Abdulhamid Hejazi Almidani, University of Stuttgart

Formulation and characterization of bio-based and sustainable microemulsions

Due to their high solubilization capacities and ultra-low interfacial tensions between water and oil, microemulsions are of great interest in chemistry and technology [1]. Thus, microemulsions are not used only in washing processes [2], but also in pharmaceuticals [3] and cosmetics [4]. In contrast to emulsions (e.g. milk or butter), microemulsions are thermodynamically stable, optically transparent mixtures of an aqueous and an oily component, in which amphiphilic molecules such as surfactants are used for stabilization. Because of their wide range of applications, large quantities of synthetic or petrochemical-based surfactants are released into soils, waters and sediments on a daily basis. Many studies show that an increased content of synthetic surfactants in the environment has a strong impact on the ecosystem [5-7]. In this context, the class of natural surfactants is becoming increasingly important as an alternative to synthetic surfactants.

Natural surfactants have a very high biodegradability due to their natural origin from bacteria, yeasts and fungi [8-9]. Furthermore, in a recent review article, Sottmann et al. emphasized that the critical micelle concentration of the biosurfactant rhamnolipid, is comparable to that of a common linear alkyl benzene sulfonate surfactant of about 10-1 mmol/L. Also, the addition of the rhamnolipid lowers the minimum surface
tension between water and air from 72 mN/m to the same value of about 30 mN/m [10]. However, the properties of aqueous surfactant systems, emulsions and microemulsions stabilized by natural surfactants have generally only been studied to a limited extent [11-14].

The aim of this PhD project is therefore to formulate efficient, but also sustainable and
environmentally friendly microemulsions of the type H2O/salt – bio-oil – biosurfactant and to
characterize their properties, starting from the application-relevant but not very sustainable
microemulsion system, consisting of an aqueous CaCl2 solution, n-octane and a surfactant mixture of commonly used synthetic surfactants. As a first step, the hydrolysis stability of the used biosurfactants will be investigated using various methods (e.g. 1H-NMR). Then, the influence of the type and size of hydrophilic head and hydrophobic residual groups of the biosurfactants on the solubilization efficiency will be investigated. Since the biosurfactants are also to be used in washing processes, their influence on the interfacial tension between water and bio-oils will also be systematically studied. In addition, the microemulsion nanostructure formed by the biosurfactant amphiphilic film will be elucidated using scattering methods such as light scattering and small-angle X-ray scattering, which is of particular interest in pharmaceutical drug delivery. Finally, existing phenomenological concepts such as the hydrophilic-lipophilic deviation (HLD) will be used to predict the influence of various parameters such as temperature and composition on the phase behavior, interfacial tension and structure of microemulsions.

Keywords: high solubilization capacity, ultra-low interfacial tension, triglyceride, biosurfactant,
rhamnolipid, microemulsion, hydrophilic-lipophilic deviation.

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