Conformation Transition Of Xanthan In Aqueous Solution And Its Rheological Research

Xanthan in aqueous solution exhibits good pseudoplastic behaviour, its viscosity decreases quickly with the increasing of shear rate. While increasing the concentration of xanthan, the viscosity increase, and all the samples can form reversible weak gel when the concentration is high enough. This structure will be easily broken by imposed shearing, and can recover when be settled.Native xanthan usually contains double helix structure. When be heated in aqueous solution, it will take a conformation transition which is called to helix to coil. In the subsequent cooling, incomplete recover of double helix chains will be found. This process was shown by oscillatory curves in my article. Generally, in heating process, the helix of xanthan disentangles into two flexible coils, the modulus will decrease. The time and degree of heating can affect the degree of dissociation. On the other hand, in cooling process, random coils will entangle with each other to form helix again, the modulus will recover in a certain extent. But the viscosity will decrease by contrast to native one because the recovered helixes are usually not complete. The temperature where the conformation transitions happen most drastically is called transition temperature Tm. The xanthan samples investigated in my article not only exhibit different viscosities and modulus, but also act differently in conformation transition in annealing process. Among them, xanthan 200, clear80, 80 and TSC are very special and exhibit some abnormal flowing curves: clear80 and 80 have no significant Tm. The modulus of 200 has abnormal increase in cooling process. The pseudoplastic property of tsc is enhanced after heating. These behaviors probably have great relationship with the substituents and ions they have in their chains.In the present of added salts (sodium hexametaphosphate (Na-HMP), KCl and CaCl2), by comparison of the rheological properties and conformation transition of sample TSC (with lowest pyruvate and calcium levels) and 80 (with higher pyruvate and calcium levels), the effect of electrolytes on the structure of xanthan and its conformation transition were investigated. It is conclude that ions like Na+,K+,Ca2+ especially Ca2+ play a important role in the double helix structure of xanthan, as well as its conformation transition. Though low concentration of ions have no significant influence on xanthan, they can decrease the repulsion force in side chains of xanthan to make chains flexible and viscosity decreasing when their concentrations are enough. On the other hand, these ions can build salt bridges among xanthan random chains, thus help the recover of double helix and increase the ordered regions and modulus. The effects of these electrolytes are accordance with the lyotropic series.