TDNMR characterization of a model crystallizing surfactant system

Complex fluids with high concentrations of crystallizing fatty acids are important for consumer skin care products. These fluids exhibit solid-like elastic behavior at rest, but yield and flow under an applied strain. Such behavior is characteristic of isotropic, space-filling networks. The processing determines the microstructure of these systems, which in turn influences their macroscopic rheological properties. The kinetics of crystal growth in model system compositions, comprising (by weight) 9% sodium dodecyl sulfate, 3% cocamidopropyl betaine, and different specified amounts of palmitic acid in H2O or D2O, were studied by time domain nuclear magnetic resonance (TDNMR) and differential scanning calorimetry (DSC). The data were analyzed using the Avrami equation; both DSC and TDNMR gave consistent Avrami exponent values of approximately 1.5 for formulations with 11 wt% and greater amounts palmitic acid content. The Avrami exponent is consistent with two-dimensional crystal growth limited both by diffusion and surface incorporation. The solids content at 25 °C was found to be proportional to the concentration of palmitic acid level above its solubility limit of ca. 2 wt%. TDNMR, combined with DSC, microscopy, and rheology provide valuable insights into the molecular structure and mechanism of crystallization in surfactant/fatty acid systems.