Dynamic Properties of Entangled Wormlike Micelles: Sodium Laurylethersulfate at High Ionic Strength-(II)

We report on the viscous behaviour of long and flexible wormlike micelles at volume fractions between Ø = 0.05–0.14 comprised of sodium laurylethersulfate (SLES) in 0.15–0.5 M NaCl equivalent of SLES concentrations between 20–50 mM at temperatures ranging from 25 °C to 45 °C by means of static and dynamic light scattering and rheology, which is an extension of previous experiments on dilute/semi-dilute solutions of SLES micelles at much lower surfactant concentrations in the presence of low salt concentrations (Clancy and Paradies, Z. Phys. Chem. 215, 4, 483–513 (2001)). The light scattering results lead to a description of the solution of wormlike SLES micelles as analogous to semi-dilute solutions of living polymers in good solvents. The dynamic properties of SLES micelles as shown by the time autocorrelation function of the scattered field was found to be bimodal, particularly between 0.15–0.5 M NaCl (25 °C). The origin and the influence of surfactant concentration and salt concentration of the bimodal behaviour are investigated in more detail and compared to theoretical models describing coupling of concentration fluctuations to viscoelasticity. Specifically the behaviour of the slow viscoelastic time as a function of temperature, salt, and SLES concentration has been extensively evaluated experimentally and compared with theoretical predictions, revealing that the terminal stress relaxation time obtained from inelastic light scattering measurements is in accord with the rheological experiments. Above the crossover concentration of SLES of Ø*∼ 0.06 the viscoelastic properties are described by an almost ideal Maxwell relaxation. The overall rheological data are in agreement with the scaling law with respect to surfactant concentration with exponents close to those predicted from a stress relaxation model applying living polymers. Furthermore, the complex elastic modulus at higher angle frequency obtained can be explained in terms of crossover between the regimes of reversible scission, and of breathing of the polymer-like SLES chains. The average increase of the chain length (¯L) with increasing SLES concentration in the presence of 0.15–0.5 M NaCl at 25 °C scales with ∼Ø^0.31–0.34 according to static light scattering measurements. The hydrodynamic correlation length, ξ_H, obtained from inelastic light scattering experiments is best approximated by ξ_H∼ C^−0.78, whereas the correlation length ξ_c scales with C^−0.48, which is a significant smaller exponent than predicted of −0.75 theoretically. – The rheological and dynamical behaviour of SLES micelles studied in this concentration range, e.g. the transition between dilute to semi-dilute regimes, including gelation shows the very different physical properties of SLES than that of its non-ethoxylated analogue and well studied sodium dodecylsulfate (SDS).