Development of an in situ rheological method to characterize fatty acid crystallization in complex fluids

Abstract

Complex fluids with high concentrations of crystallizing fatty acids are important for consumer care products. The key features of these materials are their ability to support their weight under gravity and to yield or flow beyond a critical applied strain. To better understand the post-processing behavior of these systems, we have utilized a combination of Rheology, Differential Scanning Calorimetry, X-ray scattering, and Polarized Light Microscopy to study a model formulation consisting of two synthetic surfactants and a fatty acid (n-hexadecanoic acid) in water. Homogeneous samples with high storage modulus ( G′) are realized due to the formation of a space filling solid fatty acid crystal network. A minimum of 5–7 wt% of fatty acid is required to achieve the formation of a percolating crystal network. The storage modulus, yield stress and strain of these formulations show a non-monotonic increase with fatty acid content. This is attributed to the change in the fatty acid crystal habit from large to small aspect ratio plates. We hypothesize that the fatty acid crystal network consists of crystal aggregates linked by surfactant-fatty acid gels, rendering the desired paste-like characteristics to the formulation. The combination of methods utilized in this research provides a framework for understanding crystallizing systems.