Shear and longitudinal viscosity of non-ionic C12E8 aqueous solutions

TL;DR

This study investigates the shear and longitudinal viscosity, elastic modulus, and ultrasonic absorption of C12E8 aqueous solutions, revealing two concentration-dependent structural regimes with distinct viscoelastic behaviors.

Contribution

It provides detailed measurements and analysis of the viscoelastic properties of C12E8 solutions, identifying two structural regimes and their temperature-dependent behaviors.

Findings

Presence of two concentration regimes with distinct properties

Viscoelastic behavior in surfactant-rich region follows Cole-Cole relaxation

Complex behavior in water-rich region due to aggregates and water exchange

Abstract

We present measurements of the steady shear viscosity, the longitudinal elastic modulus and the ultrasonic absorption in the one-phase isotropic liquid region of the nonionic surfactant C12E8 aqueous solutions. The overall results support the presence of two separated intervals of concentration corresponding to different structural properties. In the surfactant-rich region the temperature dependence of the steady shear viscosity follows an equation characteristic of glass-like systems. The ultrasonic absorption spectra show unambiguous evidence of viscoelastic behaviour described by a Cole-Cole relaxation formula. In the water-rich region the behaviour of the measured quantities are more complex and reflect the presence of dispersed aggregates whose size increases with temperature and concentration. An additional low frequency contribution is also observed, which is ascribed to the exchange of water molecules and/or surfactant monomers between the aggregates and the bulk solvent region.