On the shear thickening behavior of micellar aqueous solutions of cetyltrimethylammonium fluorobenzoates: Effect of the fluor position

The shear thickening behavior of micellar solutions made with homologous cetyltrimethylammonium (CTA) fluorobenzoate (FB) surfactants, in which the fluor is located in the 2-(CTA2FB), 3-(CTA3FB) or 4-position (CTA4FB) of the counterion aromatic ring, is studied here. In the temperature and range studied, the aqueous solutions made with CTA2FB do not exhibit shear thickening because 2FB is more kosmotropic than the 3FB and 4FB counterions, and larger shear rates than the ones covered here are needed to observed shear thickening in the CTA2FB system. CTA3FB and CTA4FB aqueous solutions undergo the shear thickening transition due to the formation of shear induced structures (SIS) in the shear range studied. The shear thickening transition in CTA3FB micellar solutions is stronger and appears at lower shear rates than that detected in CTA4FB ones; in addition, the intensity of shear thickening in both systems augments as the parallel-plates gap is increased. These discrepancies detected between the three surfactants studied are related to the differences among the counterion hydrophobicity i.e., 3FB− > 4FB− > 2FB−. Moreover, the penetration of the counterions into the core and charge neutralization of the surface of the surfactant aggregates induces micellar growth according to the Hofmeister-like anion series.