Stress-relaxation behavior of a physical gel: Evidence of co-occurrence of structural relaxation and water diffusion in ionic alginate gels

Stress relaxation measurements on ionically cross-linked alginate gels in an unconfined uniaxial compression are reported. Data are obtained as a function of ionic (divalent calcium) concentration and for various sample radii. Analysis of the results shows that such gels, at all investigated concentrations, relax the stress by two different concurring mechanisms: (a) an internal relaxation, most probably due to detachment and re-bonding of the cations along the alginate chains, and (b) the diffusion of water out of the gel.     

Thermal diffusion behavior of nonionic surfactants in water

We studied the thermal diffusion behavior of hexaethylene glycol monododecyl ether (C12E6) in water by means of thermal diffusion forced Rayleigh scattering (TDFRS) and determined Soret coefficients, thermal diffusion coefficients, and diffusion constants at different temperatures and concentrations. At low surfactant concentrations, the measured Soret coefficient is positive, which implies that surfactant micelles move toward the cold region in a temperature gradient. For C12E6/water at a high surfactant concentration of w1 = 90 wt % and a temperature of T = 25 degrees C, however, a negative Soret coefficient S(T) was observed. Because the concentration part of the TDFRS diffraction signal for binary systems is expected to consist of a single mode, we were surprised to find a second, slow mode for C12E6/water system in a certain temperature and concentration range. To clarify the origin of this second mode, we investigated also, tetraethylene glycol monohexyl ether (C6E4), tetraethylene glycol monooctyl ether (C8E4), pentaethylene glycol monododecyl ether (C12E5), and octaethylene glycol monohexadecyl ether (C16E8) and compared the results with the previous results for octaethylene glycol monodecyl ether (C10E8). Except for C6E4 and C10E8, a second slow mode was observed in all systems usually for state points close to the phase boundary. The diffusion coefficient and Soret coefficient derived from the fast mode can be identified as the typical mutual diffusion and Soret coefficients of the micellar solutions and compare well with the independently determined diffusion coefficients in a dynamic light scattering experiment. Experiments with added salt show that the slow mode is suppressed by the addition of w(NaCl) = 0.02 mol/L sodium chloride. This suggests that the slow mode is related to the small amount of absorbing ionic dye, less than 10(-5) by weight, which is added in TDFRS experiments to create a temperature grating. The origin of the slow mode of the TDFRS signal will be tentatively interpreted in terms of a ternary mixture of neutral micelles, dye-charged micelles, and water.     

Physico-chemical properties of alginate gel beads

Alginate gel beads are used in many applications as matrices for release or immobilisation. Until now, the parameters (such as type and concentration of cation, ionic strength and pH) of the beads formation solution in which the Na-alginate solution was dropped were not deeply studied. Therefore, in this paper, the gel formation of alginate beads and their behavior in demineralized water has been investigated carefully. The results obtained in the present study about the gel formation showed that (1) the type and the concentration of the cation play a determinant role in the gel formation phenomenon giving beads of different volumes and characteristics; (2) the weight and volume losses occurring in the ‘syneresis’ are essentially by water elimination; (3) NaCl, which gives the ionic strength of the beads formation solution, plays two roles: a competitor with calcium and a screen in the electrostatic repulsion; and (4) finally, the pH controls the gel formation process as regulator in the dissociation of the alginate and in the complexation of the calcium cations. A study by weight change dynamic analysis was also carried out. The results showed that the transfer of a bead from its formation solution into demineralized water provokes a modification of its volume, weight and stability. These results are important to understand the behavior of beads in their utilization medium.     

Thermal behavior and solid fraction dependent gel strength model of waxy oils

Thermal behavior of waxy oils is investigated using the techniques of thermogravimetric (TG) analysis and differential scanning calorimetry (DSC). Model waxy oils and real waxy crude oils are utilized. Decomposition temperatures of waxy oils are obtained using TG analysis. The effects of thermal history, wax content, and additive on the gelation process of waxy oils are investigated using DSC. The DSC method provides a measure of wax solubility as well as solid fraction. An integration method and a computation method are utilized to predict solid fraction. In addition, wax crystallization onset points are obtained at the cooling rates ranging from 1 to 20 °C min^?1. Similarly, wax dissolution endset points are obtained at heating rates ranging from 1 to 20 °C min^?1. Extrapolated onset and endset points yield wax precipitation temperature and wax dissolution temperature, respectively. Subsequently, wax solubility curves are obtained using thermodynamic computations. A wax precipitation temperature method and a wax dissolution temperature method combine thermodynamic phase behavior with onset/endset points to predict solid fraction. Both the wax precipitation temperature method and the wax dissolution temperature method can predict solid fraction of waxy oil samples. The wax precipitation temperature method and the wax dissolution temperature method are accurate when the temperature is close to the wax appearance temperature. A heat-integration method provides accurate values of the solid fraction at temperatures significantly below the wax appearance temperature. Therefore, integration method and wax precipitation temperature/wax dissolution temperature method are combined to predict solid fraction. The effect of solid fraction on yield stress is also investigated using differential scanning calorimetry and rheometry. Finally, a new solid fraction dependent gel strength model is obtained for shut in and restart of waxy crude oil pipelines.     

