STRUCTURE OF XANTHAN HETEROPOLYSACCHARIDE IN BULK AND SOLUTIONS: A NEW MODEL OF COIL STRUCTURE

The main molecular characteristics, structure in bulk and in aqueous solutions, as well as stability to thermo-oxidative degradation of the natural heteropolysaccharide xanthan, a product of the vital activity of the bacterium Xanthomonas campestris, were studied using the methods of viscometry, potentiometric titration, FTIR spectroscopy, DTGA, DSC, TEM and DLS. It has been shown that xanthan macromolecules have a high molecular weight, MvK=1170 kDa, and contain a large number of hydroxyl, carboxyl and charged carboxylate groups. An amorphous structure of a xanthan sample in the bulk state with a low glass transition temperature (Tg=27.6 °C) and a high content of non-freezing immobilized (9.6 wt.%) and adsorbed (1 wt.%) water was determined. The processes of loss of immobilized and adsorbed water during heating of the xanthan sample occurred with heat absorption, the temperatures of their maxima were Tmax=70.0–73.8 °C and 122.8 °C, respectively. The presence of 3 main stages of thermal-oxidative destruction of xanthan, starting at Td=157 °C and ending at Tend=700 °C, is shown. The destruction process at all stages developed with the release of heat, which indicates the high ability of xanthan macromolecules to oxidize. The temperatures of the maximums of individual stages of destruction were Tmax=223, 347 and 600 °C. A compact coil structure of xanthan macromolecules was established in salt-free aqueous solutions at temperatures of 20–25 °C. The small size of the macrocoils: dav=33.8 nm (TEM) or 35.5 nm (DLS), the high density of the “core” and the high ζ-potential on their surface (ζav= −73.9 mV) are shown. On this basis, a new micelle-like model of xanthan macrocoils is proposed, in which the “core” is formed by separated and hydrogen-bonded segments of the main chain, and the side branches of the polysaccharide with charged carboxyl and carboxylate groups come to the surface of the coil and form a developed double electrical layer.