MULTISCALE RHEOLOGY, PHASE BEHAVIOR AND PROCESSING OF CONCENTRATED SURFACTANT SOLUTIONS

<p dir="ltr">Surfactants are the primary active ingredients in cleaning products such as shampoos and laundry detergents. These products contain a large amount of water, essential for use but unnecessary for production and transport. As a result, there is a push toward low-water, high-active formulas that reduce packaging waste and transportation costs. These also enable the inclusion of additives otherwise unstable or insoluble in aqueous solutions. However, making concentrated formulations is more complicated than simply removing water, as conventional production methods require increased energy input and can compromise the performance of highly viscous pastes. This work developed multiscale structure-property-processing relationships for concentrated surfactant feedstocks, linking their rheology, phase behavior, and processability to the microstructure and processing conditions.</p><p dir="ltr">An essential step in formulating surfactant-based products is looking up phase diagrams. Traditional methods to generate these are slow and blind to phase-evolution kinetics. A novel dynamic-diffusive interfacial transport (D-DIT) method was developed for in-situ identification of phases and water content across an entire isotherm in one experiment. The phase evolution to equilibrium of a non-ionic surfactant C₁₂E₆was tracked and transitions in confined environments differed from bulk behavior. The micellar-to-hexagonal transition concentration (C*) depended on the volume ratio of water to surfactant. D-DIT was also used to develop time-resolved relationships across a range of processes, including surfactant droplet dissolution and aqueous polymer film drying.</p><p dir="ltr">To maintain high surfactant activity, reduce viscosity, and simplify processing, the effects of temperature variations and formulation aids on the bulk behavior of model, industrially-relevant 70 wt.% lamellar SLE_nS solutions were investigated. At low temperatures, unexpected shear-induced crystallization was observed above the equilibrium crystallization temperature, accompanied by a peak in complex viscosity. Three additive-driven approaches were demonstrated to improve processability. Linear-chain alcohols (C₂-C₅) partitioned into inter-bilayer water layers, dehydrating headgroups and inducing lamellar-to-micellar transitions. Short-chain polyols formed higher-viscosity hexagonal or mixed phases via hydrogen bonding, which weakened on heating to yield low-viscosity fluids. Within the lamellar phase, salt promoted shear-induced crystallization above the equilibrium temperature, while propylene glycol suppressed crystallization and enhanced wall slip.</p><p dir="ltr">Lab-scale material relationships developed for concentrated lamellar pastes and other complex aqueous fluids were validated using an instrumented pilot-scale pipe loop. Benchtop rheometry and 1D velocity profiles from ultrasound speckle velocimetry (USV) were compared with pressure-based viscosity measurements and 2D velocity profiles from in-line magnetic resonance imaging (MRI). Reliable viscosity correlations were observed above shear rates equal to 1 s^-1. MRI effectively captured bulk velocity profiles, while the USV was more sensitive near the walls. Short-chain linear alcohols induced simple-shear velocity profiles, while propylene glycol enhanced wall slip; both reduced pressure drops and enabled higher flow rates.</p><p dir="ltr">Outcomes from these studies bridge the gap between lab-scale characterization and industrial-scale processing of concentrated feedstocks, providing robust experimental frameworks for future predictive modeling. Overall, this dissertation advances the efficient processing and development of sustainable, next-generation surfactant-based consumer products.</p>