Aerobic immobilized cells in alginate gel particles of variable density

Applied Biochemistry and Biotechnology - Immobilized cells ofAcetobacter sp. were used as a model system of aerobic cells to study the influence of gel particle density in fermentation carried out...     

Removal of some nitrophenol contaminants using alginate gel beads

Biopolymers such as alginates are commonly used to remove the cationic contaminants from wastewaters. The major component of the alginate is the alginic acid, a linear, binary heteropolymer of β-d-mannuronate and -l-guluronate residues. In this study the fundamental aspects in the preparation of alginate beads and the effects of salt, sodium alginate concentrations and two cationic surfactants (dodecyltrimethylammonium bromide, cetyltrimethylammonium bromide) on the domains of binding isotherms were investigated. The alginate cross-link complexes with metal ions can exist either as homogeneous clear solutions or precipitates or as spherical beads. The applicability of the calcium and calcium–iron alginate gel beads for removal of some nitrophenols from aqueous solutions was studied. The sorption and kinetic experiments were conducted under different values of pH, initial concentration of nitrophenols and the amount of alginate gel beads. The removal efficiency of contaminant increases with the increasing of the pH and the quantity of alginate beads and decreases with the increasing of initial contaminant concentration. The uptake of nitrophenols occurs rapidly in the first 12 h, followed by a slow process that takes about 72 h. According to the egg-box model of gelation mechanism the cavities formed in the alginate gel capture the cationic contaminants. The adsorption equilibrium data obtained for nitrophenols derivatives at various pH and initial solid sorbent amount were applied to the two classical models, i.e. Langmuir and Freundlich, and the isotherm parameters were calculated.     

Gel kinetic characteristics and creep behavior of polymer microspheres based on bulk gel

Abstract

Deep profile control technology of polymer microspheres has become a widely used new method in improving oil recovery in heterogeneous reservoirs. The viscoelastic property of polymer microspheres plays an important role in the deformable migration behavior. In this study, a new method of measuring the viscoelastic properties of polymer microspheres based on bulk gel was proposed. Using mechanical rheometer and microrheometer, the effects on the storage modulus and gel performance were systematically researched. The creep-recovery test was applied to characterize the creep behavior of different polymer microsphere bulk gel. The results show that the storage modulus of polymer microspheres could be controlled by adjusting the agent concentration in the synthetic reaction. Moreover, the kinetic equation of gel time of polymer microspheres bulk gel and reaction temperature was established: ln(GT)?=?3289.18(1/T)-9.33. Elastic strain index was put forward as a new parameter to characterize the viscoelasticity of polymer microsphere in creep-recovery test. Finally, relationship between elastic strain index and storage modulus was constructed and a classification criterion of polymer microspheres with different viscoelasticity was proposed based on a large number of creep-recovery results. The research could provide a good theoretical guidance and technical support for the understanding of viscoelasticity of polymer microspheres.     

High-Alkaline protease fromBacillus PB92 entrapped in calcium alginate gel

High-alkaline protease (HAP) has been entrapped in Manugel DMB (an alginate gel) and assayed with two sizes and types of substrates: neutral protein casein and synthetic chromogenic tripeptide substrate, Z-Gly-Pro-Cit-PNA. Increasing the concentration of calcium chloride used for capsule formation decreased the measured enzyme activity with both substrates. Capsules were found to be stable in water for long periods of time, but they dissolved in both phosphate and carbonate-bicarbonate buffers. The pH vs activity profiles of encapsulated enzyme showed pH optima between 10 and 11 with both substrates. The calcium alginate matrix surrounding the enzyme was quite effective in stabilizing the enzyme at 20–25 °C and even more so at 4°C. Enzyme stability at 50 °C was quite impressive, some enzyme activity being evident even after remaining for 1 wk at this temperature in water. Increasing concentrations of sodium dodecyl sulfate (SDS) were also found to inhibit the protease progressively, whereas a polyhexamethylene biguanidium chloride (PHMBH⁺Cl^-) and SDS:PHMBH⁺Cl^- combination showed the opposite effect. Optical microscopy, especially polarized light microscopy, provided a sensitive physical means of ascertaining some of the structural properties (sphericity, disorganization or organization, distinct layer enveloping the capsules, intensity of the maltese cross) of the capsules with and without enzyme before and after different chemical treatments and the presence or absense of the substrate.     

