Dielectric spectroscopy of concentrated cosmetic W/O-emulsions: possibilities to distinguish product changes caused by coalescence, sedimentation and variation of ingredients

Synopsis A quick result whether a newly developed cosmetic water‐in‐oil (W/O)‐emulsion shows constant sensory behavior (stable) or whether it changes its behavior over time (instable) is an important aspect in cosmetic research. In order to observe changes as quickly as possible, analytic methods are used. An established method is rheology, a sensitive method that gives direct information on sensory aspects. Additional information concerning the kind of instability allows a more focused improvement of formulation in the case of instabilities. In order to gain this information, an additional analytic method with a more ingredient specific focus is needed. In this article, the possibilities of using dielectric spectroscopy in order to get additional information are discussed. For concentrated W/O‐emulsions a dependence of emulsion behavior from volume median droplet diameter dv50 is visible by both methods: rheology and dielectric spectroscopy. A variation of droplet size distribution at constant volume median droplet diameter dv50 does not change the results for the examined emulsions. Quantitative information about mean droplet size is possible with calibration. Because of their different physical forci, rheology and dielectric spectroscopy complement each other in high‐sensitive detection of coalescence. In contrast to the mechanical properties of W/O‐emulsion, dielectric spectroscopy gives additional information concerning some reasons of change in emulsion structure. Mechanism like sedimentation and coalescence can be distinguished and the phase where changes take place (oil phase or water phase) can be located. The latter is possible by a new parameter – the correlation between maximum of dielectric loss ɛ′′(fR) and the relaxation frequency fR. Their correlation can be described by a simple power law. By coupling rheology and dielectric spectroscopy, an improvement in fast emulsion development and production control may be achieved without losing the advantage of a quick and easy measurement procedure. Résumé Savoir rapidement si une émulsion H/E récemment développée montre un comportement constant (stable) ou si elle en change dans le temps, est un aspect important dans le domaine de la recherche cosmétique. Afin d'observer ces changements aussi rapidement que possible, des méthodes analytiques sont utilisées. Une méthode établie est la rhéologie, méthode sensible qui donne une information directe sur des aspects sensoriels. Une information complément