The effect of NaCl addition on the rheological behavior of cetyltrimethylammonium p-toluenesulfonate (CTAT) aqueous solutions and their mixtures with hydrophobically modified polyacrylamide aqueous solutions

In this paper, the influence of NaCl addition, up to very large concentrations, on the rheological properties of cetyltrimethylammonium p-toluenesulfonate (CTAT) solutions and their mixtures with two hydrophobically modified polyacrylamides (HMPAM) has been studied under simple shear. The CTAT concentrations employed were above the critical rod concentration. As salt is added to CTAT aqueous solutions, the zero-shear viscosity first increases, goes through a maximum, and at very high ionic strengths increases once more. The overlap concentration of worm-like micelles decreases as the concentration of NaCl increases. The results are explained by the salt addition-induced growth of worm-like micelles and salting out effects at the highest contents of NaCl. The influence of ionic environment on the rheological properties of CTAT with two HMPAM solutions with different contents of hydrophobic moieties was also studied under simple shear. When NaCl is added to HMPAM/CTAT solutions, the same trends observed in CTAT/NaCl solutions were repeated but the viscosity increases were largely magnified. The large viscosity enhancements with salt increments in HMPAM/CTAT solutions were explained by the formation of an interpenetrated network of hydrophobically modified polymer chains and worm-like micelles with hydrophobic sequences embedded within its structure.