Reducing excess water in surfactant solutions can combat the increasing scarcity of water. The goal of this study was to characterize how the microstructure and rheological properties of concentrated surfactant solutions change with decreased water content and addition of common additives. The microstructure of concentrated 70 wt.% sodium laureth sulfate (SLES) surfactant in water was characterized by small angle x-ray scattering (SAXS) and birefringence through a crossed-polarized optical shear cell. The addition of a model desiccant (sodium chloride, NaCl), plasticizer (propylene glycol, PG), and crystallizer (cetyl stearyl alcohols, CS) changed the microstructure of the fluid. One dimensional SAXS profiles indicated multiple phase transitions between flat lamellae sheets, micelles, and hexagonal phases with the addition of different additive concentrations. Pure 70 wt.% SLES was composed of lamellar sheets while a transition from lamellae to hexagonal phase occurred at NaCl concentrations of 1-2 wt.% and PG concentrations of 10-20 wt.%. Phase characterization was supported by birefringent shear results of the 70 wt.% SLES showing shear thinning and weak birefringence - indicative of lamellar sheet phases. Samples containing high concentrations of NaCl, or PG did not flow under gravity and were strongly birefringent - indicative of a hexagonal phase. The correlation between changes in bulk phase structure, measured viscosity, and birefringence demonstrates that when combined SAXS and cross-polarized optical shear are powerful methods to explore the structure-processing relationships of concentrated surfactant fluids.

Microstructure evolution of concentrated surfactant solutions with varying desiccant, plasticizer, and crystallizer additives

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