Comparison of oleyl and elaidyl isomer surfactant–counterion systems in drag reduction, rheological properties and nanostructure

Schematic view of cis- and trans-forms of surfactant hydrocarbon chains. [Display omitted] ► Presented the first drag reduction, rheological and nanostructure comparisons of unsaturated (cis/trans) cationic surfactant/counterion systems. ► 1H NMR results indicate that the hydrocarbon chain of the cis surfactant is more favorable for micelle growth than the trans surfactant. ► The oleyl surfactant drag reducing system had greater resistance to mechanical shear than the elaidyl surfactant. ► Confirmed that viscoelasticity is not required for a surfactant system to be drag reducing. ► High extensional viscosity is a major factor in the drag reduction mechanism. Compared with quaternary ammonium cationic surfactants with saturated alkyl chains, quaternary ammonium cationic surfactants with one double-bond in their alkyl chains, when mixed with appropriate counterions (in certain molar concentration ratios, ξ), can reach much lower effective drag-reduction temperatures, while maintaining the upper drag-reduction temperature limit of the corresponding saturated drag reducing surfactant solutions. No previous study has compared the effects of cis- vs. trans-unsaturated alkyl hydrocarbon tail configurations (oleyl vs. elaidyl) trimethyl ammonium chloride cationic surfactants at different counterion/surfactant concentration ratios on micellar nanostructures, 1H NMR spectra and on rheological and drag-reduction behavior of their solutions. Since neither pure oleyl (cis-) nor elaidyl (trans-) trimethyl ammonium chloride surfactants are commercially available, they were synthesized and their 5 mM solutions with NaSal counterion at concentrations of 5 mM, 7.5 mM and 12.5 mM were studied.