Thermal Stability Of Gels Research Articles

Improvement of casein / κ-carrageenan composite gel properties: Role of locust bean gum concentration

Processed cheese analogues (PCA) are popular among consumers worldwide for their diverse flavors and forms. Casein/κ-carrageenan (CA/KC), as the main structural basis of common PCA, suffers from syneresis and thermal instability, which could be well ameliorated by locust bean gum (LBG) as a thickening, non-gelatinizing polysaccharide. However, the mechanism by which LBG improves the gel properties of CA/KC composites has not yet been clarified. In this study, the CA/KC/LBG ternary composite gels were constructed and the effect of LBG concentrations (0–0.5%) on the CA/KC binary gel properties were investigated. Compared with the control CA/KC gel, the addition of low LBG concentrations (0.1–0.3%) significantly increased the water-holding capacity, strength and thermal stability of gels (P < 0.05), while the addition of high LBG concentrations (0.4%–0.5%) weakened the gel properties. To reveal the influence mechanism of LBG, the water migration, molecular interactions and microstructure of gels were compared. The results confirmed that the addition of LBG induced the migration of water molecules from free state to bound state and enhanced the hydrogen bonding interactions between CA/KC gel molecules. In addition, scanning electron microscopy results showed that the addition of low concentrations of LBG induced the unfolding of KC macromolecules into chains, which optimized the gel network structure of KC. The formed CA/KC/LBG ternary composite gels showed more homogeneous and denser gel network structure than the control. In conclusion, the addition of an appropriate amount of LBG could well improve the thermal stability of CA/KC binary gel and the sense of storage stability of PCA products.

Preparation and Mechanism of Stability for High-Temperature and High-Salinity Gels

Summary By using an acrylamide (AM)-acrylamide propyl sulfonate (AMPS) copolymer crosslinked with hydroquinone (HQ) and hexamethylenetetramine (HMTA), a stable gel was prepared at 150℃ in simulated brine with a salinity of 25×104 mg/L. The gelation time, strength, and thermal stability of gel were investigated, and the results indicated that the dosage of the crosslinker was the key factor controlling the stability and gelation time of gel. When the polymer concentration was more than 0.7 wt% and the concentrations of HQ and HMTA were more than 0.2 wt%, respectively, a gel aged under high temperature and high salinity for 180 days could be obtained, but its gelation time was short and could be extended by adding lauramidopropyl hydroxy sulfobetaine (LHSB). When the crosslinker concentration was low, the gel would exhibit syneresis problems after aging at 150℃. Cryo-scanning electron microscopy (cryo-SEM), 13C nuclear magnetic resonance (13C-NMR), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR) analyses showed that the presence of AMPS in the crosslinked polymer controlled the gel resistance to high temperature and high salinity. Using AM-AMPS copolymer with high AMPS content and regulating the crosslinker concentration can effectively inhibit the intramolecular catalytic hydrolysis of AMPS and enhance the stability of gel. The research results could guide the design and development of gels for conformance control in high-temperature and high-salinity reservoirs.

Rheological and gelling properties of Nicandra physalodes (Linn.) Gaertn. pectin in acidic media

Nicandra physalodes (Linn.) Gaertn. polysaccharide (NPGP) was previously recognized as a pectic polysaccharide, with a high galacturonic acid content (87.8%) and a low methoxylation degree (28%). In the present study, it was found that NPGP can form self-supporting gels when cooling its heated solutions (2.0%, w/v) acidified by citric acid. It was demonstrated that the decrease in pH led to the suppression in electrostatic repulsions between the pectin chains, thereby promoting pectin chain-chain association mainly through hydrogen bonding. As the pH decreased from 3.2 to 2.4, the gel strength and gel thermal stability were continuously increased. Moreover, it was shown that sucrose addition slightly promoted the gelation and gel thermal stability of NPGP, but the effect of monovalent ions (Na+) and divalent ions (Ca2+) was not significant. Conclusively, our results indicate that NPGP is a new gelling polysaccharide that shows great potential in formulation of acidic gel foods.

