Κ-carrageenan In Aqueous Solution Research Articles

Associative behaviour of κ-carrageenan in aqueous solutions and its modification by different monovalent salts as reflected by viscometric parameters

The viscometric behaviour of κ-carrageenan in aqueous solutions and in the presence of monovalent salts was investigated at 25 °C. Coil, helix or double helix conformations were induced by cooling hot κ-carrageenan solutions under appropriate ionic conditions. A new viscometric approach was used for modeling the behaviour of κ-carrageenan solutions. The intrinsic viscosity, [η], is markedly changed by the presence of different monovalent salts (NaCl, NaI and CsI). In pure water, the intrinsic viscosity amounts to 48 dL·g−1. In 0.1 M NaCl solutions (single helix state) [η] is 6.2 dL·g−1, whereas in 0.1 M NaI (double helix conformation) it is approximately twice as large. In 0.1 M CsI (dissimilar cation and counter-ion) the intrinsic viscosity is three times larger, suggesting the formation of the associated κ-carrageenan helices. Stepwise association of κ-carrageenan helices was followed in presence of NaI/CsI mixtures of different compositions. The value of Smidsrød-Haug stiffness parameter (B) measured for κ-carrageenan in NaCl solutions is 4.47 × 10−2, higher than that of DNA (5.5 × 10−3), but lower than those reported for carboxymethyl cellulose (6.3 × 10−2), indicating that the chain conformation is moderately rigid.

Thermoreversible gelation and scaling behavior of Ca2+-induced κ-carrageenan hydrogels

The effect of calcium cations (Ca2+) on thermoreversible gelation properties, microstructure, and scaling behavior of κ-carrageenan in aqueous solution was studied by micro-differential scanning calorimetry (micro-DSC), field emission scanning electron microscopy (FESEM) and rheology. It was found that κ-carrageenan molecules self-associated into fibrils, and the diameter and number of fibrils increased with increasing Ca2+ ion concentration. The formation and melting of κ-carrageenan hydrogel was thermally reversible and extremely sensitive to Ca2+ ions. The viscoelastic property at the gel point was examined using the Winter-Chambon criterion to obtain the critical relaxation exponent n and the critical gel strength Sg. n and Sg were found to decrease and increase with increasing Ca2+ ion concentration respectively. The normalized gel strength, Sg/Ca2+, was not independent of Ca2+ ion concentration, suggesting a strong effect of Ca2+ ions on both size and density of junctions formed during gelation. A schematic diagram has been proposed to explain how Ca2+ ions could facilitate the coil-helix transition, leading to the formation of more aggregates acting as junctions.

Thermoreversible gelation and scaling laws for graphene oxide-filled κ-carrageenan hydrogels

The effects of graphene oxide (GO) and ammonia functionalized graphene oxide (AGO) on microstructure, thermoreversible and critical gelation properties of κ-carrageenan in aqueous solution were studied by micro-differential scanning calorimetry (micro-DSC), field emission scanning electron microscopy (FESEM) and rheology. The presence of GO or AGO was able to decrease the gel formation and melting temperatures and reduce gel strength. The effect of AGO was more evident than that of GO. It was observed by FESEM that κ-carrageenan gel was induced by the formation of a fibrillar network, and fibrillar size and density decreased with increasing GO or AGO content. The viscoelastic properties at the gel point were examined using the Winter-Chambon criterion to obtain the critical relaxation exponent n and the critical gel strength Sg. n increased but Sg decreased with increasing GO or AGO content. The more obvious increase in n and decrease in Sg were observed in the AGO/κ-carrageenan solution, which further demonstrated the stronger weakening effect of AGO. A schematic diagram has been proposed to explain the effect of AGO on the gelation of κ-carrageenan, where AGO sheets attracted a number of κ-carrageenan chains through hydrogen bonding and electrostatic interaction between sulfate groups of κ-carrageenan and amine groups of AGO. As a result, the number of free κ-carrageenan chains in solution decreased and the coil-helix transition and aggregation of helices of κ-carrageenan were affected negatively.

Thermoreversible gelation and viscoelasticity of κ-carrageenan hydrogels

The gelation behavior of κ-carrageenan in aqueous solution was studied by microdifferential scanning calorimetry and rheology. It was found that the formation and melting of κ-carrageenan hydrogels were thermally reversible and extremely sensitive to κ-carrageenan concentration. In comparison with the crossover of G′ and G″, the extrapolation method based on multiwave oscillation and Winter–Chambon criterion were able to give more accurate critical gel temperature Tc. At the gel point, the critical relaxation exponent n was almost constant whereas the critical gel strength Sg increased with κ-carrageenan concentration. In the stable gel state, the plateau modulus Ge depended on temperature according to a power-law scaling, Ge ∝ ε2.2, where ε is the relative distance (=|T − Tc|/Tc) and independent of κ-carrageenan concentration. The presence of potassium ions shifted the formation and melting temperatures of κ-carrageenan hydrogel to higher temperatures, and the temperatures for gel formation and melting increased with increasing potassium ions' content.

