Oil-induced anomalous thermoresponsive viscoelasticity in fluorinated surfactant systems.

We have studied the rheology and structure of a mixed nonionic fluorinated surfactant, perfluoroalkyl sulfonamide ethoxylate, C(8)F(17)SO(2)N(C(3)H(7))(CH(2)CH(2)O)(n)H abbreviated as C(8)F(17)EO(10), and perfluorodecalin (C(10)F(18)) or perfluoropolyether oil, (C(3)F(6)O)(n)COOH, in an aqueous system using rheometry and small-angle X-ray scattering (SAXS) techniques. In the absence of oil, the viscosity of surfactant solutions (10 and 15 wt %) first decreases slightly and then more strongly with temperature. Addition of a small amount of fluorinated oil to the wormlike micellar solution disrupts the network structure and decreases the viscosity sharply at lower temperature indicating a rod-sphere transition. The trend of the viscosity curve changes gradually and an anomalous viscosity maximum as a function of temperature appears. It is found that perfluoropolyether oil decreases the viscosity more effectively than perfluorodecalin. The generalized indirect Fourier transformation (GIFT) analysis of the SAXS data confirmed the formation of long rod-like particles in an oil-free, surfactant/water system at 20 degrees C. Addition of a trace amount of fluorinated oils induces modulation in the structure of the micelles and eventually short rods or spherical particles are formed. The decreasing trend in the viscosity with oil concentration is thus attributed to the microstructure changes induced by the added oils.