Polysaccharides hysteresis

Food-grade CMC is a cellulose carboxylic acid ether with an optimum DS = 0.4-0.7. The higher the DS within this range, the more hydrophilic is the polyanion. Uniformity of substitution makes CMC more compatible with dissolved salts and less inclined to thixotropy than uneven distribution (Feddersen and Thorp, 1993). This gum does not precipitate from a 50% ethanol solution. Below approximately pH 4 in water, the polyanions revert to the un-ionized, water-insoluble acid. CMC viscosity-hysteresis has already been described (Fig. 2 in Chapter 3). CMC dispersions and films have the extra advantage of transparency relative to many other polysaccharide dispersions. The films are resistant to oils, grease, and organic solvents (Hercules, Inc., 1980). 

Adsorption of polymers, including proteins, polysaccharides, and so on, is usually irreversible with respect to changes in the concentrations of these components in solution. It implies that, within the time of the experiment, the ascending branch of the adsorption isotherm F/c ) does not coincide with the descending branch, or, in other words, that the adsorption isotherm contains a hysteresis loop. This situation is schematically depicted later in Figure 15.13. 

Bosker et al. studied the interfacial behavior of polystyrene-block-maltodextrins at the air-water interface via Langmuir-Blodgett technique [124]. Owing to the slow adsorption/desorption of the polysaccharide chains and block copolymer aggregation, the systems show interfacial pressure hysteresis between consecutive compression and expansion cycles of a monolayer. 

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