Adsorption of soluble surfactants at the air-water interface

Abstract

Surfactants are amphiphilic molecules widely used for many applications especially in flotation in the mining industry. Flotation, as an effective method to recover valuable products from ores, relies on the selective attachment between bubbles and value mineral particles that is controlled by using surfactants. Surfactants adsorbed at the air-water interface determine the bubble properties such as its size, stability and surface potential. Therefore, a better understanding of the surfactant adsorption is the key to optimise the flotation process and improve separation efficiency. However, current theories about surfactant adsorption are far from complete: classical adsorption models cannot effectively predict experimental surfactant density distribution and surface potential. This work examines surfactant adsorption at the air-water interface and its effect on surface properties. We apply experimental techniques, modellings and simulations to quantify the surfactant behaviour. We develop a new adsorption model which provides a much more accurate picture of surfactant adsorption at the air-water interface.We point out that the surfactant has a wide distribution at the air-water interface as determined by various techniques. This distribution cannot be simplified by one or more infinitely thin adsorption layers as assumed by classical models. Those models cannot accurately predict the density distribution and surface potential of the surfactant right at the interface. The assumption of classical models only works for surfactant several nanometres away from the surface. Only our model can accurately describe the surfactant distribution and surface potential at the surface. By applying this model to different systems, we provide realistic explanations for the experimental surface potential of surfactant solution, disjoining pressure of thin liquid film, and counterion specific effect of surfactant.Chapters 2-3 study the limitations of classical models. In Chapter 2, we review several classical adsorption models and experimental techniques measuring surface properties. We find that the measured and predicted surface potential are significantly different as classical adsorption models failed to predict this property effectively. This is because classical models assume a very thin adsorption layer and it does not represent the realistic one at the air-water interface. We also demonstrate that different experimental techniques measure the surface potential at different depth and those values are not directly comparable. In Chapter 3 we evaluate the stability and fitting qualities of classical models. We find that many models are not mathematically stable and the estimated model parameters are unreliable. The model stability decreases significantly with the number of adjustable parameters in the model. Therefore, it is not wise to extend classical models to address the realistic adsorption layer because of the decreased reliability caused by additional parameters.Chapters 4-6 are about the development and application of our new adsorption model. In Chapter 4, we develop a new adsorption model for ionic surfactants that incorporates the effect of adsorption layer thickness. This model accurately reproduces the experimental density distribution of surfactant at the air-water interface, and it provides a better prediction of the surface potential than classical models. In Chapter 5, we apply our model for the thin liquid film formed by a surfactant solution. This new model reproduces experimental disjoining pressure of foam films very well over a wide surfactant concentration range. We point out that the electrostatic potential decreases slower in the presence of a thick adsorption layer. Therefore, the electrostatic potential is higher in the thin liquid film than predicted by classical models. In Chapter 6, we combine sum-frequency generation spectroscopy, mathematical modelling, and molecular dynamics simulation to explain the origin of the counterion-specific effect for surfactant. We find that both headgroup-counterion and alkyl fragments-counterion affinities contribute to the counterion-specific effect. Surfactant sodium dodecyl sulphate behaves like a large ion, and it prefers to bind with large counterions. Therefore, large counterions enhance the surface adsorption and lower the surface tension the most.In summary, this work develops a better understanding of the surfactant adsorption at the air-water interface. We use modelling, experimental techniques and simulations to quantify the adsorption of surfactant and highlighted the significant effect of the adsorption layer thickness. The surfactant adsorption is important for both scientific research and practical applications such as mineral flotation. To further investigate this topic, the exact ion distribution of surfactant at the air-water interface needs to be determined by advanced experimental techniques and simulations.