Holger Hammerich

Water/oil microemulsions from binary ionic-nonionic surfactants

The addition of a second surfactant to an otherwise thermodynamically stable microemulsion causes interfacial and bulk effects: the combination of AOT and C12E5 supports the formation of planar interfaces (considering the molecular geometries of the surfactants) which induces a sphere to oblate ellipsoid (≈ disk) transition ensuing a lamellar liquid crystalline phase. Heats of adsorption of C12E5 (adsorbed on an H2O/AOT/i-C8H18 interface) corroborate the existence of two different interfacial states. The phase transition was inferred from combining conductometric and electro-optic Kerr effect studies.

On the temperature-dependent viscosity of the L2 phase in water/ aerosol OT/isooctane (WO) thermodynamically stable emulsions

Abstract Remarkable temperature-dependent viscosities within the L2 phase of water/Aerosol OT/isooctane ( W O ) thermodynamically stable emulsions were observed. Their trend is exactly opposite to the temperature dependence of simple liquids. A straightforward theoretical model was proposed which describes surprisingly well those parts of the experimental plots which were considered by the model. The model is based on observations with electrooptical Kerr effect and quasi-elastic light-scattering experiments in these systems.

CHARGE TRANSPORT AS SUITABLE TOOL TO STUDY NEAR-PERCOLATION PHENOMENA

Electrical conductivity measurements in non-ionic (CmE5, m=10, 12) percolated w/o microemulsions with added carrier electrolytes (NaCl and/or AOT) serve as empirical tools to trace near percolation limit phenomena. Salient effects are observed if AOT (structurally a strong electrolyte) is dissolved in the w/o interface of the nanodroplets. They are due to the fact that AOT becomes a typical weak electrolyte since it is by its very nature confined to the pentaethylene glycol/water mixture in the interface. The results indicate further a continuous phase transition from spheres to cylinders a conclusion already earlier arrived at (see H.F. Eicke, W. Meier, H. Hammerich, Colloids Surfaces A 118 (1996) 141).

Experimental evidence of universality for interacting percolation

Varying the concentration of cosurfactant in a microemulsion shifts the percolation threshold of the electrical conductivity but does not change the behaviour of the conductivity near the threshold as a function of temperature. The same effect is observed if one adds various alcohols.