Globular protein gelation

In this paper, significant advances in the structure and rheological properties of globular protein gels have been described. The achievements of scattering methods, which have provided information over a range of distances from the monomer to the micron scale, are reviewed. The kinetics of gelation measured by rheological methods have also been discussed, including currently applied models for the gelation time, the gel elastic modulus and the critical gel concentration.     

Viscosity divergence and gelation

A new model of gelation is presented in which the viscosity is observed to diverge without the introduction of shear elasticity when a critical concentration of suspended particles is reached. This viscosity divergence results from the hydrodynamic interaction between the suspended particles and can be explained using the Random Resistance Network model. A comparison of the data and with other models is also presented.     

Donor-acceptor-promoted gelation of polyaromatic compounds

Low molecular mass organogels are nonconventional polymeric structures in which a minute amount of low molecular weight compound can reversibly gelify the whole solution without forming covalent bonds between the monomers. In this article, we demonstrate that certain electron acceptors (taking dinitrobenzoates as model compounds) that are incapable of gelifying the solvent on their own can assemble as much as a 15-16-fold larger amount of polyaromatic hydrocarbons (PAHs) and form two-component donor-acceptor organogels in different solvents. At the core of the long-range order stand donor-acceptor pairs. We assess our claims by detailed 1H NMR, spectrophotometry, fluorescence, and time-resolved fluorescence methods. The thermodynamics of the gelation process is described on the basis of temperature dependent 1H NMR studies. We believe that, in this case, 1H NMR provides direct quantification of the dissolved concentrations of the different species and therefore provides a direct way to measure the enthalpy, entropy, and free energy associated with gel formation.     

Generalized statistical gelation theory

Gel points in random polymerizations of the general type Σ_iRA + Σ_jRB in which A-groups react with A- and B-groups, and B-groups react only with A-groups are considered. (The symbols Σ_i and Σ_i signify that the A- and B-bearing reactants RA and RB can be mixtures of monomers of different functionalities, denoted generally as f_ai and f_bj.) The usual case of A-groups reacting only with B-groups is a special case of the present theory. The effects of chemical kinetics, the competitive reaction of A- and B-groups, are separated from the generalized statistical condition for gelation. The former are used to define reaction curves and the latter, gelation curves. Both types of curve are represented as p_a as a function of p_b. For a given polymerization, gelation occurs when the reaction curve and the gelation curve intersect. When A-groups react only with B-groups, the gel points are those for the usual type of Σ_iRA + Σ_jRB polymerization, and, in the limit of A-groups only reacting with A-groups, the gel points are those for Σ_iRA self polymerizations.     

Thermoreversible gelation of polyacrylonitrile/dimethylformamide-solution

The thermoreversible gelation of semi-diluted atactic polyacrylnitrile (PAN)/dimethylformamide (DMF)-solutions has been studied. The structural features of PAN/DMF-gels, formed by supercooling have been investigated by DSC-, x-ray- and swelling measurements. A new structural model has been introduced to describe the morphology of the junction zones of PAN/DMF-gels and to also explain the gelation behavior of PAN/DMF-solutions as the structural features of PAN/DMF-gels. The junction zones of a PAN/DMF-gel have been defined as ordered junction zones.A gelation enthalpy of about H=–6 kJ/mol supports the idea that an ordered junction zone is formed by intermolecularly neighboring stereoregular parts of atactic PAN chains due to a nucleation process in the solution. It can be defined as a strongly distributed fringed micelle.     

Rheological investigation of starch polysaccharide gelation

Small-amplitude dynamic measurements of aqueous starch polysaccharide solutions are performed by a Bohlin controlled-stress rheometer with air bearing. Three classes of starch polysaccharides–native starches, fractions of starches and hydrolysed starches–are compared in their molecular composition and rheological properties during and after the gelation process. Viscoelastic properties of solutions and gels are recorded in dependence on temperature and time, yielding storage and loss moduli during and after sol-gel transition. Hot concentrated aqueous solutions are cooled down from 90 °C to 5 °C at a rate of 1 °C/min. Measurements are carried out at 0,1 Hz and 5% strain amplitude deformation. The typical course of the moduli of gelling starch polysaccharide solutions shows a liquid-like behavior (G” > G') in the upper temperature level between 60 and 90 °C, a jumpwise increase with ensuing intersection of G' and G” below 60 °C and a solid-like behavior (G' > G”) at lower temperatures with a slightly in time growing storage modulus. Storage and loss moduli depend on molecular composition and concentration of the substance. The process of starch polysaccharide aggregation is discussed with regard to theories of physical gelation by Ross-Murphy and Winter.     

