Thermoresponsive viscoelasticity of concentrated solutions with a fluorinated hybrid surfactant

Temperature-dependent viscosity of an aqueous solution of sodium 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexanesulfonate (FC6-HC4), a fluoro-hybrid type surfactant, which has a fluorocarbon chain and a hydrocarbon chain in a molecule, was examined. The FC6-HC4 solution showed viscoelastic behavior at concentrations around 10 wt.%. Further study on the temperature dependence of the viscosity of a 10 wt.% solution revealed that the viscosity increased about 102 times, showing a maximum as temperature rose from 6 to 36°C, and then decreased about 10−4 times with further rising at temperatures up to 64°C. Moreover, tan δ, a parameter for visoelasticity, exhibited a minimum at 36°C. The phenomenon found in this work was reversible with respect to temperature change. The study revealed the importance of the chemical structure in which the numbers of carbon atoms are 6 and 4 in the fluorocarbon and hydrocarbon chains, respectively, for the hybrid surfactant to show the thermoresponsive viscoelastic behavior. Temperature-dependent visicosity increase was found to be observed during the process where the size of assemblies of surfactant molecules decreased with rising temperature and when the probability of collision is highest between small and large molecular assemblies.