Regensburg 2007 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 21: POSTER: Dynamics and Diffusion

CPP 21.6: Poster

Wednesday, March 28, 2007, 16:00–18:30, Poster B

Determination of microscopic interaction constants with unprecedented accuracy — •Felix Lehmkühler, Michael Paulus, Simone Streit, and Metin Tolan — Experimentelle Physik I/DELTA, Universität Dortmund, Otto-Hahn-Straße 4, 44221 Dortmund

The poly(propylene glycol) 4000 (PPG4000) - isobutane interface was studied by x-ray reflectivity measurements. The interface was investigated under different pressures between 1.0 bar and 2.6 bar which is close to the condensation pressure of isobutane at the temperature of 289K. While adsorption of molecular thin films of isobutane is expected for hydrophobic interacting systems like glycerol-isobutane or water-propane, the formation of isobutane films on PPG4000 surface is suppressed by the high miscibility of isobutane and PPG4000. Thus no evidence for the formation of isobutane films was found.

The surface roughness of the substrate was estimated to vary between 0.4 nm at 1 bar nitrogen and 0.8 nm at 2.6 bar isobutane pressure. This is explained by the change of surface tension of the PPG4000-isobutane mixture with isobutane pressure. Ideal solution of isobutane in PPG4000 regarding to Raoult's Law is assumed to describe the measurements. The surface tension is calculated with the capillary waves theory to confirm the ideal solution with theoretical models for surface tension behavior of binary liquid mixtures. The Parachor-method and gradient theory are applied. Furthermore, the gradient theory allows to determine microscopic interaction contants qualitatively and quantitatively.