Rheological Properties and Scaling Laws of κ-Carrageenan in Aqueous Solution

Rheological properties, gel network structures, and scaling laws for κ-carrageenan in aqueous solution were studied by rheology and field emission scanning electron microscopy (FESEM). The FESEM micrographs verified that the κ-carrageenan gels were formed by the formation of fibrils. The critical gel concentration, cg, obtained using the Winter–Chambon criterion, was found to be proportional to temperature as expressed by cg ∼ T0.85 where T is the temperature in °C. At the gel point, the critical relaxation exponent n obtained was a constant (n = 0.62) and independent of temperature. The critical gel strength Sg increased with increasing cg. A constant gel strength Sg/cg was obtained by normalizing Sg with cg to eliminate the effect of temperature, showing a unique character of κ-carrageenan in aqueous solution during gelation. The molecular structure of the junctions at the gel point was analyzed using the modified Eldridge–Ferry model, which supported the similarity of the fractal structure in the κ-car...

Dilute iota- and kappa-Carrageenan solutions with high viscosities in high salinity brines

We evaluated the rheological characteristics of ι- and κ-Carrageenan in aqueous solutions. Viscosities strongly increased with increasing polymer concentration or salinity. Monovalent Na + cations were more effective in increasing viscosity than divalent Ca 2+ cations. A more complex brine containing Na +, Mg 2+ and Ca 2+ cations also showed high viscosities at a high salinity. We observed shear thinning behavior for these polysaccharide solutions. We explain these rheological phenomena with molecular processes, specifically the conformation change of the ι-Carrageenan from random coil to double helix. In the context of enhanced oil recovery, ι-Carrageenan solutions can reach much higher viscosities at high salinities than standard polyacrylamide solutions. Moreover, Carrageenans are renewable, nontoxic, green substances.

Small-angle neutron scattering study on irradiated kappa carrageenan

The structure of gamma-ray-irradiated κ-carrageenan in aqueous solutions was investigated in terms of small-angle neutron scattering. The scattered intensity, I( q), of non-irradiated κ-carrageenan solutions (5 wt%) was well described with an Ornstein–Zernike (OZ)-type function with the correlation length of 85 Å, indicating that the κ-carrageenan solution behaves just as a polymer solution in the semi-dilute regime. By increasing the irradiation dose (100 kGy), I( q) changed to a power-law function with the scattering exponent of −1.84. Further increase in dose results in a recovery of OZ-type function. This indicates that a progressive cleavage of κ-carrageenan chains takes place randomly, leading to a self-similar structure at 100 kGy. This is followed by further segmentation of κ-carrageenan chains.

Effect of solvent quality and ions on the rheology and gelation of κ-carrageenan

The rheology and conformational helix transition of κ-carrageenan in aqueous solutions of glycerol and sorbitol at low water activity are studied. Carrageenan forms a hydrated gel in water but a weakly flocculated network of partially hydrated particles in glycerol. The melting transitions of carrageenan in glycerol/water solutions show a single peak in the complex moduli at low glycerol concentrations, and two peaks when the glycerol concentration is higher than 40% (w/w). These are thought to be due to the breakage of hydrogen bonds (low temperature peak) and calcium ion dissociation (high temperature peak). The helix–coil transition is shifted to higher temperatures in solutions with higher glycerol or sorbitol content. The temperature of the conformational transition corresponds with that of gelation for all samples. However, there is more hysteresis between the cooling and heating curves (for both OR and rheology) at high water content. This suggests that gelation occurs through helix aggregation in water but the lower solubility of carrageenan results in a more random network of helices in low water activity solvents.

On the Molar Mass of κ-Carrageenan in the Course of Conformational Transition from the Disordered to the Fundamental Ordered Form

Weight-average (Mw) molar mass data were obtained by light scattering on aqueous solutions of a sample of κ-carrageenan with a relative molar mass of about 157 000. They were compared with literature data of several samples, having Mw values from 35 000 to 720 000, using a variety of methods. Temperature and concentration of supporting 1:1 electrolyte were chosen so as to span across the disorder-to-order conformational transition, monitored by the change of optical activity. The present comparative study indicates that the single helix is the fundamental ordered (secondary) conformation of κ-carrageenan in aqueous solution. This conclusion appears to be universally valid on the basis of a very large number of cases, including polymer samples of different average molar mass, in the presence of different counterions and co-ions, and at different temperatures. The single-helical structure is the conformational “building-block” of a higher (“tertiary”) ordered structure that stems from the association of (...

Electric field orientation of sonicated κ-carrageenan in aqueous solution: ionic polarization of loosely bound counterions

The electric field orientation of κ-carrageenan in aqueous solution was investigated by the reversing-pulse electric birefringence method at 633 nm and 33°C. The observed field orientation was attributed to the fast-induced polarization of the counterions (K + ions) in the ionized polymer domain. Since the charge density parameter ξ of non-helical and elongated κ-carrageenan is 0.66, the fast-induced ionic polarization is probably due to the counterions loosely bound by Debye-Hückel type electrostatic forces, but not due to the condensed counterions.