Colloidal Interactions at Oil-Water Interfaces
Colloids are particles in the size range from about 10 nm to 10 µm, which are dispersed in a medium. Characteristic to them is that they are on one hand small enough to experience pronounced Brownian motion and on the other hand large enough to regard the surrounding medium as homogeneous. In daily life we meet them in form of foods, paints or cosmetic products, however at our institute we preferentially investigate well defined spherical particles made out of polystyrene or other polymers.
The properties of colloidal dispersions and emulsions result from microscopic interactions of colloidal particles among themselves and external boundaries. In a preceding Master thesis a new total internal reflection microscope (TIRM) was set up, that for the first time is capable of measuring interactions between colloidal probe particles and liquid-liquid interfaces. Using evanescent light scattering on single particles, the interactions of particles at an oil-water interface can be determined with sub 10 femtonewton (10-15 N) force resolution.
The above figure shows measured interaction potentials of attractive van der Waals forces competing with repulsive electrostatic interactions at different electrolyte concentrations. The fits according to DLVO-theory show excellent agreement with the data and the potential of the method for precise measurement of interactions.
Within this thesis in particular the electrostatic interactions with the oil water interface should be characterized further in the presence of very special salts (like NaBPh4). These salts have anions, which prefer the oil phase while the kations dissolve only in the aqueous phase. This results in interesting charge effects (Donnan potentials) at the oil-water interface.
The aim of the thesis is the first direct measurement of these potentials for colloidal particles.
If you are interested, please contact: Dr. Laurent Helden (E-Mail), 2. Physikalisches Institut, Raum 5-549, Tel. 685-652-19.