Viscoelastic Wormlike Micelles in Mixed Nonionic Fluorocarbon Surfactants and Structural Transition Induced by Oils

摘要

Formation and rheological behavior of viscoelastic wormlike micelles in an aqueous solution of a mixed system of nonionic fluorocarbon surfactants, perfluoroalkyl sulfonamide ethoxylate, C8F_(17)SO2N(C3H7)-(CH2CH2O)_nH (abbreviated as C8F_(17)EO_n, n = 20 and 3), was studied. A partial ternary phase diagram of water/C8F_(17)EO_(20)/C8F_(17)EO3 was constructed at 25 °C by visual inspection through crossed polarizers. In the water/surfactant binary system, C8F_(17)EO_(20) forms an isotropic micellar solution over a wide concentration range (～80 wt %). The micellar solution of the C8F_(17)EO_(20) can solubilize a significant amount of C8F_(17)EO3, and the solubility increases with increasing C8F_(17)EO_(20) concentration. With successive addition of C8F_(17)EO3 to the aqueous C8F_(17)EO_(20) solution, viscosity increases rapidly, and a viscoelastic solution is formed. The viscosity of the viscous sample was ～5th order of magnitude of pure water. The viscoelastic solution follows the Maxwell model typical of wormlike micelles at low-frequency region. With further addition of C8F_(17)EO3 the viscosity decreases, and phase separation occurs. Addition of perfluoropolyether oil, (C3F6O)_nCOOH, to the viscoelastic solution decreases the viscosity monotonically until phase separation. On the other hand, when perfluorodecalin oil, C_(10)F_(18), is added, viscosity first decreases and attains a limiting value before excess oil phase separates out. The viscosity decrease in water/surfactant/oil systems is possibly caused by the microstructural transition in the network structure. Small-angle X-ray scattering (SAXS) measurements were performed to complement the rheological data. It has been found that the C8F_(17)EO3 induces one-dimensional growth to the C8F_(17)EO_(20) micelles. On the other hand, when (C3F6O)_nCOOH is added, wormlike—sphere type transition is more likely to occur.