Under pressure: testing the compressional stress stiffening of synthetic detergents

With environmental impact weighing on the minds of shoppers, solid soap bars offer a green alternative to liquid soaps, which typically require plastic packaging. Synthetic detergents, or syndets, are an emerging option to replace traditional fatty acid-based soaps because they work better for solid bars and have uses in cosmetics and pharmaceutics. However, bars are harder to manufacture from synthetic detergent than traditional soaps.

To help improve their production, Ciccone et al. carried out a rheological characterization of syndets.

“Syndets exhibit interesting rheological behaviours due to their high viscosity and heterogeneous and complex structure,” said coauthor María del Pilar Noriega Escobar. “Rheological characterizations and experimental research are required to improve extrusion efficiency and machine design and prevent product problems.”

The team studied the material response in the shear direction when subjected to an axial compressive stress. Tests were run at three different stages of syndet production with samples of differing moisture content.

Syndet formulations showed a compressional shear stiffening by up to two orders of magnitude within the instrumental limits of applicable compressional stresses. The results also showed a slight reduction in yield stress with moisture. This helps explain some of the difficulties in the processing of such synthetic detergents.

“Further work is needed to deepen understanding of the dependency of syndet-based formulations on temperature variations, specific types of ingredients, and composition,” del Pilar Noriega Escobar said.

The team plans to continue studying syndets, focusing on rheological and thermal characterizations. Ultimately, they hope to fully understand the complex behaviors of syndets to solve production problems.

Source: “A rheological characterization of synthetic detergent formulations,” by Giuseppe Ciccone, Simeon Skopalik, Claire Smart, Senol Gezgin, David Ridland, Manosh C. Paul, María del Pilar Noriega Escobar, and Manlio Tassieri, Physics of Fluids (2022). The article can be accessed at https://doi.org/10.1063/5.0099145 .