Thermal behavior of n-alcohols, benzene, and toluene in water urea and water ethyleneglycol systems

Relations are suggested for calculating the heat characteristics of alcohols, benzene, and toluene in aqueous solutions of urea and ethylene glycol. The solution heat of an alcohol shows greater dependence on the glycol concentration, while the heat capacity of solution, on the urea concentration. For n-butanol in aqueous glycol, the characteristic temperature at which the solution heat of the alcohol is zero decreases as the glycol concentration increases. The dependence of the characteristic temperature of 1-butanol on the urea concentration has a maximum.     

Thermomechanical analysis of calcium alginate hydrogels in water

Spherical hydrogels of Ca alginate (CaAlg) were prepared by conversion of Na alginate (NaAlg) to CaAlg in CaCl₂ aqueous solution. The degree of conversion was varied by changing the conversion times. NaAlg samples with mannuronic and guluronic component ratios (M/G ratio) 0.21 and 0.88 were used. The degrees of conversion and water contents (W_c=mass of water in CaAlg/mass of dry CaAlg) were measured by weighing. The degrees of conversion from NaAlg to CaAlg increased with increasing immersion time and the content of G. W_c of CaAlg hydrogels decreased with increasing immersion time and levelled off at 75 g g^-1 when immersion time exceeded about 10 min. Viscoelastic properties of CaAlg hydrogels in water were measured using a thermomechanical analyzer (TMA) equipped with a quartz compression probe. Dynamic modulus (E') of CaAlg with M/G=0.88 increased with increasing immersion time when immersion time is less than 5 min. When immersion time exceeded 5 min, E' reached a constant value at 4.5*10⁴ Pa. In contrast, E' of CaAlg with M/G=0.21 increased with increasing immersion time. This suggests that densely crosslinked hydrogels are formed when the guluronate sequence of the samples is rich and Na ions are fully converted to Ca ions. This revised version was published online in July 2006 with corrections to the Cover Date.     

Properties of calcium-induced gel beads prepared with alginate and hydrolysates

Calcium-induced alginate gel beads (Alg-Ca) containing alginate hydrolysate, such as the guluronic acid block (GB), was prepared and the drug release profiles were investigated under simulated gastrointestinal conditions. The addition of GB to Alg-Ca altered its rheological properties. A model drug (hydrocortisone) was incorporated at 78% of its theoretical yield within the dried Alg-Ca containing 5% GB and it was gradually released from the beads in JP XIV 1st medium for disintegration test (pH 1.2), while it was rapidly released with disintegration of the gel matrix in JP XIV 2nd medium (pH 6.8). In contrast, for Alg-Ca containing GB and chitosan, disintegration was not observed in these media and the drug release rate was markedly different. These results demonstrate that the release profiles of drugs incorporated into Alg-Ca can be controlled by adding these polysaccharides.     

Effects of natural polysaccharide addition on drug release from calcium-induced alginate gel beads

Calcium-induced alginate gel beads (Alg-Ca) containing various polysaccharides, including an alginate hydrolysate, were prepared and the drug release profiles were investigated. Hydrocortisone (HC) was gradually released from Alg-Ca into the mimic gastric fluid, while in intestinal fluid, it was quickly released with the dissolution of Alg-Ca. However, with Alg-Ca containing 5% chitin (CT), dissolution of Alg-Ca was not observed, and release of HC showed apparent zero-order kinetics. Furthermore, addition of the alginate hydrolysate altered the HC-release profile for Alg-Ca.     

Antimicrobial finishing of cotton textile based on water glass by sol–gel method

A low temperature and cost-effective process for antimicrobial finishing of cotton textiles has been developed by sol–gel method. The antimicrobial treatment was performed by treating cotton textile with silica sols from water glass and then with silver nitrate solution. The antimicrobial activity was determined by using E. coli as a model for Gram-negative bacteria. The results showed that the treated textile has an excellent antimicrobial effect and laundering durability. SEM analysis showed coarse surface morphological change on the water glass treated cotton textile. The residual concentration of silver ion on fabrics was informed by ICP-MS. XPS results indicated that two different states of silver were present on the surface of the antimicrobial textile.     

