Hydrophobic Solubilization in Aqueous Colloidal Systems - Mechanistic Insights into Protein-Based Solubilization and Additive-Modified Surfactant-free Microemulsions

The solubilization of hydrophobic compounds in water is a common challenge in industry and daily life. In this work, different solubilization strategies are examined, and experimental findings are linked to schematic mechanistic concepts. This aims to contribute to more sustainable solubilization methods. In the first chapter, the topic is introduced, and a comparative overview of the most important techniques is provided. In the second chapter, the solubilization of commercially available, denatured plant protein isolates in aqueous media is investigated. Three isolates from pea, chickpea, and potato are subjected to various physical and chemical modification methods, as well as combinations of these, to identify the solubility-limiting factors hydrophobicity and aggregation. The protein solubility of all tested isolates is significantly improved by targeting both non-covalent and covalent bonds: combining SDS with pH shifting destabilizes insoluble aggregates through protein unfolding and enhances interaction with SDS. A schematic solubilization mechanism is proposed, which may also apply to other plant protein isolate classes. In the third chapter, pea protein isolate is used to solubilize the polyphenol curcumin in water. Using a well-established pH-shifting method, supersaturated curcumin-loaded pea protein isolate nanoparticles are prepared. While the curcumin concentration immediately after preparation is 1.39 ± 0.093 mg/mL, it decreases to only 0.0528 ± 0.0088 mg/mL after 24 hours of continuous stirring. Fundamental studies into supersaturation reveal an underestimated influence of the nanoparticle preparation method on curcumin binding. Precipitation of the polyphenol from the largely inert protein carrier is attributed to either aggregation of tightly packed curcumin molecules on the carrier surface or their isomerization followed by aggregation. Based on these findings, the prepared nanoparticles are proposed as a supersaturated drug-delivery system (SDDS). A schematic solubilization mechanism is proposed, which may also apply to the solubilization of curcumin with other plant protein isolate classes. Emphasizing the formulation's kinetic and stable states allows for diverse applications in both food and pharmaceutical industries. In the fourth chapter, the effect of low-concentration surface-active additives on the binodal and the mesostructure of the surfactant-free microemulsion EtOH/1-Octanol/Water is investigated. While the addition of nonionic surfactants does not notably affect the binodal, the addition of ionic surfactants and an ionic hydrotrope causes a significant decrease. Building on previous research, the impact of an additive on the binodal is proposed to depend on whether it carries a charge, its solubility in the individual solvent compounds, and, for ionic surfactants, the headgroup's hydration. Changes in the mesostructure are assessed by evaluating SAXS data. This approach maintains the system’s solubilization capacity for a hydrophobic compound while increasing water content and reducing the amount of organic solvent by adding small amounts of ionic compounds.