CHBE Seminar by Reinhard Miller: Surfactant Adsorption at the Water/Alkane Interface - Competitive and Cooperative Effects Between Surfactant and Alkane Molecules

Abstract: The theory of surfactant adsorption at water/oil interfaces was based for a long time on models derived for the water/air interface. This was the main reason for the fact, that many experimental observations could not be adequately described. Therefore, a number of observations, peculiar for water/oil interfaces, were mainly explained by the penetration of oil molecules into the interfacial layer.

Recently, a competitive mechanism for the adsorption of surfactant at the water/oil interface was proposed, which provides a much better description of experimental data. This idea assumes that oil molecules compete with the surfactant molecules at the interface. However, this picture does not allow explaining why the interfacial tension of the water/oil interface decreases significantly already at extremely low surfactant concentrations in the aqueous phase. It was found that this phenomenon is caused by a cooperativity of surfactant and oil molecules in the interfacial layer. Cooperativity in this sense means that already few surfactant molecules adsorbed at the interface can induce a significant ordering of oil molecules in the interfacial layer. And in turn, this new interfacial structure attracts further surfactant molecules to adsorb. 

A refinement for the theoretical description of experimental data was finally achieved by applying suitable adsorption models for the two adsorbing compounds, i.e. a Frumkin adsorption model for the adsorption of oil molecules, and for the adsorption of surfactant molecules, depending on the surfactant’s molecular structure, a Langmuir, Frumkin, or reorientation model is suitable.

Biography: Reinhard Miller studied math in Rostock and colloid science in Dresden, Germany. Miller worked first at the Academy of Sciences in Berlin, and from 1991 to 2019 at Max Planck Institute of Colloids and Interface in Potsdam. Since then he has worked as a senior scientist at the Technical University Darmstadt. His main scientific interests are dynamics and thermodynamics of adsorption of surfactants, proteins, polymers, particles and their mixtures at liquid interfaces, dilational and shear surface rheology, and the formation and stabilization of foams and emulsions. He published his scientific results in about 650 papers in peer-reviewed journals and many books and book chapters. He is the editor of the journals Advances in Colloid and Interface Science and Colloids and Interfaces.

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