Influence Of Gelation Research Articles

Enhancing anti-washout behavior of cement paste by polyacrylamide gelation: from floc properties to mechanism

Polyacrylamide (PAM) is one of the commonly used anti-washout admixtures (AWAs) in cement-based materials. However, PAM chains tend to be contracted under attacks from alkaline and metal ions in cement paste, resulting in low efficiency of washout resistance enhancement. Herein, the gelation of PAM is implemented to surmount this obstacle, in which the influence of gelation on the floc properties is evaluated by the light scattering method and the enhancing mechanism is explored as well. Unlike the contraction of PAM chains in cement paste, the resulting PAM-based hydrogel can unfold a three-dimensional network that enlarges the contact surface with cement particles and promotes the bridging interaction. As a result, the gelation of PAM essentially improves the floc strength and helps maintain its originally compact floc structure with an unchanged fractal dimension of approximately 2.50 under the elevated shear rates. Furthermore, fresh cement paste with the addition of 0.006% PAM-based hydrogel achieves a decrease of 10% in washout mass loss compared to that with the same dosage of PAM. The result of breakage-regrowth experiment shows the PAM-based hydrogel facilities a rapid reaggregation with a recovery factor of 76%, while the flocs in PAM modified cement paste are irreversible after breakage as its recovery factor is only 37%. This understanding is of great importance to chemical admixtures where gelation is a promising way to promote their interaction with cement particles. Moreover, floc properties provide a new perspective on understanding the early-stage behavior of fresh cement paste.

Cellulose Aerogel Microparticles via Emulsion-Coagulation Technique.

Cellulose aerogel microparticles were made via emulsification/nonsolvent induced phase separation/drying with supercritical CO2. Cellulose was dissolved in NaOH-based solvent with and without additives in order to control solution gelation. Two emulsions, cellulose solution/oil and cellulose nonsolvent/oil, were mixed to start nonsolvent induced phase separation (or coagulation) of cellulose inside each cellulose droplet leading to the formation of so-called microgels. Different options of triggering coagulation were tested, by coalescence of droplets of cellulose solution and cellulose nonsolvent and by diffusion of nonsolvent partly soluble in the oil, accompanied by coalescence. The second option was found to be the most efficient for stabilization of the shape of coagulated cellulose microgels. The influence of gelation on particle formation and aerogel properties was investigated. The aerogel particles' diameter was around a few tens of microns, and the specific surface area was 250-350 m2/g.

Influence of gelation on the retention of purple cactus pear extract in microencapsulated double emulsions.

A purple cactus pear (Opuntia ficus-indica) extract (CP) was encapsulated in double emulsions (DE) gelled with gelatin (DE-CP-G) and with gelatin and transglutaminase (DE-CP-GT), as well as in a DE with a liquid external aqueous phase (DE-CP), in order to study the retention of betanin as colorant agent. Both gelled DEs showed a predominantly elastic behavior, in contrast with DE-CP. The degradation rate constant of betanin was significantly higher in DE-CP-GT (90.2 x 10−3 days-1) than in DE-CP-G (11.0 x 10−3 days-1) and DE-CP (14.6 x 10−3 days-1) during cold-storage (4 °C). A shift towards yellow color was found in all the systems during cold-storage (4 °C) and after thermal treatment (70°C/30 min), especially in DE-CP-GT, denoting a higher degradation of betanin. Betalamic acid, cyclo-Dopa 5-O-β-glucoside, 17-decarboxy-betanin and neobetanin were identified by UHPLC-MS/MS as degradation products of betanin.

Influence of gelation and calcination temperature on the structure-performance of porous VOX-SiO2 solids in non-oxidative propane dehydrogenation

VOX-doped porous SiO2 solids (VOX-SiO2) were synthesized at different gelation and calcination temperatures through an in-situ one-step synthesis and evaluated by non-oxidative propane dehydrogenation (PDH), exhibiting excellent catalytic performance. VOX-SiO2 catalysts were characterized by XRD, low-temperature N2 adsorption/desorption, SEM, TEM, UV–Vis, Raman, XPS, NH3-TPD, H2-TPR, and TGA. The structural and performance results for the catalysts reveal that gelation temperature has only a slight effect on the surface acidity, textural properties and VOX species dispersion of VOX-SiO2 catalysts within a reasonable temperature range (30–60 °C). The 60-VOX-SiO2-580 catalyst, with a gelation temperature of 60 °C, a calcination temperature of 580 °C, and prepared by the one-step synthesis method, obviously shortens gelation process time by half compared to catalyst gelled at 30 °C and exhibits superior catalytic performance for PDH. Over high gelation temperature (≥75 °C) leads to the loss of integrity and decreases catalytic activity of the VOX-SiO2 catalysts. Compared with the VOX-MCM-41 catalyst prepared by a one-step hydrothermal method, VOX-SiO2 catalysts exhibit better catalytic activity and stability. Importantly, at a reaction temperature of 580 °C, the 60-VOX-SiO2-580 catalyst exhibits remarkable long-term catalytic stability in the eight reaction-regeneration cycles for PDH. Its propane conversion shows a slow decrease from 55% to 45% for a 4 h reaction duration, but the propene selectivity remains at approximately 91%. The results of this work will be beneficial for further industrial development and application of porous VOX-SiO2 solids in catalysis.

Influence of gelation on particle shape in sheared biopolymer blends

The morphology of biopolymer droplets gelled under shear as the dispersed phase of a two-phase mixture of biopolymers in aqueous solution (or “biopolymer blend”) was investigated experimentally. The process involves shearing the liquid biopolymer blend either at a steady stress till gelation, for creation of ellipsoidal gelled particles, or a stress step-up process for creation of high aspect ratio particles. The resulting particle shapes are analyzed and compared against models for affine deformation conditions [Janssen (1997)] and steady shear conditions [Maffetone and Minale (1998)]. Although they are only strictly valid for Newtonian fluid phases, application of these models to a situation where one phase is gelling, has provided insight into the morphological changes occurring in the shear-gelation process.