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Aqueous solutions containing sodium lauryl ether sulphate (SLES) and cocamidopropyl Betaine (CAPB) with a constant total surfactant concentration (10 wt%) but different volume mixing ratios were prepared. A remarkable increase in the solutions' viscosity at a volume mixing ratio of 34 v/v% SLES to 66 v/v% CAPB was observed. By increasing the volume ratio of SLES (r-value) in the binary mixture, the viscosity of 10 wt% SLES, which is close to that of water, increases to a maximum, but then drops down drastically at high r values (>45 v/v%). The maximum viscosity (V max ) is 2.8 Pas, whereas the minimum is 2 mPas. The rheological behavior at V max has the remarkable feature of a simple Maxwell fluid over a large frequency range with one relaxation time. By adding a microemulsion (Plantasil Micro), the value of V max decreases extremely, whereas adding cosurfactants like isodecyltrietheleneoxide (IT 3 ) or salts, like calcium chloride, leads to a pronounced increase in the value of V max . At 0.76 wt% CaCl 2 , V max is 140 Pas and 150 Pas in the presence of 1 wt% IT 3 . By increasing the SLES volume mixing ratio in the solution the maximum viscosity is shifted to higher CaCl 2 amounts. The increasing viscosity can be explained by a transformation of the micelle shape from spherical to rod-like. Whereas adding cosurfactants or salts leads to an increased entanglement between the rod-like micelles. Consequently, higher microemulsion amounts can be added before reaching the minimum viscosity.

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Products for personal care and home care have to fulfil many different requirements regarding performance, aesthetics, costs and safety. This has consequences for the use of new technologies such as microemulsions for this kind of applications. If, for example, just one of these parameters (e.g. performance) is sufficient or even excellent while the other parameters do not meet the requirements, then the new technology will not be applied. This example reflects the challenges of applying microemulsions in cosmetic products and in detergency. Although there have been numerous basic studies with microemulsions which show superiority in certain aspects of performance, broad applications of microemulsions in detergency and cosmetic products are not yet achieved. The reasons for this are manifold and depend on the specific application. From an economical point of view the high surfactant concentration is one aspect. However, the fact that the formation of a microemulsion depends on various parameters such as the type of oil, the chosen surfactant, the temperature and the electrolyte content to mention just a few is even more important.As discussed in Chapters 1 and 3 of this book, the formation of microemulsions can be controlled by adjusting the temperature, the electrolyte content or the hydrophiliclipophilic balance by varying the ratio of different surface active agents. All these techniques are applicable in cosmetics and detergency and have specific advantages and disadvantages. For example, most consumer products require a temperature stability which is usually not achieved with temperature-induced microemulsions. Therefore, no general rule exists for the most suitable type of microemulsion but it depends on the application. In this chapter, several examples are given which describe the status of different applications in cosmetics and detergency. Microemulsions in cosmeticsMicroemulsions are used in many cosmetic products and are in the focus of current industrial and university research activities. The reasons and motivations are manifold. The following is a summary of their utilisation in several cosmetic product categories with many references to publications and patents. Common aspects of microemulsions Microemulsions: Background, New Concepts, Applications, Perspectives. Edited by Cosima Stubenrauch

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