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

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Physicochemical and engineering aspects, 2011-09, Vol.388 (1), p.12-20
• Main Authors: Thareja, Prachi, Street, Carrie B., Wagner, Norman J., Vethamuthu, Martin S., Hermanson, Kevin D., Ananthapadmanabhan, K.P.
• Format: Article
• Language: English
• Subjects: Chemistry; Crystallization; Crystals; Exact sciences and technology; Fatty acid; Fatty acids; Fluid flow; General and physical chemistry; Mathematical models; n-Hexadecanoic acid; Network; Networks; Palmitic acid; Rheology; Storage modulus; Strain
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/j.colsurfa.2011.07.038

[Display omitted] ► Effect of fatty acid (palmitic acid) crystallization on the rheology and microstructure of a surfactant-fatty acid mixed formulation. ► Use of Starch Pasting Cell (SPC) as a reliable way for rheological characterization of the crystallizing formulation. ► Change in crystal habit with the increase in fatty acid. ► Non-monotonic increase in modulus and yield stress with fatty acid content. 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.