasing, which is attributed to the number of aggregation junctions increasing, responsible for the increase of the contribution of the reversible network to the viscosity increase, the β value. In addition, a thermothickening behavior for the HMPAM/PNIPAM mixed solution is observed with increasing temperature over 15–30 °C, which is consistent with the large increase of the Huggins coefficient of HMPAM in the presence of PNIPAM from 1.95 to 7.59 as temperature increases from 25 to 30 °C. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 709–715, 2005     

Experimental Study and Numerical Modeling of Rheological and Flow Behavior of Xanthan Gum Solutions Using Artificial Neural Network

This study investigated effect of temperature, concentration, and shear rate on rheological properties of xanthan gum aqueous solutions using a Couette viscometer at temperatures between 25°C and 55°C and concentrations of 0.25 wt% to 1.0 wt%. The Herschel–Bulkley model described very well the non-Newtonian behavior of xanthan gum solutions. Shear rate, temperature, and concentration affected apparent viscosity and an equation was proposed for the temperature and concentration effect valid for each shear rate. This article also presents an artificial neural network (ANN) model to predict apparent viscosity. Based on statistical analysis, the ANN method estimated viscosity with high accuracy and low error.     

Growth and characterization of 0.1 and 0.25 zinc magnesium ammonium sulphate

Mixed crystals of 0.1 and 0.25 zinc magnesium ammonium sulphate were grown by slow evaporation of aqueous solution at room temperature. The bright and transparent crystals obtained were characterized by thermal (TG–DTA), FTIR and XRD analyses. A fitting decomposition pattern for the compound was formulated on the TG curve which shows two stage mass losses between 133 and 478.75 °C. In this temperature range, DTA curve shows exothermic peaks supporting the formulated decomposition pattern. The FTIR spectra show the vibration frequencies due to the formation of zinc magnesium ammonium sulphate mixed crystals. Detailed structural analysis of the compound is under progress.     

Structure and rheology of hyperbranched and dendritic polymers. II. Effects of blending acetylated and hydroxy‐terminated poly(propyleneimine) dendrimers with aqueous poly(ethylene oxide) solutions

The effect of adding acetylated poly(propyleneimine) dendrimers to the structure and rheology of aqueous solutions of high molecular weight poly(ethylene oxide) (PEO) was investigated by rheology and small‐angle neutron scattering in a temperature range of 10–40 °C. In the semidilute regime, the steady shear rheology of PEO solutions was unmodified by the addition of dendrimers at a comparable weight concentration. At the highest concentrations studied, the addition of acetylated dendrimers suppressed the onset of a low‐frequency elastic modulus at the lowest temperature investigated. For comparison, the addition of PEO of a comparable molecular weight at the same weight fraction resulted in a milder suppression but, unlike the dendrimers, greatly increased the solution viscosity. The addition of acetylated dendrimers to a semidilute PEO solution at 10 °C substantially reduced the solution turbidity. These effects on the rheology and optical properties were confirmed by small‐angle neutron scattering measurements of the molecular structure of the mixture. Additional SANS measurements in the dilute regime (0.1 wt % PEO) showed quantitatively that the dendrimers decorated the PEO chains in a necklace structure, such as that observed previously for micelles. The results suggested a mechanism of rheology modification whereby the dendrimers disrupted the association network structure in the PEO solution at lower temperatures by preferentially associating with the PEO chains in solution. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 874–882, 2000     

A Study of the Effects of Aeration and Agitation on the Properties and Production of Xanthan Gum from Crude Glycerin Derived from Biodiesel Using the Response Surface Methodology

The effects of aeration and agitation on the properties and production of xanthan gum from crude glycerin biodiesel (CGB) by Xanthomonas campestris mangiferaeindicae 2103 were investigated and optimized using a response surface methodology. The xanthan gum was produced from CGB in a bioreactor at 28 °C for 120 h. Optimization procedures indicated that 0.97 vvm at 497.76 rpm resulted in a xanthan gum production of 5.59 g L^?1 and 1.05 vvm at 484.75 rpm maximized the biomass to 3.26 g L^?1. Moreover, the combination of 1.05 vvm at 499.40 rpm maximized the viscosity of xanthan at 0.5 % (m/v), 25 °C, and 25 s^?1 (255.40 mPa s). The other responses did not generate predictive models. Low agitation contributed to the increase of xanthan gum production, biomass, viscosity, molecular mass, and the pyruvic acid concentration. Increases in the agitation contributed to the formation of xanthan gum with high mannose concentration. Decreases in the aeration contributed to the xanthan gum production and the formation of biopolymer with high mannose and glucose concentrations. Increases in aeration contributed to increased biomass, viscosity, and formation of xanthan gum with greater resistance to thermal degradation. Overall, aeration and agitation of CGB fermentation significantly influenced the production of xanthan gum and its properties.     

