Surfactant Adsorption Layers: Experiments and Modeling.

During recent years, great progress has been made in understanding the adsorption of surfactants at liquid interfaces. In addition to tensiometry, new efficient methodologies have been developed, in particular interfacial selective optical methods which allow direct access to the adsorbed amounts and interfacial layer compositions. In addition to these new experimental tools, the thermodynamic description by equations of state now allows one to provide a quantitative picture of surfactant interfacial layers.

A Multistate Adsorption Model for the Characterization of CDMPO Adsorption Layers at the Aqueous Solution/Air Interface.

Experimental data for tridecyl dimethyl phosphine oxide (CDMPO) adsorption layers at the water/air interface, including equilibrium surface tension and surface dilational viscoelasticity, are measured by bubble and drop profile analysis tensiometry at different solution concentrations and surface area oscillation frequencies. The results are used to assess the applicability of a multistate model with more than two possible adsorption states. For the experiments with single drops, the depletion of surfactant molecules due to adsorption at the drop surface is taken into account.

New view of the adsorption of surfactants at water/alkane interfaces - Competitive and cooperative effects of surfactant and alkane molecules.

The theoretical description of the adsorption of surfactants at interfaces between aqueous solutions and oil was based over a very long time on models derived for the solution/air interface. Thus, most of the experimentally observed peculiarities could not be specifically considered but were merely interpreted in terms of a penetration of oil molecules into the alkyl chain layer of the adsorbed surfactant molecules. These penetrating oil molecules enhance the surfactant adsorption as compared to the water/air interface.

Particular Behavior of Surface Tension at the Interface between Aqueous Solution of Surfactant and Alkane.

A two-component interfacial layer model was employed to describe the experimental results obtained for various surfactants. In contrast to the previous works, here it is shown that the adsorption activity of alkane depends on its interaction with the adsorbed surfactant and is proportional to the surface coverage by this surfactant. Also, it is assumed that this increase of the adsorption activity parameter is limited by some maximum value.

Dilational interfacial rheology of tridecyl dimethyl phosphine oxide adsorption layers at the water/hexane interface.

The dilational visco-elasticity of surfactant adsorption layers was measured at low frequencies by the drop profile analysis tensiometry using oscillating drops. As the studied non-ionic surfactant CDMPO (tridecyl dimethyl phosphine oxide) is soluble in water and in hexane, the partitioning of the surfactant between the two solvents had to be taken into consideration. The diffusion controlled exchange of matter theory was generalized in order to take into consideration the curvature of the interface, the diffusional transport in both adjacent bulk phases as well as the transfer across the liquid interface.