Mechanical, thermal stability and microstructural properties of emulsion-filled gels: Effect of sugar beet pectin/soy protein isolate ratio

The current study focused on how matrix ratio affected the gel properties and microstructures of sugar beet pectin (SBP)/soy protein isolate (SPI)-based gels with and without emulsion. Roles of emulsion stabilized by Tween 20 and different ratios of matrices in gelation induced by laccase and heat treatment were evaluated. The textural, rheological and thermal stability properties of either gels or gels containing emulsion were enhanced with increasing level of SBP and weakened with increasing concentration of SPI. With the increase of SBP concentration, the emulsion addition showed no significant influence on the textural parameters of gels. However, the thermal stability of gel was weakened because of the emulsion addition. The E/P212 showed improved textural and rheological properties compared with the gel without emulsion. All gels exhibited a frequency-independent behavior, and the water holding capacity of each gel was about 90%. Moreover, microstructure analysis indicated that the well-filled honeycomb gel was formed in E/P212. The gelation behavior was mainly dominant by SBP, and the underlying gelation mechanism was that the SBP was cross-linked to promote the gelatinization reaction and rate of SPI.

Preparation and characterization of thermo-sensitive gel with phenolated alkali lignin

Thermo-sensitive gel exhibits great potential industrial application. It has been widely used in tissue repair, drug release and water purification for its property of phase transition in response to external stimuli, reusability and biocompatibility. In this study, a novel lignin-based thermo-sensitive gel was synthesized with alkali lignin by two steps. Firstly, phenolated lignin (PPAL) was synthesized with purified alkali lignin (PAL) catalyzed by sulfuric acid. Subsequently, thermo-sensitive gel was achieved by thermal polymerization of phenolated alkali lignin and N-isopropylacrylamide (NIPAAm). Furthermore, the prepared hydrogels were characterized in terms of thermal behavior, interior morphology and their swelling behavior. Compared with PAL-based gel, the obtained PPAL-based gel exhibits a higher crosslinking density and lower critical solution temperature (LCST) due to the increase of reaction site and smaller space volume of the hydrophobic side groups grafted on NIPAAm. TGA data revealed that thermal stability of gel was enhanced (50% weight loss at ~380 °C) by using lignin as precursor. SEM images showed that a more regular interior morphology, better compressive strength was also found (PPAL0.05, 11.15 KPa). Furthermore, the swelling ratio of PPAL-based gel was lower than that of PAL-based gel due to its more complex structure.

Mechanism of Polyacrylamide Hydrogel Instability on High-Temperature Conditions.

A gel system composed of acrylamide (AM), N,N′-methylenebisAM (BIS), and ammonium persulfate ((NH4)2S2O8) was developed and applied extensively in reservoirs to reduce water cut and increase oil production in mature fields. However, this gel system suffers from thermal stability loss and syneresis at high temperatures that reduces its ability to control water flow. It has been widely accepted that the loss of gel thermal stability can be explained via three aspects: the rupture of polymer chains, the breakage of cross-linker chains, and hydrolysis of polymer. The mechanism of hydrogel syneresis through polymer hydrolysis has been investigated extensively in other publications. However, research on the other two mechanisms is quite limited. In this article, we conduct a series of experiments to demonstrate how the rupture of polymer and cross-linker chains leads to the hydrogel instability at high temperatures. Viscosity and energy-dispersive system measurements suggested that polyAM chains were disrupted by the oxidation reactions involving free radicals. The method to measure the cross-linking degree was established and in combination with X-ray photoelectron spectroscopy measurements, the results showed that cross-linker chains were broken as a result of weaker C–N bond resulting from positively charged mesomethylene carbon and hydrolysis of amide groups on the cross-linker. Because of the application of deionized water in the experiments, nuclear magnetic resonance and FTIR measurements showed that the hydrolysis degree of polymer was weak. Hence, our results verified that breakage of polymer and cross-linker chains led to the rupture of the gel network at high temperature. Besides, cross-linker chains may play a more important role in the thermal stability of the gel, which explains some work into high-temperature-resistant gels.

Gel properties and interactions of Mesona blumes polysaccharide-soy protein isolates mixed gel: The effect of salt addition

Effect of different salt ions on the gel properties and microstructure of Mesona blumes polysaccharide (MBP)-soy protein isolates (SPI) mixed gels were investigated. Sodium and calcium ions were chosen to explore their effects on the rheological behavior and gel properties of MBP-SPI mixed gels were evaluated by using rheological, X-ray diffraction, protein solubility determination, and microstructure analysis. Results showed that the addition of salt ions change the crystalline state of gels system, the crystal of gel was enhanced at low ion concentrations (0.005–0.01 M). The two peaks of gel characteristic at 8.9° and 19.9° almost disappeared at high salt ions concentrations (0.015–0.02 M), and new crystallization peaks appeared at around 30° and 45°. The elasticity, viscosity, gel strength, water holding capacity, and thermal stability of gel were increased at low ion concentration. Results showed that the main interactions which promoted gel formation and maintain the three-dimensional structure of the gel were electrostatic interactions, hydrophobic interactions, and disulfide interactions.