Molecular origin for the thermal stability of schizophyllan

The non-Newtonian behavior, dynamic viscoelasticity and optical rotation of schizophyllan solutions were measured with a rheogoniometer and polarimeter. Schizophyllan showed shear-thinning behavior below 0.3%, but plastic behavior above 0.5%. Gelation occurred for native and sonic degraded schizophyllan at concentrations of 0.8 and 2.0% at 0 °C, but melted when the temperature reached 5 °C, which was estimated to be a transition temperature. However, the dynamic modulus stayed at a large value during an increase in temperature up to 85 °C. A small decrease in the dynamic modulus of schizophyllan was observed upon addition of urea (4.0 M). A transition temperature was observed when the temperature of schizophyllan reached 60 ° in 0.1 N NaOH and in 85% dimethyl sulfoxide solution, respectively. Though the optical rotation remained constant during a decrease in temperature up to 15 °C, it increased with further decrease in temperature.The thermostability of the dynamic viscoelasticity of schizophyllan may be essentially attributed to an intramolecular hydrogen bonding in aqueous solution. A possible mode of an intramolecular hydrogen bonding for schizophyllan was proposed.     

Study on the temperature-induced sol–gel transition of cellulose/silk fibroin blends in 1-butyl-3-methylimidazolium chloride via rheological behavior

Temperature-induced sol–gel transition of cellulose/silk fibroin/1-butyl-3-methylimidazolium chloride ([BMIM]Cl) was studied from the viscosity and dynamic modulus of the mixtures. The shear thinning behavior of the mixture solution was very obvious with a decrease in temperature. The curves of storage modulus G′ and loss modulus G″ were parallel when the temperature was below 20 °C, indicating that a gel structure exists in the system. The sol–gel transition process was described according to Winter and Chambon’s theory. The gel structure of the mixture system was loosened with the increase of silk fibroin concentration.     

Cellulose–silica composite aerogels from “one-pot” synthesis

Cellulose–silica composite aerogels were prepared via “one-pot” process: aqueous solutions of cellulose–8 wt% NaOH and sodium silicate were mixed, coagulated and dried with supercritical CO₂. The system was studied both in the fluid and solid (dry) states. Cellulose and sodium silicate solutions were mixed at different temperatures and concentrations; mixture properties were monitored using dynamic rheology. The gelation time of the mixture was strongly reduced as compared to that of cellulose–NaOH solutions; we interpret this phenomenon as cellulose self-aggregation inducing partial coagulation due to competition for the solvent with sodium silicate. The gelled cellulose/sodium silicate samples were placed in aqueous acid solution which completed cellulose coagulation and led to in situ formation of sub-micronic silica particles trapped in a porous cellulose matrix. After drying with supercritical CO₂, an organic–inorganic aerogel composite was formed. The densities obtained were in the range of 0.10–0.25 g/cm³ and the specific surface area was between 100 and 200 m²/g. The silica phase was shown to have a reinforcing effect on the cellulose aerogel, increasing its Young’s modulus.     

Surfactant-free dimer fatty acid polyamide/montmorillonite bio-nanocomposites

Polyamide bio-nanocomposites were successfully prepared using a surfactant-free approach. The clay morphology was fixed by dispersing the ammonium ion-exchanged clay in acetic acid. This was mixed with an acetic acid solution of the polyamide, and the composite was recovered by precipitation with water. The composites featured a mixed morphology containing some exfoliated clay sheets together with nano-sized clay tactoids. Bio-nanocomposites containing as much as 27.5 wt% clay were obtained. At this filler level, and depending on the temperature, the modulus was up to nine times higher than that of the parent polymer. Addition of clay also increased the glass transition temperature by as much as 5 °C. This indicates that the high interfacial surface area, presented by the clay platelets dispersed in the matrix, significantly impaired the polymer chain mobility.     

TG–DTA, UV and FTIR spectroscopic studies of urea–thiourea mixed crystal

A mixed crystal of urea–thiourea was grown by slow evaporation of aqueous solution at room temperature. The bright and transparent crystals obtained were characterized by thermogravimetric–differential thermal analysis (TG–DTA), UV and FTIR spectroscopic analyses. A fitting decomposition pattern for the title compound was formulated on the TG curve which shows a two stage weight loss between 200 and 750 °C. In this temperature range DTA curve shows exothermic peaks supporting the formulated decomposition pattern. The UV and FTIR spectra show the characteristic absorption, vibration frequencies due to urea–thiourea mixed crystals. Detailed structural analysis of the compound is under progress.     

