Nonequilibrium effects on the association of an anionic polysaccharide and a cationic surfactant

Mixtures of oppositely charged polyelectrolytes and surfactants play a crucial role in various industrial applications,. The interactions between these components influence key physicochemical properties, making it essential to develop a thorough understanding of the mechanisms governing their association. In particular, their bulk interactions and adsorption at fluid/fluid interfaces are of great interest due to their impact on emulsification, foaming, and other stabilization processes. However, the study of these systems remains challenging, as their complexation is often influenced by nonequilibrium phenomena, leading to kinetically trapped aggregates that exhibit distinct physicochemical properties compared to their equilibrium counterparts.

Among the many polyelectrolyte–surfactant systems, the interaction between sodium alginate (SA) and hexadecyltrimethylammonium bromide (CTAB) serves as a model for investigating these phenomena. Sodium alginate, a natural anionic polysaccharide, interacts strongly with the cationic surfactant CTAB, forming aggregates with diverse morphologies depending on factors such as concentration, ionic strength, and mixing protocol. The resulting structures exhibit a wide range of sizes and interfacial behaviors, significantly affecting their adsorption at fluid/fluid interfaces. Understanding these processes is critical for optimizing their use in industrial applications, where controlling interfacial properties is key to achieving desired functionalities.

This study aims to provide, by combining bulk characterization techniques (electrophoretic mobility, fluorescence spectroscopy, conductimetry and UV/visible spectrosctopy) with surface tension measurements, new insights into the complexation mechanisms of SA-CTAB mixtures, with a special focus on the formation of nonequilibrium states and their impact on interfacial properties. Our results highlight the importance of kinetic effects in determining the final properties of polyelectrolyte–surfactant complexes. In fact, the presence of Marangoni stresses during the preparation of the solution results in a very different association process compared to that appearing when Marangoni stresses are absent. This presents strong implications in the nature of the systems, which can influence the final application.