Effects of additives on the rheology and phase behavior of lamellar-structured concentrated surfactant solutions

Structure-property-processing relationships for model lamellar structured 70 wt% SLE_nS solutions were developed with a combination of rheometry, cross-polarized optical microscopy, calorimetry, small angle X-ray scattering, and rheo-ultrasonic speckle velocimetry. Additives were utilized to maintain high surfactant activity, reduce bulk viscosity and simplify processing. While the bulk flow behavior of neat SLE_nS solutions was similar, the effect of some additives was sensitive to the degree of ethoxylation. Linear-chain alcohols (C₂-C₅) partitioned into inter-bilayer water layers, dehydrating surfactant headgroups and inducing lamellar-to-micellar transitions. Short-chain polyols formed higher-viscosity hexagonal and mixed phases at room temperature through hydrogen bonding with surfactant headgroups. Heating beyond the upper temperature limit weakened these interactions, resulting in low-viscosity solutions. Within the lamellar phase, common salt promoted shear-induced crystallization above the equilibrium temperature range. Propylene glycol suppressed shear-induced crystallization and promoted wall-slip under shear, forming lubrication layers near the wall. These strategies offer practical levers to tune rheology and microstructure of concentrated surfactant systems, with the datasets developed providing a foundation for future modeling. Outcomes from this study inform the sustainable design and efficient processing of concentrated surfactant-based products.