Light-activated ionic gelation of common biopolymers

Biopolymers such as alginate and pectin are well known for their ability to undergo gelation upon addition of multivalent cations such as calcium (Ca(2+)). Here, we report a simple way to activate such ionic gelation by UV irradiation. Our approach involves combining an insoluble salt of the cation (e.g., calcium carbonate, CaCO(3)) with an aqueous solution of the polymer (e.g., alginate) along with a third component, a photoacid generator (PAG). Upon UV irradiation, the PAG dissociates to release H(+) ions, which react with the CaCO(3) to generate free Ca(2+). In turn, the Ca(2+) ions cross-link the alginate chains into a physical network, thereby resulting in a hydrogel. Dynamic rheological experiments confirm the elastic character of the alginate gel, and the gel modulus is shown to be tunable via the irradiation time as well as the PAG and alginate concentrations. The above approach is easily extended to other biopolymers such as pectin. Using this approach, a photoresponse can be imparted to conventional biopolymers without the need for any chemical modification of the molecules. Photoresponsive alginate gels may be useful in creating biomaterials or tissue mimics. As a step toward potential applications, we demonstrate the ability to photopattern a thin film of alginate gel onto a glass substrate under mild conditions.     

Interfacial routes to colloidal gelation

Colloidal gelation is a rheological transition from fluid-like to solid-like viscoelasticity in a particulate suspension and is often instigated by causing the net interparticle interaction to be attractive. In this article, three routes to colloidal gelation that have been discovered recently and involve interfacial phenomena at a fluid interface are reviewed. As in conventional systems, gelation is due to a percolating particle network that imparts elasticity to the mixture, but the network formation involves interfacial particle jamming or bridging, or capillary interactions along or across interfaces, in a mixture of immiscible fluids. Gelation imparts mechanical stability to these multiphase mixtures and paves the way for their use as templates for the synthesis of functional, microstructured materials and composites. The gel mechanical properties are mediated by the interfacial forces and the mixture's microstructure, and therefore show different dependencies on particle volume fraction across the three systems.     

Amylose gelation in low-concentration conditions

Amylose gels of low concentration (1–2 wt.-%) were studied by means of a rheological method and turbidity measurements. The effects of temperature and concentration were investigated. The kinetics of turbidity evolution and of the storage modulus G' variations were strongly dependent of both parameters. The overall gelation process could be described in three steps: first, an induction period, followed by a sharp variation of G' and turbidity, and finally a slow variation of G'. These observations confirm that amylose gelation proceeds through a phase separation process.     

Temperature-induced gelation in dilute nanofluids

We report a temperature-induced gelation in dilute nanofluids containing surfactant capped iron oxide and alumina particles of average diameter ~10 nm. We observe a dramatic enhancement in the elastic modulus, viscous modulus, and viscosity, by 3-6 orders of magnitude for a volume fraction (φ) less than 0.035, above a critical shear rate ( ?γ(c)) and temperature (T(c)). The T(c) follows a weak power law scaling with φ as T(c) ~ φ(β), where the scaling exponent β is found to be -0.24. The observed gel-like transition at elevated temperature is attributed to strong van der Waals attractions on the kT energy scale due to poor solvent conditions, which is reminiscent of the phase behavior reported in polymer-coated colloids.     

Thermoreversible gelation of solutions of vinyl polymers

The thermoreversible gelation of solutions of isotactic poly(methyl methacrylate) is investigated. Amorphous gels can be prepared in l-butanol by a combination of a liquid-liquid demixing with an upper critical demixing temperature and a glass transition. Annealing of the demixed solutions above their glass transition temperature T_G, results in the formation of a crystalline gel. In oxylene, crystalline gels are formed by a liquid-liquid demixing with an lower critical demixing temperature and an annealing at room temperature. Very fast gelation is observed to occur far below room temperature in solvents like 2-butanone.     

