Polysaccharides concentration regimes

Figure 4-6 Illustration of Dilute and Concentrated Regimes in Terms of Log c[tj (coil overlap parameter) against Log t)sp = [( o >ls)l>ls] (Vsp = specific viscosity) slope of 3.3 for entangled polysaccharide chains dissolved in good solvents and 4.1 for polymers with specific intermolecular associations.

Fig. 5 Correlation of material characteristics with polysaccharide concentration. Concentration regimes [cq very dilute, close-to-ideal solutions A dilute B beyond overlapping concentration C dense systems (condensed/solid phase) volume ratio 4> = ] and the consequences on observed material qualities dependence on isolated molecule properties at close-to-ideal conditions (cq) and increasing dominance of effects of supermolecular structures with increasing concentration. (Graphics Macromedia Fireworks.) (View this art in color at www.dekker.com.)...

For combinations of surfactants and HPMC, Figure 38 shows that surfactant micelles create more intermolecular chain entanglement as the concentration of SDS increases than was seen in Figure 37 for SDS and HPC. In Figure 38, entanglement requires the polymer concentration, Cp, to exceed a critical overlap concentration, c (i.e., the polymer must be in the concentrated regime), or the polysaccharide tends to collapse and the viscosity drops. It is currently felt that at sufficiently high surfactant concentrations, both HPC and HPMC become fully extended and the lipophilic domains become enshrouded in surfactant micelles. 

The correlation of material characteristics with de facto concentration of polysaccharides is best discussed for three regimes and limiting concentrations (Fig. 5). 

In the regime exceeding c (c - (j) = 1), concentration (c) is typically replaced by volume fraction (cj)), and the polysaccharide fraction becomes equivalent... 

Polymeric thickening agents used in foods typically are well soluble polysaccharides, with an excluded volume parameter [j clearly above zero. This implies that especially the dilute and semidilute regimes are often of importance. Viscosity of very dilute solutions has been discussed in Sections 6.2.2 and 6.3.2. For higher concentrations, the reduced viscosity (j sp/c) is higher, as is true for any system (see Figure 5.5), but for polymer solutions the viscosity increases far stronger with concentration as soon as the chain overlap concentration is reached. 

It is not uncommon for bacteria to produce slime (polysaccharide) as a response to excess O2 (Wilkinson, 1958). Hill (1971) was first to show that the slime production associated with massive colony formation of Derxia gummosa as induced by a low oxygen regime. At 0.05 atm O2 abundant massive colonies were observed on agar but only a few were present at O2 = 0.2 atm. The massive colonies fixed N2 whereas the small colonies were inactive probably because excess O2 inhibited N2 fixation. Slime production can be shown to protect N2-fixing M. flavum against O2. When the polysaccharide was removed from the medium the O2 concentration for maximal acetylene reduction was 0.02 atm compared with 0.025 atm in the presence of slime. A gum-free strain at the same cell concentration had an optimal O2 concentration of 0.015 atm. Presumably extracellular polysaccharide diminished the 02-solution rate and capsular polysaccharide may also have impeded O2 uptake into the cell (Yates, 1977). 

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