Influence of ionic strength on complex formation between poly(ethylene oxide) and cationic surfactant and turbulent wall shear stress in aqueous solution

We investigate the influence of ionic strength on the interaction between poly(ethylene oxide) (PEO) and cationic surfactant, hexadecyltrimethylammonium chloride (HTAC), and the consequent effect on turbulent drag reduction in aqueous PEO/HTAC solutions. Conductivity and surface tension data for PEO-HTAC in aqueous solution indicate that salt stabilizes binding of HTAC micelles to the polymer. Dynamic light scattering analysis indicates an increase in hydrodynamic radius for HTAC micelles in aqueous salt solution. In contrast, salt reduces the hydrodynamic radius of PEO-HTAC complexes. The latter observation is consistent with contraction of the PEO-HTAC complex via electrostatic screening. For the measurement of turbulent drag reduction in a Couette cell, our data indicate that the minimum wall shear stress in aqueous HTAC solutions occurs at an optimum HTAC concentration, close to CMC, and this optimum concentration value decreases with increasing ionic strength. This result suggests a lowering of the CMC in turbulent flow. For aqueous PEO-HTAC mixtures, the minimum wall shear stress occurs at an optimum PEO concentration smaller than that of pure PEO solutions, and this optimum concentration value increases with ionic strength. Our findings provide evidences that the turbulent wall shear stress does not always scale inversely with the hydrodynamic volume of the polymer–surfactant complex.