Dynamics of liquid crystalline gelation of DNA

DNA liquid crystalline gel (LCG) films have been prepared by immersing DNA aqueous borate solutions sandwiched between two circular glass plates into cobalt chloride solutions. The time courses of the thickness, the weight fractions of DNA and cobalt cations, and the birefringence and turbidity of the film consisted of outer DNA LCG and inner DNA amorphous gel or solution have been measured. To clarify the mechanism of the process forming LCG, the theory based on the nonequilibrium thermodynamics with "moving boundary picture" [Langmuir 2005, 21, 8155-8160] was modified, and the results were analyzed by the modified theory. It was found that the growth process of DNA LCG consists of two dynamics: cobalt cation diffusion-limited process at the early stage and the DNA circumstance change limited process at the later stage.     

Thermoreversible gelation with junctions of variable multiplicity

Effect of junction multiplicity on the phase behavior of physical gels is studied with special attention to the interference phenomena between gelation and phase separation. Three important models of the junction multiplicity are detailed, each representing cross-linking by hydrogen bond, microcrystallization and dipole aggregation. Singularities accompanying the gelation transition are found to be analogous to those of the Bose-Einstein condensation.     

A lattice model of vitrification and gelation

In this work we introduce a simple lattice model with T-shaped molecules in two dimensions that exhibits a rich range of morphological behaviors. Depending on the volume fraction and quench path, this system can adopt uniform liquid, solution, and phase-separated states, as well as inhomogeneous glass or gel-like states, as revealed by dynamic mean-field simulations. An important characteristic of this system is the existence of a large number of degenerate low-energy states with small barriers that leads to a broad, kinetically explored landscape. The mean-field stability and phase diagram of this model is constructed and provides a useful guide for understanding the complex behaviors of the system. One striking feature is that there is a cascade of instabilities that converge to mark the onset of what we identify as the glass transition. Both dynamic mean-field and Monte Carlo simulations reveal glass-like relaxation dynamics. Our results lead to a picture of gelation as a continuation of the glass transition into the two-phase region, or equivalently, as an incomplete phase separation arrested by the onset of the glass transition.     

Temperature-induced gelation of concentrated silicon carbide suspensions

Due to the steric barrier provided by the adsorption of the dispersant hypermer KD1 (a polyester/polyamine condensation polymer), stable and low-viscosity suspensions of SiC, Y(2)O(3), and Al(2)O(3) powder mixtures could be prepared in methyl ethyl ketone (MEK)/ethanol (E) solvent with solids loading as high as 60 vol%. The solvency of the dispersant in MEK/E decreased dramatically on cooling. Steady shear viscosity and oscillatory measurements were performed as a function of temperature for suspensions with different solids loading. The viscosity and elastic modulus of suspension increased with decreasing temperature and became more sensitive with the increase of solids loading. The suspensions with solids loading higher than 40 vol% could be solidified with decreasing temperature, but gelation temperature and gelation stiffness decreased with decreasing solids loading. The 60 vol% solid-loaded suspension was a stable and free-flowing fluid at 20 degrees C and gradually transformed to a very highly viscous and elastic system upon cooling to about 13 degrees C. Complete solidification occurred when the temperature was decreased to 5 degrees C. The gelation mechanism was mainly based on the collapse of the adsorbed layer as the temperature decreases, which induced incipient flocculation and formed a stiff network. The gelled body was further strengthened by separation of the dispersant from the suspension.     

