(64f) Impact of like-Charge Surfactants on Compressional Mechanical Properties of Particle-Laden Interfaces

The role of particles in properties of interfacial systems has intrigued scientists since its discovery in the early 1900’s. Particles of proper wettability tend to populate fluid interfaces leading to a decrease in contact area between the two fluids, which lowers the overall free energy of the system. This characteristic is responsible for the formation of the so-called Pickering emulsions, which have been extensively discussed in the literature. The attention over the past few years has turned towards systems composed of particles along with another well-studied species, surfactants. Surfactants are amphiphilic molecules that stabilize interfaces by decreasing their interfacial tension. The presence of both species in vicinity of fluid interfaces can either lead to a synergistic effect or a competitive behavior, where each scenario has consequences for interfacial binding and stabilization of the interface. Majority of studies found in the literature are focused on opposing-charge particle and surfactant systems, where the surfactant adsorption on the particle surface results in a change of the particle wettability, which could potentially improve the particle adsorption. However, few studies have focused on the interface when it is populated with like-charged species. For instance, in systems containing both anionic surfactant and negatively charged colloidal silica particles dispersed in the bulk and, no particle adsorption to the air/water interface was observed. However, an increase in the surfactant adsorption to the interface was reported in presence of the particles. In contrast, a study on negatively charged polystyrene particles directly deposited at the air/water interface showed that the presence of like-charged surfactants in the aqueous sub-phase affects particle interactions at the interface, resulting microstructure of the particles at the interface, and the response of such particle-laden interface to shear stresses. Our goal is to clarify the role of the like-charged surfactant (sodium dodecyl sulfate) on the interfacial behavior of liked-charged colloidal particles (silica particles) at the air/water interface; in particular, their mechanical properties when subjected to external stresses. In this work, we have studied a number of silica particle characteristics (e.g., size, roughness, and surface functional groups) in presence of sodium dodecyl sulfate (below and above the critical micelle concentration) and investigated their impact on the resulting interfacial behavior. Specifically, we have examined the efficacy of particle binding to the interface, mechanical response of the resulting interface to compressions, and the resulting collapse mode. Our findings indicate that that the order with which surface-active species are added to the system impacts the microstructure and properties of the resulting interfacial layer. In addition, unique differences are captured when comparing the interfacial behavior of colloidal and fumed silica particles in presence of surfactants.

ACKNOWLEDGEMENTS

Support from the NSF under grant #1934513 is gratefully acknowledged.