大茴香醛改性海藻酸钠凝胶的制备及释药性能

在酸催化下使海藻酸钠与对甲氧基苯甲醛(又名大茴香醛)发生缩醛化反应,使其疏水改性,并将其制备成凝胶。利用红外(FTIR)、荧光、透射电镜(TEM)、扫描电镜(SEM)、紫外、热重分析(TGA)对产物进行了表征。结果表明,大茴香醛成功地与海藻酸钠发生了反应。该凝胶可以作为药物载体对牛血清白蛋白进行包埋释放,结果发现,改性后的产物其载药率和缓释性能比未改性的海藻酸钠有了一定的提高。     

Thermal expansion of a viscoelastic gel

It was previously shown [J. Non-Cryst. Solids, 130, 157 (1991)] that the permeability of a saturated gel can be determined from a measurement of its rate of expansion during a change in temperature. The existing analysis assumes that the solid network of the gel behaves elastically, but in many cases the gel is likely to be viscoelastic, especially when the gel exhibits syneresis. In this paper, the expansion kinetics are determined for a gel with a viscoelastic (VE) network. Sample calculations for a silica gel containing ethanol indicate that a naive application of the elastic analysis could result in errors exceeding a factor of two in the estimate of the permeability of a VE gel. However, in such cases, there are qualitative features in the experimental data that alert the experimenter to the existence of significant VE relaxation. Therefore, it is possible to avoid errors by careful examination of the data.     

Uptake behavior of americium on alginic acid and alginate polymer gels

The uptake behavior of Am³⁺ was investigated by using alginic acid and alginate polymer gels. The affinity of radioactive nuclides for carboxyl groups in gel matrices increased in the order of Na⁺ < Cs⁺ < Co²⁺ < Pd²⁺, Sr²⁺, Fe³⁺ < Eu³⁺, Am³⁺. Among alginate gels, calcium alginate exhibited relatively higher uptake rate and distribution coefficient of Am³⁺. The column packed with calcium alginate beads was effective for the removal of trace amounts of Am³⁺ from acidic nitrate solutions.     

The gel swelling properties of alginate fibers and their applications in wound management

Calcium alginate fibers have a novel gel‐forming capability in that, upon the ion exchange between sodium ions in the contact solution and calcium ions in the fiber, the fiber slowly transforms into a fibrous gel. This paper reviews the principles of the gel‐forming process for alginate fibers and analyzed the gelling behavior of various types of alginate fibers. The absorption characteristics of alginate wound dressings were analyzed and it was found that alginate wound dressings absorb a large quantity of liquid into the fiber structure, in addition to those held between the fibers in the textile structure. This gives rise to the unique gel blocking properties of alginate wound dressings. In addition, alginate wound dressings also have novel hemostatic and antimicrobial properties as well as the ability to promote wound healing. They are now widely used in the management of highly exuding wounds such as leg ulcers, pressure sores, and surgical wounds. Copyright © 2007 John Wiley & Sons, Ltd.     

Recent advances on hydrogels based on chitosan and alginate for the adsorption of dyes and metal ions from water

In contemporary times, water resources have become increasingly scarce and suffer from anthropogenic pollution sources with an organic and inorganic origin that are products of industrial, agricultural, and everyday waste. Contamination with heavy metals and dyes in wastewater is considered a risk for water sources that can leak into underground and surface sources, leading to increased biological and chemical contamination. The pollutant removal process is performed by adsorption treatment methods, which is the most common method, and it is considered an effective method with a high and economical removal rate.In this review, we discuss the use of biobased hydrogel adsorbents in the removal of organic dyes and metal ions from water. The literature indicates that hydrogels exhibit rapid absorption kinetics and a dye removal absorption capacity that can reach more than 100 mg/g and sometimes more than 2000 mg/g, with a metal adsorption capacity ranging from 38 mg/g to more than 440 mg/g. These results are discussed and compared by taking into account hydrogel materials that contain biopolymers such as alginate, chitosan or both. In general, absorption depends mainly on biobased materials, which have a natural origin and can be utilized to synthesize hydrogels to remove pollutants, dyes and heavy metals. Chitosan and alginate are prominent materials for this use and they can be incorporated with other components to obtain hydrogels or nanocomposite materials with different efficacies to remove dyes and metal ions.