Organic gels prepared from side‐chain liquid crystalline polymers without the spacer

A series of side‐chain liquid crystal polymers (SCLCPs) without the spacer, named poly[ω‐(4′‐n‐alkyl oxybiphenyl‐4‐oxy)methacrylate (PMBiCm, m = 1, 2, 4, 6, 8, 10, 12, 14, 16, and 18), have been synthesized. The novel polymer organogels were prepared by introducing PMBiCm into common organic solvents. Solubility and gel properties of polymer organogelators differ widely according to the nature of the solvents. In aromatic solvents, PMBiCm completely dissolved in solvent due to good compatibility between biphenyl mesogen group and aromatic solvents. Poly[ω‐(4′‐n‐alkyl oxybiphenyl‐4‐oxy)methacrylate were still insoluble in polar solvents such as acetone, ethanol, DMF, ethylene glycol, and n‐butanol. This behavior resulted from mismatch of solubility parameter between PMBiCm and solvent. Considering the factors of solvent, we have systematically studied 3 organic solvents with different polarities (butyl acetate, n‐butyl amine, and n‐heptane). It is found that the length of the alkoxy tail chain of the SCLCPs has significant influence on gelability and gel thermal stability. In further studies discussed by UV‐Vis spectroscopy, the results revealed that the π‐π stacking interaction of the biphenyl mesogens might be the key factor for guiding the self‐assembly processes and the polymer gel formation. This work is useful to comprehending physical mechanism of polymer organogels. Meanwhile, those expand SCLCPs to a wide range of applications.

Fluid loss control mechanism of using polymer gel pill based on multi-crosslinking during overbalanced well workover and completion

Recently, polymer gel used as fluid loss pill for well workover and completion is reported more, while it mainly focuses on using single crosslinking mechanism. To the best of our knowledge, polymer gel based on multi-crosslinking is commonly used in the aspect of water shutoff. The theory of multi-crosslinking for preparing polymer gel has rarely been used in overbalanced well workover and completion. In this paper, we conduct experimental study to reveal the physical process and fluid loss control mechanism of the polymer gel pill based on multi-crosslinking during overbalanced well workover and completion. We propose the design philosophy of the multi-crosslinking based on two or three crosslinking reactions under different conditions. In this paper, the first gelation reaction of the target polymer gel is between a kind of high-temperature resistance acrylamide based polymer (abbreviated as “SPAM”) and fresh chromium acetate as metallic-ion crosslinker to quickly form a pre-crosslinking gel with strength of around Code C at surface temperature of 30°C. It aims at guaranteeing pumping performance and restricting free water for lowering the risk of fluid loss in inner tube wall and or the separation of polymer and crosslinker during pumping process. Subsequently, with the increasing of wellbore temperature during the pumping process, the pre-crosslinking gel will gradually become mature (when gel reaches at the final gel strength) to form high strength gel by the use of either polyethyleneimine (PEI) or hexamethylenetetramine (HMTA) or both as organic crosslinker(s) through covalent bonding. The mature polymer gel can realize the complete crosslinking structure to guarantee gel thermal stability. The mature polymer gel pill based on multi-crosslinking formulated with a combination of SPAM, fresh Chromium acetate, PEI and HMTA indicates an obvious viscoelasticity characteristic at high temperature of 150°C, while exhibiting a notable elastic performance for the formula without crosslinker HMTA. The fluid loss evaluation experiment result shows that the mature polymer gel based on multi-crosslinking can really act as fluid loss pill to withstand high positive pressure during overbalanced well workover and completion. Whilst there is some degree of permeability damage for the treated core, but not too high. Besides, the scanning electron microscope (SEM) is further employed to reveal the fluid loss control mechanism of the mature polymer gel at the micro level. This study provides an avenue to implement polymer gel pill based on multi-crosslinking for fluid loss control during overbalanced well workover and completion.

Covalent immobilization of microbial naringinase using novel thermally stable biopolymer for hydrolysis of naringin.

