Rheophysics of thixotropic pastes: The rheology and microstructure of systems containing surfactants and a crystallizing fatty acid

Soft solids with crystal networks as the underlying structure appear in a wide variety of areas and applications, including many consumer products. Understanding how the microscopic structure is related to the macroscopic properties in these systems poses an interesting and relevant topic. The goal of this work, to gain a fundamental understanding of the relationships between molecular structure and interactions, crystallization, and macroscopic rheological properties of model system formulations containing surfactants and a crystallizing fatty acid, has been accomplished via a comprehensive study linking molecular composition, formulation, and processing to final properties through the use of a variety of techniques, including rheology, time domain nuclear magnetic resonance (TDNMR), differential scanning calorimetry, X-ray scattering.;The model system was developed by examining industrial samples with a crystalline network structure capable of supporting its own weight at rest but able to flow under a relatively low level of applied deformation. The model system was comprised of palmitic acid (PA), sodium dodecyl sulfate (SDS), cocamidopropyl betaine (CAPB), and H2O or D2O. PA, the crystallizing component, serves as the main structural component, and the surfactants (SDS and CAPB) allow for the formation of macroscopically homogeneous formulations.;Crystallization occurs through a mixed mechanism of diffusion and surface incorporation. The model system formulations are comprised of PA crystals with adsorbed surfactant and water and a mixed PA-surfactant mesophase. Formulations with sufficient PA and surfactant content cooled form solid crystalline networks. Crystal size transitions from long to short plate-like crystals with increase in PA content, a consequence of an increase in supersaturation. Neutralization of the fatty acid by adjustment of the pH to approximately 7 results in the loss of crystal network formation. Adjustment of the pH to approximately 8-9 allows for the faster formation of a crystal network with wide plate-like and fibrillar crystals. TDNMR measurements and studies with varying surfactant concentration demonstrate the important role of a mixed PA-surfactant gel phase in stabilizing the crystal network. Finally, application of a cell model provides quantification of the development of material properties and further insights into the physical mechanism of crystal network development.