Studies on gelation of soybean globulin solutions

Thermal denaturation of soybean globulin fraction (SBGF) in diluted solution (protein concentration 0.15–0.63%) has been studied by the method of differential adiabatic scanning calorimetry. SBGF thermograms have two maxima. The low temperature maximum is consistent with denaturation of 7S component, while the high temperature maximum with denaturation of 11S components of this fraction. In the investigated range of protein concentrations the thermodynamic parameters (temperature and enthalpy) of denaturation of SBGF and its main components are constant. This fact suggests that differential adiabatic scanning calorimetry gives information purporting a change in the protein state at molecular level. The temperatures and enthalpies of denaturation of the main SBGF components linearly rise with increase of NaCl concentration. The slope of dependences of denaturation temperature on salt concentration,K _s, is extremely large (nearly 20 K · l/mole). The elementary thermodynamic theory of lyotropic effects in thermal denaturation of proteins has been developed based on the two-state model and linear approximation of protein-salt interactions by means of the corresponding second virial coefficient. It shows that the dependences of thermodynamic parameters of thermal denaturation on salt concentration should be linear in the initial section. This conclusion is consistent with the experiment. The differences of enthalpies and entropies of transferring denatured and native forms of the main SBGF components from water into NaCl solution have been determined. They are positive and their quantity increases linearly with salt concentration. This fact is consistent with the concept to the effect that the main factor of salt influence on thermal denaturation of SBGF is confined to a decrease of protein hydration. The effect of protein nature on the quantity of lyotropic effect in thermal denaturation has been considered. Using simple considerations as a basis, the dependence of the ratio betweenK _s and the denaturation temperature in water has been obtained, which characterizes the lyotropic effect, on the molar fraction of hydrophobic residues in the protein molecule. This dependence is linear and the lyotropic effect rises with increase in the content of hydrophobic residues. It is satisfactorily consistent with the experimental data on NaCl effect on thermal denaturation temperature for ichthyocol gelatin, ribonuclease, lysozyme, 7S and 11S SBGF components. An extraordinary strong influence of NaCl on thermal denaturation temperatures for the main SBGF components can be accounted for by a relatively high content of hydrophobic residues.     

Controlling the gelation temperature of biomimetic polyisocyanides

The gelation temperature and mechanical properties of aqueous ethylene glycol-decorated polyisocyanide solutions strongly depends on the length of the glycol tail. Copolymerisation of monomers with different tail lengths allows for precise engineering of the gel properties.     

Water gelation of an amino acid-based amphiphile

The water immobilization by a simple amino acid-containing cationic surfactant was investigated. A variety of techniques, such as (1)H NMR spectroscopy, circular dichroism (CD), steady-state fluorescence spectroscopy, and field-emission scanning electron microscopy (FESEM) were applied to determine the formation and architecture of the hydrogel. The new gelator with a minimum gelation concentration (MGC) of 0.3 % w/v shows prolonged stability and a low melting temperature (39 degrees C). (1)H NMR experiments revealed that intermolecular hydrogen bonding between the amide groups and pi-pi stacking of the indole rings are the two regulating parameters for gelation. Furthermore, fluorescence studies with 8-anilino-1-naphthalenesulfonic acid (ANS) as the probe indicate the participation of hydrophobicity during gelation. The luminescence study using both ANS and pyrene, along with FESEM results, indicate a critical concentration, well below the MGC, at which fibres begin to form. These cross-link further to give thicker fibers, leading to the formation of a hydrogel (0.3 % w/v). This new hydrogelator expresses high supramolecular chirality, as evidenced by the CD spectra. In addition, the gelator molecule was found to be nontoxic up to a concentration of 4 mM (0.2 % w/v). The high supramolecular chirality, prolonged stability, low melting point, and biocompatibility of the molecule make it a focus of chemical and biological interest.     

Mechanical selfsimilarity of polymers during chemical gelation

Network polymers near their gel point exhibit selfsimilar mechanical behavior, as expressed by power law relaxations. The range of selfsimilarity is defined by two limiting length scales. The upper limit is the correlation length, defined by the linear size of the typical cluster, and a lower limit, roughly given by the size of one preformed linear chain, i. e., the mean distance between crosslinks. The correlation length increases with the approach to the gel point, and diverges at the critical extent of reaction, i. e., the gel point where the infinite cluster is formed. Above the gel point, it decreases again with further crosslinking. Dynamic mechanical measurements of the complex modulus at the gel point show a power law in the frequency dependence over the entire frequency range, monitoring selfsimilarity. Swelling effects reduce the fractal dimension of the percolation cluster form 2.5 to 2. It is shown how the power law G() ^1/2, found by experiment, is connected to the structure of the polymeric cluster.Presented at the Physikertagung 1987 in Berlin.