Dynamic mechanical analysis of ethylene tetrafluoroethylene (ETFE) foils in use for transparent membrane buildings

Glass transition temperature and tan delta (the ratio of loss modulus to storage modulus) are indispensable parameters for determining appropriate application range of ETFE foils. In this study, ETFE foils in terms of specimen number, material direction and thickness were investigated with dynamic mechanical analysis (DMA) over a temperature range of -70-100 °C at frequencies of 0.1, 1, and 10 Hz. Glass transition temperatures were obtained with storage modulus, loss modulus and tan delta curves. It is found that frequency effect on glass transition temperature was proportional and that frequency effect was more significant than material direction effect. Moreover, a comparison study showed that elastic modulus determined with quasi-static experiments was greater than storage modulus calculated with dynamic mechanical experiments. To propose suitable glass transition temperature ranges for engineering application, an approach to determine confidence interval based on statistical analysis was employed. The resulting intervals with confidence coefficient of 95% were 31.2–32.7 °C, 60.5–66.4 °C and 79.6–83.3 °C for storage modulus, loss modulus and tan delta, respectively. In general, this study could provide useful observations and values for evaluating dynamic mechanical properties of ETFE foils.     

Pore-Scale Studies on the Stability of Microfoam and the Effect of Parameters on Its Bubble Size

This paper presents a new method to prepare microfoam with excellent stability and high by using a sandpack foam generator. The micromorphology of microfoam were analyzed, and average bubble diameter and uniformity of microfoam were studied by microscope. The stability of xanthan gum-stabilized microfoam and common microfoam at the pore scale was also compared. The results showed that a highly uniform microfoam ranging in size from 10 to 100 µm in diameter with a variable coefficient less than 10% was successfully prepared. The bubble size of the microfoam could be controlled by solution viscosity, gas and liquid flow rate, temperature, and backpressure. The bubble size of microfoam decreased and became uniform with the increase of solution viscosity, total flow rate, and backpressure. The bubble size increased slightly and became non-uniform with the increase of temperature, while the concentration of foaming agent had little effect on the bubble size when above 5000 mg/L. The xanthan gum in the solution increased the viscosity and thickness of liquid membrane, so xanthan gum-stabilized microfoam maintained better stability within microconfined media than common microfoam under condition of 160 g/L salinity, 90°C, and 6 MPa backpressure.     

New approach for synthesis of poly(ethylglyoxylate) using Maghnite-H+, an Algerian proton exchanged montmorillonite clay,as an eco-catalyst

In this works, we have explored a new method for a green synthesis of poly(ethylglyoxylate) (PEtG). This method consists on using a montmorillonite clay called “Maghnite-H⁺” as an eco-catalyst to replace triethylamine which is toxic. Cationic polymerization experiments are performed in bulk conditions at three temperatures (?40°C, 25°C, 80°C) and in THF solutions at room temperature (25°C). At 25°C, an optimum ratio of 5 wt% of catalyst leads to molar masses up to 22000 g/mol in THF solutions. Polymerizations in bulk conditions lead to slightly lower masses than experiments conducted in THF solutions. However, bulk polymerization of ethyleglyoxylate remains a preferable method in order to avoid the use of a solvent and therefore to stay in the context of green chemistry. The structure of obtained polymers are characterized and confirmed by ¹H and ¹³C NMR. Thermogravimetric Analysis (TGA) shows an enhanced thermal stability for end-capped PEtG compared to non-terminated PEtG. The best conversion rate (92%) is observed in bulk conditions at 25°C for a reaction time of 48h. An activation energy could be calculated from bulk experiments (Ea = 6.9 kJ/mol). An interesting advantage of Maghnite-H⁺ is an easy recoverage by a simple filtration from the polymer solution.     

pK a Constant Determination of Two Triazole Pesticides: Tebuconazole and Penconazole

We determined the acidity constants of tebuconazole and penconazole, two fungicides from the group of 1,2,4-triazoles. Potentiometric titrations were performed in a 20 % (v/v) acetonitrile/water mixture at 25 °C and at a fixed ionic strength (KNO₃, 0.1 mol·dm^?3). The pK _a values (representing thermodynamic constants) were determined to be 5.0 ± 0.1 and 5.2 ± 0.1 for tebuconazole and penconazole, respectively. These values could be used in pure water solutions to consider the protonated or deprotonated forms when studying the field behavior of these fungicides. Molecular modeling calculations allowed identifying the N4 atom as the protonation site.     

Investigational Studies on Highly Purified Fenugreek Gum as Emulsifying Agent

In this study, highly purified galactomannan containing fenugreek gum was isolated by newly reported method and investigated for its surface and emulsification property. Comparative studies were carried out with other galactomannan containing natural emulsifiers like locust bean gum, guar gum, and non-galactomannan anionic xanthan gum. The results revealed that highly purified fenugreek gum has better surface and interfacial tension reducing property among all gums used in this study. Emulsion prepared with 0.6% highly purified fenugreek gum showed greater reduction in droplets size with greater surface area compared to guar gum, locust bean gum, and xanthan gum emulsion. Zeta potential values indicated that highly purified fenugreek gum emulsion showed greater repulsive forces and was able to form more stable emulsion compared to other gums. No coalescence or phase separation was observed during storage.