Surfactant–Particle Interactions

Motivation

Surface-active agents such as collectors and frothers often coexist with particles in multiphase processes, particularly in flotation cells. The interfacial properties of such mixtures often differ significantly from those of the individual constituents. Because these effects are strongly coupled, the behavior of a mixed interfacial system cannot be predicted from the behavior of surfactants or particles alone.

Surfactants and particles can interact strongly with each other at fluid interfaces. These interactions may lead to the formation of mixed interfacial layers, nanoparticle–surfactant complexes, particle networks, or viscoelastic surface films. Such layers can substantially modify hydrodynamic boundary conditions and material transport at bubbles and droplets, especially under dynamic flow conditions. Understanding these mechanisms is essential for designing reagent schemes that promote selective attachment while avoiding excessive froth stability, poor drainage, or uncontrolled interfacial immobilization.

Goals

• Determine how surfactant and particle charge and concentration influence the interfacial properties of the system.
• Distinguish true electrostatic effects from ionic-strength effects.
• Identify the role of the surfactant-to-particle ratio in controlling nanoparticle–surfactant complex formation and interfacial activity.
• Quantify how mixed interfacial layers modify surface rheology, including surface viscoelasticity and interfacial mobility.
• Connect surfactant–particle interactions to flotation subprocesses such as attachment, coalescence, and froth stabilization.
   

Techniques

• Profile analysis tensiometry for equilibrium and dynamic surface-tension measurements.
• Large-amplitude compression experiments of adsorbed interfacial layers.
• Surface shear and dilatational rheology.
• Zeta-potential measurements to evaluate surfactant adsorption and charge reversal.
• Dynamic light scattering for complex and particle size measurements.
• Theoretical analysis of equivalent surfactant concentration, critical micelle concentration, and ionic-strength effects.

Results

For oppositely charged systems, such as negatively charged silica particles, ranging from nanoscale to ultrafine particles, with the cationic surfactant CTAB, the surfactant-to-particle ratio is the key control parameter. Below a critical ratio, surfactant molecules are mainly adsorbed onto the particles, forming particle–surfactant complexes and strongly reducing the amount of free surfactant in solution. In this regime, the surface tension remains relatively constant because free surfactant molecules are largely depleted.

Above the critical ratio, free surfactant molecules coexist with particle–surfactant complexes. Both species can contribute to the interfacial layer, leading to co-adsorption and more complex interfacial behavior. Compression experiments show that the particle-containing layer can collapse or restructure, while dynamic surface-tension measurements reveal the contribution of free surfactant adsorption.

For similarly charged systems, such as negatively charged silica particles with the anionic surfactant SDBS, the apparent enhancement of surfactant surface activity is mainly caused by changes in ionic strength rather than direct electrostatic repulsion between particles and surfactants. Particles and added electrolyte both reduce the critical micelle concentration and shift the surface-tension behavior. This distinction is important because it changes how such systems should be interpreted and optimized.

Overall, the results show that mixed surfactant–particle systems can regulate surface tension, interfacial elasticity, and surface mobility through several mechanisms, including surfactant depletion, complex formation, co-adsorption, charge screening, and particle-network formation. These mechanisms directly influence film drainage, bubble coalescence, bubble–particle attachment, and froth stability.

Publications

• Eftekhari, M., Schwarzenberger, K., Javadi, A., & Eckert, K. The influence of negatively charged silica nanoparticles on the surface properties of anionic surfactants: Electrostatic repulsion or the effect of ionic strength? Physical Chemistry Chemical Physics, 2020.
• Eftekhari, M., Schwarzenberger, K., Karakashev, S., Grozev, N., & Eckert, K. Interaction between silica nanoparticles and CTAB surfactants at the air–water interface: The influence of the ratio of surfactants to nanoparticles. Journal of Colloid and Interface Science, 2023.
• Eftekhari, M. The effect of ultrafine particles on the interfacial and hydrodynamic properties of multiphase systems. Doctoral dissertation, TU Dresden, 2023.