Naringinase induced from the fermented broth of marine-derived fungus Aspergillus niger was immobilized into grafted gel beads, to obtain biocatalytically active beads. The support for enzyme immobilization was characterized by ART-FTIR and TGA techniques. TGA revealed a significant improvement in the grafted gel’s thermal stability from 200 to 300 °C. Optimization of the enzyme loading capacity increased gradually by 28-fold from 32 U/g gel to 899 U/g gel beads, retaining 99 % of the enzyme immobilization efficiency and 88 % of the immobilization yield. The immobilization process highly improved the enzyme’s thermal stability from 50 to 70 °C, which is favored in food industries, and reusability test retained 100 % of the immobilized enzyme activity after 20 cycles. These results are very useful on the marketing and industrial levels.

Hydrogels of sodium alginate in cationic surfactants: Surfactant dependent modulation of encapsulation/release toward Ibuprofen

The interaction of cetyltrimethylammoium bromide (CTAB) and its gemini homologue (butanediyl-1,4-bis (dimethylcetylammonium bromide), 16-4-16 with biocompatible polymer sodium alginate (SA) has been investigated in aqueous medium. Addition of K2CO3 influences viscoelastic properties of surfactant impregnated SA via competition between electrostatic and hydrophobic interactions. Viscosity of these polymer-surfactant systems increases with increase in concentration of K2CO3, and a cryogel is formed at about 0.5M K2CO3 concentration. The thermal stability of gel (5% SA+0.5M K2CO3) decreases with increase in surfactant concentration, a minimum is observed with increase in 16-4-16 concentration. The impact of surfactant addition on the alginate structure vis-à-vis its drug loading capability and release thereof was studied using Ibuprofen (IBU) as the model drug. The hydrogel with 16-4-16 exhibits higher IBU encapsulation and faster release in comparison to the one containing CTAB. This higher encapsulation-cum-faster release capability has been related to micelle mediated solubilization and greater porosity of the hydrogel with gemini surfactant.

The influence of solvents on sol–gel derived calcium aluminate

High purity calcium aluminate (CA) samples were derived by a sol–gel method using aluminium-sec-butoxide (ASB) and calcium nitrate tetrahydrate (CN). In order to facilitate the controllable synthesis process an ethyl acetoacetate (EAA) was used as a chelating agent for ASB (EAA:ASB=3:2). Synthesis was performed using different solvents: ethanol (EtOH), 2-propanol (PrOH), and 2-butanol (BuOH) in order to influence the early stage of CA sol–gel synthesis. The gels were examined using Fourier transform infrared (FTIR) and Nuclear Magnetic Resonance (NMR) spectroscopy. Al was found to be highly prone to change of coordination states as a consequence of different synthesis parameters and thus various processes occurring; oligomerisation, hydrolysis, condensation, and chelation.The derived gels were dried and thermally treated to form CA. The thermal evolution, crystallisation behaviour and final microstructure of dried gels were monitored by means of Differential Thermal and Thermogravimetric Analysis (DTA/TG), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) with Energy dispersive X-ray spectroscopy (EDS), particle size distribution and N2 adsorption/desorption isotherms. It was determined that solvents influence gel composition, structure and particularly the morphology of CA precursor powders. The influence of solvents on the properties of sols and gels also reflects in the properties of calcined CA powders like phase composition, crystallisation parameters and the morphology.EtOH based sample is characterised with emphasised condensation stage on behalf of which gel thermal stability is enhanced. Thereby final CA microstructure may differ upon calcination. EtOH based sample yields Scherrer crystallite size of 150Å and bimodal particle size distribution with size between 5 and 20µm while specific surface area was 5m2g−1, differing to samples where other solvents were used.It was shown that better utilisation of the CA sol–gel synthesis process can be achieved by simple modification of synthesis parameters.

