Hubert Motschmann | PubMed Health

PubMed2006

Aqueous ionic and complementary zwitterionic soluble surfactants: molecular dynamics simulations and sum frequency generation spectroscopy of the surfaces.

An aqueous ionic surfactant, 1-dodecyl-4-(dimethylamino)pyridinium (DMP) bromide, and the corresponding zwitterion 2-[4-(dimethylamino)pyridinio]dodecanoate (DPN) were explored by means of molecular dynamics (MD) simulations and, for the ionic system, by infrared-visible sum frequency generation (IR-vis SFG). The molecular structure of the interfacial layer was investigated for the ionic and zwitterionic systems as a function of surfactant concentration, both in water and in salt (KF or KBr) solutions, by MD simulations in a slab geometry. The buildup of the surface monolayer and a sublayer was monitored, and density and orientational profiles of the surfactants were evaluated. The difference between the ionic and zwitterionic systems and the effect of the added salt were analyzed at the molecular level. The results of MD simulations were compared to those of nonlinear optical spectroscopy measurements. IR-vis SFG was employed to study the DMP ionic surfactant in water and upon addition of simple salts. The influence of added salts on the different molecular moieties at the interface was quantified in detail experimentally.

Ions↗

PubMed2004

A study of kinetic molecular exchange processes in the medium frequency range by surface SHG on an oscillating bubble.

The dilatational properties of fluid surfaces and interfaces have been comprehensively investigated in recent years. For example, an improved oscillating bubble device provided experimental results that allow for critical testing of established surface models, such as the Lucassen/van den Tempel (LvdT) model. The comparison of the LvdT model with the oscillating bubble experiments demonstrates a mismatch between the model parameters. For example, near the CMC or the limit of solubility the calculated parameters of surfactant solutions become unrealistically large. The deviation can be explained by the introduction of more detailed surface models, in particular by the modification of the effective thickness of the surface layer, its internal structure and the molecular exchange processes between these structures. For the verification of such processes an experimental setup was realized which allows for an independent determination of the instantaneous adsorption state at the surface of an oscillating bubble inside a surfactant solution. The setup utilizes the Second Harmonic Generation (SHG)--effect at the air-solution interface generated by the light of a pulsed LASER. The set-up is described in detail, and the results of a first experimental series are presented and discussed in this paper. As system, aqueous solutions of the fluortenside F381 were used.

Journal Article↗