Cation effects on sol–gel and gel–sol phase transitions of κ-carrageenan–water system

Abstract

Sol–gel and gel–sol phase transitions of κ-carrageenan in pure water and in KCl solution were studied using photon transmission technique. Photon transmission intensity, I_tr, was monitored against temperature to determine the sol–gel and gel–sol temperatures (T_sg and T_gs) and activation energies (ΔH_sg and ΔH_gs). It was observed that T_gs was notably higher than T_sg due to the hysteresis on the phase transition loops. T_gs and ΔH_gs values were also higher for gels containing KCl than for those without KCl. The increase in carrageenan content caused an increase in both critical temperatures and activation energies for the gels prepared in pure water and in KCl solution. Increases in the KCl/carrageenan ratio, raised both T_gs and T_sg. Similarly ΔH_sg was elevated by the increase in cation content of the gel. These results were interpreted as the formation of stronger gels in the presence of KCl in water.

Introduction

It is well known that polymer gels are created from polymer networks and solvents i.e. the polymer network envelopes the liquid and prevents it from flowing out. In other words, the gel polymer network acts as a container that keeps a large amount of liquid. Gels are classified by the strength of the cross-linkages. Some gels are cross-linked chemically by covalent bonds, whereas others are cross-linked physically by hydrogen or ionic bonds and by the physical entanglement of polymer chain [1], [2]. In general, gels formed by chemical bonding are irreversible gels since they cannot be dissolved again. However moderate heating can reversibly dissolve a physically cross-linked gel. The process of gelation upon cooling is a sol–gel phase transition and the reversible gelation process is called a gel–sol phase transition [3]. Gel's phase transitions can be effected by solvent composition, pH changes, ion composition changes in the gel and electric field. Many of the natural polymer gels fall into the class of physical gels, among which red algae attracted attention for various applications.

Red algae produce a wide range of galactose based polysaccharides, one of which (carrageenan) has achieved great interest because of its applications in food and other industries. κ, ι and λ carrageenans are three major types which differ in number and position of sulfate ester substituents they contain. Among them, κ carrageenan has proved to be one of the best media for entrapping cells. This is a copolymer of β-Δ-galactose sulphate and 3,b anhydro-α-Δ-galactose (Fig. 1a) which is readily available as a food additive [4], [5]. In general, the carrageenan family is known to form thermally reversible gels as a function of temperature and gel inducing agents. These are usually cations such as K⁺ [5]. The sol–gel phase transition of κ-carrageenan solutions occurs as a result of coil-to-helix conformational transition upon cooling. Further decrease in temperature results in aggregation between ordered helices. This cooperative process has been generally explained as a ‘domain model’ based on the presence of double helices in the functional zones of carrageenan gel network. The addition of cation solutions at an adequate concentration appears to improve the gel strength or the elastic modulus of κ-carrageenan gels via enhancing conformational ordering and subsequent aggregation [6], [7]. Detailed information on molecular structures of carrageenan determined by X-ray fiber diffraction analysis may be found in a review by Millane et al. [8]. As an important characteristic of these systems the decrease in the degree of sulphation from ι-carrageenan to κ-carrageenan, makes the gels less elastic, more rigid and brittle and they require lower concentration of polysaccharides for their formation. In addition, a decrease in anhydrogalactose content increases both the potassium ion sensitivity and the gelling capacity.

The microviscosity of the hydrophobic microdomains, the dynamic rheology and molecular conformation in the melting process of gel-like systems containing κ-carrageenan were studied by means of fluorescence intramolecular excimer forming probe, by dynamic viscometry and by optical rotation, respectively [9]. Helical conformation was considered necessary for aggregation and gelation to occur and the formation of a helical structure was promoted by adding an appropriate electrolyte to a sufficient ionic strength and/or by lowering the temperature. Several groups studied the phase transition temperatures, rheological properties and gel-network characteristics during sol–gel and gel–sol transitions of κ-carrageenan-salt solution [10], [11], [12], [13], [14]. The effectiveness of increasing T_sg and T_gs at various salt concentrations were examined by following the sequence of K⁺>Ca²⁺>Na⁺ in KCl, CaCl₂ and NaCl solutions respectively.

In this work, sol–gel and gel–sol transitions of carrageenan in pure water and in KCl solution were studied using photon transmission technique. This technique was previously used to monitor the formation of acrylamide gels in various crosslinker and water concentrations [15], [16]. In both cases, I_tr decreased dramatically at a certain reaction time. This is attributed to the formation of microgels in the system. Recently, energy release during gelation of acrylamide was observed using the same technique [17]. Here, it was observed that during the sol–gel transition of carrageenan in both pure water and KCl solution, I_tr presented a sigmoidal decrease. However, on reheating during the gel–sol transition, I_tr intensity increased by following another sigmoidal path back to its initial position, forming a hysteresis loop. T_sg and T_gs values were determined for each hysteresis loop studying various carrageenan concentrations in both systems (with and without K⁺). It was observed that both T_gs and T_sg values were elevated by raising the carrageenan concentration. T_gs and T_sg values also increased by raising the KCl content for a given carrageenan concentration. ΔH_sg and ΔH_gs were measured and it was observed that they were increased by elevating carrageenan and KCl contents for given KCl and carrageenan concentrations respectively.