Dynamic adsorption and tension behavior of surfactants at air-liquid interfaces

Chien-Hsiang Chang, Purdue University

Abstract

Dynamic adsorption and tension behavior of surfactants at air/liquid interfaces is important in foaming, coating flows for thin film production, wetting, and other processes. A bubble surfactometer has been used to study the adsorption dynamics of several nonionic and ionic surfactants under both constant-area and pulsating-area conditions. A new model, which considers the enhancement or activation energy barrier of adsorption/desorption, is applied to octanol, sodium dodecylsulfate, and some novel surfactants used in wool-dyeing applications. The results indicate strong effects of molecular interactions, such as electrostatic repulsion, between interfacial monolayer and adsorbing molecules. As the interface area oscillates sinusoidally, the dynamic tensions fluctuate and their amplitudes vary with the oscillation frequency. The tension amplitudes are related to the exchange rates of molecules to and from the interface. The nonequilibrium tensions can be substantially lower than the equilibrium tensions. This phenomenon was observed quite generally for surfactants and proteins studied. The low nonequilibrium tensions can be accounted for by an adsorption hysteresis mechanism that is predicted by a newly developed convective mass transfer model, without need to consider surface dilatational rheology effects. Models have been extended to help in understanding dynamic competitive adsorption of mixed surfactants at interfaces. Data and models reveal several interesting mixture phenomena, having implications for foam-based separations from dilute solutions and for lung surfactants used for the treatment of the respiratory distress syndrome.

Degree

Ph.D.

Advisors

Franses, Purdue University.

Subject Area

Chemical engineering

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