Covalent immobilization of β‐galactosidase on carrageenan coated with chitosan

Abstractβ‐Galactosidase was covalently immobilized to carrageenan coated with chitosan for the hydrolysis of lactose. The chitosan‐carrageenan polyelectrolyte interaction was found to be dependent on the chitosan pH. At pH 4, the chitosan reached its maximum binding of 28.5% (w/w) where the chitosan surface density was 4.8 mg chitosan/cm2 g of carrageenan gel disks, using Muzzarelli method. Glutaraldehyde was used as a mediator to incorporate new functionality, aldehydic carbonyl group, to the bio‐polymers for covalent attachment of β‐galactosidase. The enzyme was covalently immobilized to the biopolymer at a concentration of 2.73 mg protein per g of wet gel. FTIR proved the incorporation of the aldehydic carbonyl group to the carrageenan coated with chitosan at 1720 cm−1. The optimum time for enzyme immobilization was found to be 16 h, after which a plateau was reached. The enzyme loading increased from 2.65 U/g (control gel) to 10.92 U/g gel using the covalent technique.The gel's modification has shown to improve the carrageenan gel thermal stability as well as the immobilized enzyme. For example, the carrageenan gel treated with chitosan showed an outstanding thermal stability at 95°C compared with 35°C for the untreated carrageenan gel. Similarly, the immobilization process shifted the enzyme's optimum temperature from 50°C for the free enzyme towards a wider temperature range 45–55 °C indicating that the enzyme structure is strengthened by immobilization. In brief, the newly developed immobilization method is simple; the carrier is cheap, yet effective and can be used for the immobilization of other enzymes. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Lactose Hydrolysis by β-Galactosidase Covalently Immobilized to Thermally Stable Biopolymers

Lactose has been hydrolyzed using covalently immobilized beta-galactosidase on thermally stable carrageenan coated with chitosan (hydrogel). The hydrogel's mode of interaction was proven by Fourier transform infrared spectroscopy, differential scanning calorimetry (DSC), and Schiff's base formation. The DSC thermogram proved the formation of a strong polyelectrolyte complex between carrageenan and chitosan followed by glutaraldehyde as they formed one single peak. The modification of carrageenan improved the gel's thermal stability in solutions from 35 degrees C to 95 degrees C. The hydrogel has been proven to be efficient for beta-galactosidase immobilization where 11 U/g wet gel was immobilized with 50% enzyme loading capacity. Activity and stability of free and immobilized beta-galactosidase towards pH and temperature showed marked shifts in their optimum pH from 4.5-5 to 5-5.5 and temperature from 50 degrees C to 45-55 degrees C after immobilization, which reveals higher catalytic activity and reasonable stability at wider pHs and temperatures. The apparent K(m) of the immobilized enzyme increased from 13.2 to 125 mM, whereas the V(max) increased from 3.2 to 6.6 micromol/min compared to the free enzyme, respectively. The free and immobilized enzymes showed lactose conversion of 87% and 70% at 7 h, respectively. The operational stability showed 97% retention of the enzyme activity after 15 uses, which demonstrates that the covalently immobilized enzyme is unlikely to leach. The new carrier could be suitable for immobilization of other industrial enzymes.

Preparation, characterization and proton-conductivity of silica gel containing 71 wt.% 12-tungstogermanic heteropoly acid

Silica gel containing 71 wt.% 12-tungstogermanic acid was prepared by means of sol–gel method. Infrared (IR) spectrum and X-ray powder diffraction (XRD) pattern revealed that the Keggin structure characteristic of GeW 12O 40 4− anion was present in the silica gel skeleton. The change of the characteristic peaks in the IR spectra was investigated. Thermogravimetry (TG) curve shows that there are three steps of weight loss. Differential thermal analysis (DTA) curve shows that the thermal stability of gel is lower than that of pure 12-tungstogermanic acid. The silica gel exhibited considerably high proton conductivity. It is 4.00 × 10 −3 S cm −1 at room temperature (16°C).

Ｂ２Ｏ３‐Ｎａ２Ｏ‐Ａｌ２Ｏ３系のゲルの熱的安定性とガラス化領域

The thermal stability of gels in the B2O3-Na2O-Al2O3 system was evaluated by two methods; (1) observation of the change of amorphous region after heating up to 200-800°C at a heating rate of 1°C/min and keeping for 2h, and (2) determination of the crystallization temperature Tc from the DTA curve of gels with heating rate of 5°C/min and calculation of the ratio of Tc to the liquidus temperature Tl, Tc/Tl. After heating gels up to 800°C, the original gel region, which extends over a wide area including Al2O3, was reduced to only a small amorphous region near the Al2O3 rich area. The Tc/Tl ratio showed the highest value of more than 0.8 in the composition range B2O3=55-90mol% and Al2O3=0-20mol%, while it was less than 0.6 value in the Al2O3 rich composition area. The compositional dependence of Tc/Tl ratio was compared with the variation of the glass-forming region obtained from melt at various cooling rates Q in the range of logQ=-2 to 5. It was recognized that the gel region having a high Tc/Tl value corresponds to a stable glass region obtainable under slow cooling. It seems that some close relationship exists between the Tc/Tl of a gel and the critical cooling rate Q* for glass formation of a melt having the same composition.