Rheological Properties of Wormlike Micellar Solutions Being Available in Wide Temperature Range in Sucrose Palmitate Systems

Abstract

We investigated the melting point of the hydrated solid and rheological property of wormlike micellar solutions in the water/sucrose monopalmitate (C₁₆SE)/tri (oxyethylene) dodecyl ether (C₁₂EO₃) system when a part of C₁₆SE was substituted with the surfactants which have lower Krafft point than C₁₆SE. The melting point of the hydrated solid of the water/C₁₆SE/C₁₂EO₃ system is around 34°C. When C₁₆SE is substituted with sodium dodecyl sulfate (SDS), the melting point of the hydrated solid decreases below 5°C at α (weight fraction of SDS in C₁₆SE+SDS mixture) being greater than 0.3 and the zero shear viscosity (η₀) increases from the original nonionic system. When C₁₆SE is substituted with octa (oxyethylene) dodecyl ether (C₁₂EO₈), the melting point of the hydrated solid decreases down to 24°C and the maximum zero shear viscosity (η₀^max) decreases with the increase in α (weight fraction of C₁₂EO₈ in C₁₆SE+C₁₂EO₈ mixture). η₀^max is dramatically increased by the substitution of C₁₆SE with SDS. With further increase of α, η₀^max decreases gradually. The dynamic rheology data of highly viscous wormlike micellar solutions fit well to the Maxwell’s mechanical model of viscoelastic material, indicating the formation of rigid network of entangled wormlike micelles. Shear (plateau) modulus G₀ is almost unchanged, whereas relaxation time τ_R shows similar behavior to η₀^max. Hence, the behavior of η₀^max can be explained from the behavior of τ_R by considering the equation relating these quantities, η₀=G₀τ_R. Since τ_R is proportional to the length of wormlike micelles, the behavior of η₀^max can be explained in terms of the change in the length of wormlike micelles caused by the addition of SDS.