Microstructure and rheology of mixed ionic surfactants

Personal care products are complex blends of several surfactants which are typically anionic, zwitterionic and nonionic. Formulations are often created with little understanding of the interactions between these different types of surfactants. Here a model system comprised of sodium dodecyl sulfate (SDS), sodium lauryl ether sulfate (SLES), and cocomonoethanolamide (CMEA) represents the industrial surfactant blend almeo. The effect of the addition of a zwitterionic surfactant, tegobetaine, to this blend was investigated. An understanding of the phase behavior of the four component mixture was gained by building up the complexity of the mixture one surfactant at a time. Nested ternary and pseudo-ternary phase maps were created to determine the phase boundary for the dilute one phase isotropic region that is useful for formulation. The nonionic surfactant in almeo, CMEA, greatly affected the overall phase behavior of the mixture, decreasing the one phase isotropic region of phase space. Small angle neutron scattering (SANS) was used to probe the microstructures present in the dilute one phase region and to understand the structural evolution that occurs upon addition of tegobetaine. The effect of these structural parameters on the bulk solution properties was established using rheology. Rheology was also used to characterize the dynamics and material properties of the wormlike micelle solutions that exist at higher surfactant concentrations.;Wormlike micelle (WLM) solutions of sufficient length and concentration will entangle, which leads to viscoelasticity. Alternatively, branching can lead to the formation of a viscoelastic network. The viscoelasticity of these WLM solutions makes them relevant from a wide range of applications ranging from personal care products to oil drilling. Scaling relationships for properties of WLM exist, which are useful in product development. However, these relationships were developed for linear entangled micelles, not branched networks, and the effect of branching is unknown. Here neutron spin echo (NSE) was used to gain a better understanding of the dynamics of both entangled and branched WLM solutions and how they relate to the bulk and network properties.