I measured the viscosity of both pure alginate at 3% (w v-1) and a mixed solution of alginate at 3% (w v-1) +1% (w v-1) sodium chloride. Interestingly, instead of ending up with an enhancement in viscosity, I could see that the viscosity dropped slightly at low shear rates. This trend was then followed with a slower rate until both alginate and alginate-NaCl solutions reached to the similar final viscosity at higher shear rates about 300 S-1.
Viscosity can be highly sensitive to ion concentration due to screening effects which change the range and strength of the electrostatic interactions between the molecules in your sample. Salt concentration is actually used in products like shampoo to fine tune the viscosity and properties of the material.
With regard to the difference in your viscosity curves, I would hypothesise that at low shear rate, the viscosity is strongly controlled by electrostatic molecular interactions, however these interactions break down at higher shear rates, where the size and shape of the molecules become the defining feature. This would explain why there is a difference in viscosity at low shear rates and less so at higher shear rates.
As a matter of interest, how are you measuring your sample viscosity?
We often like to think of solvents as being able to suspend an unlimited variety of solutes without interaction, but this isn't true. One species will impact the others. Salts aren't the only things that do this. Dissolved gasses also impact each other. The solubility of Nitrogen is not independent of concentration of Oxygen and vice versa.
Dear all,
besides the fruitful aforementioned contributions, this is a typical shear-thinning behavior. In addition to that, ion specific binding and power are to be considered. At high shear it is the main chain that contributes to the viscosity values due to alignment under the effect of shear. Regards
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