Beyond Clean: Unraveling Phase Behavior and Rheology of Soaps

Soaps are some of the oldest surfactants known to humans and remain indispensable in modern health, personal care, and sustainability agendas. Still, the simple chemistry of salts of fatty acids reveals a rich, multiscale landscape of self-assembly and flow behavior that controls soap processing and sensorial attributes. This review integrates phase behavior with rheological analysis to demonstrate how distinct microstructures, ranging from micellar to liquid-crystalline phases, possess unique rheological signatures. We further demonstrate how the coexistence and combination of these phases in a product such as a soap bar introduce additional levels of structural and rheological complexity. In addition, we highlight how linear viscoelastic analysis resolves relaxation hierarchies and how yield stress and thixotropic responses are inherent to the multiphase brick-and-mortar microstructures in products such as soap bars. The complex rheology of soap bars necessitates constitutive models that incorporate elasticity, plasticity, and time-dependent structure to predict extrusion or stamping performance. We also explore how the microstructure relates to cleaning efficacy, foam richness, lather dynamics, and rinse feel by coupling rheology measures with in-use dilution behavior. Finally, we discuss open questions, challenges, and future directions for developing more sustainable products.