Short haired wormlike micelles in mixed nonionic fluorocarbon surfactants

Abstract

We have studied the rheological behavior of viscoelastic wormlike micellar solution in a mixed system of nonionic fluorinated surfactants, perfluoroalkyl sulfonamide ethoxylate, C8F17SO2N(C3H7)(CH2CH2O)nH abbreviated as C8F17EOn (n = 10 and 20). Above critical micelle concentration, the surfactant, C8F17EO20 forms small spherical micelles in water and the viscosity of the solution remains constant regardless of the shear rate, i.e., the solutions exhibit Newtonian behavior. However, upon successive addition of the C8F17EO10 the viscosity of the solution increases and at certain C8F17EO10 concentration, shear-thinning behavior is observed indicating the formation wormlike micelles. Contrary to what is expected, there is a viscosity increase with the addition of the hydrophilic C8F17EO20 to C8F17EO10 aqueous solutions at certain temperature and concentration, which could be attributed to an increase in rigidity of the surfactant layer and to the shifting of micellar branching to higher temperatures. The oscillatory-shear rheological behavior of the viscoelastic solution can be described by Maxwell model at low frequency. Small-angle X-ray scattering (SAXS) measurements confirmed the formation of small spherical micellar aggregates in the dilute aqueous C8F17EO20 solution. The SAXS data shows the one-dimensional growth on the micellar size with increase in the C8F17EO10 concentration. Thus, the present SAXS data supports the rheological data.