Thermal hysteresis and reversibility of gel–sol transition in agarose–water systemsa)

Thermal hysteresis and stability of agarose-water gelling systems were studied by techniques providing information at different microscopic levels. Results are discussed in terms of the two distinct orders (molecular and supramolecular) predicted in the current model of gels of this type, i.e.,branched double helices of polysaccharide chains and bundles of associated double helices. The two types of order appear to be lost through two distinguishable transitions. Supramolecular order disappear gradually over a temperature span of about 50° when the temperature of the gel is increased. Over the same temperature span, it appears reversibly restorable without hysteresis although with long kinetics. In agreement with other data available in the literature, molecular order instead appears and disappears through much sharper, largely hysteretical transitions occuring with considerably quicker kinetics. Mimicking of thermal hysteresis in supramolecular ordering appears to be only a consequence of the fact that this ordering requires the existence of molecularly ordered ’’building blocks’’ to occur. The wider interest of systems of this type lies in their being recognized model systems of a variety of biologically organized structures.