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Current methods to develop surfactant phase diagrams are time-intensive and fail to capture the kinetics of phase evolution. Here, the design and performance of a quantitative swelling technique to study the dynamic phase behavior of surfactants are described. The instrument combines cross-polarized optical and short-wave infrared imaging to enable high-resolution, high-throughput, and in situ identification of phases and water compositions. Data across the entire composition spectrum for the dynamics and phase evolution of a binary aqueous non-ionic surfactant solution at two isotherms are presented. This instrument provides pathways to develop non-equilibrium phase diagrams of surfactant systems—critical to predicting the outcomes of formulation and processing. It can be applied to study time-dependent material relationships across a diverse range of materials and processes, including the dissolution of surfactant droplets and the drying of aqueous polymer films. 

Structure-property-processing relationships for model lamellar structured 70 wt.% SLEnS 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 SLEnS 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. 

Material relationships at low temperatures were determined for concentrated surfactant solutions using a combination of rheological experiments, cross-polarized microscopy, calorimetry, and small angle X-ray scattering. A lamellar structured 70 wt% solution of sodium laureth sulfate in water was used as a model system. At cold temperatures (5 °C and 10 °C), the formation of surfactant crystals resulted in extremely high viscosity. The bulk flow behavior of multi-lamellar vesicles (20 °C) and focal conic defects (90 °C) in the lamellar phase was similar. Shear-induced crystallization at temperatures higher than the equilibrium crystallization temperature range resulted in an unusual complex viscosity peak. The effects of processing-relevant parameters including temperature, cooling time, and applied shear were investigated. Knowledge of key low-temperature structure–property-processing relationships for concentrated feedstocks is essential for the sustainable design and manufacturing of surfactant-based consumer products for applications such as cold-water laundry.