Surfactant adsorption kinetics - emulsion stability, World Congress on Emulsion 2010

Contribution to the 5th world congress on emulsion CME
12 - 14 October, Lyon, France

Interrelation between surfactant adsorption kinetics, interfacial rheology and emulsion stability

T. Sobisch1, N. Mucic2, J. Krägel2, A.V. Makievski3, R. Miller² and D. Lerche1

1 LUM GmbH, Rudower Chaussee 29 (OWZ) 12489 Berlin / Germany
info@lum-gmbh.de, www.lum-gmbh.com
² MPI für Kolloid- und Grenzflächenforschung, Am Mühlenberg 1, 14424 Potsdam / Germany
3 SINTERFACE Technologies, Volmerstrasse 5, 12489 Berlin / Germany


Abstract
The right choice of emulsifier is an ongoing challenge in emulsion technology. The time scale of the emulsification process has a strong impact on the emulsifier efficiency, which can only become effective if the adsorption kinetics are fast enough to avoid coalescence of freshly formed droplets. Additionally, the interfacial rheology at the oil-water interface determines the final emulsion stability.
The interrelation between surfactant adsorption kinetics, interfacial rheology and emulsion stability was investigated in model systems paraffin oil / water / nonionic surfactants, where the EO-chain length was varied.
The kinetics and thermodynamic properties of the surfactants at the oil/water interface were measured by using drop profile analysis tensiometry. The same tool is applied for the investigation of the interfacial dilational rheology by recording the interfacial tension response to harmonic area compressions-expansions of the studied drop.
Emulsions were prepared with a dissolver type homogenizer. Stability of emulsions was investigated as function of time of processing, surfactant concentration and HLB value using multisample analytical centrifugation. Not only the extent of phase separation but also the kinetics are directly measured in-situ.
Emulsion stability increases with time of emulsification. The emulsifier with 5 EO units is far more effective than that with 12 EO units. Adsorption equilibrium at the interface is reached in seconds. The kinetics of adsorption show clearly a depletion effect of surfactant molecules from the aqueous phase, which is especially pronounced for the emulsifier with 5 EO units. Nevertheless, at higher surfactant concentration the interfacial tension is lower for the more lipophilic surfactant.


Keywords: emulsion stability, adsorption layer properties, analytical centrifugation, emulsion preparation, emulsifier selection, oil-in-water