Design of tailor-made curcumin Pickering particles for the stabilization of water-in-oil emulsions

Emulsions stabilized by solid particles are a specific type of emulsion in which solid or colloidal particles are used as emulsifiers; these particles adsorb, at oil-water interfaces more strongly than traditional surfactants. These emulsions are known as Pickering emulsions and represent a valid alternative to traditional one due to their improved stability, biological compatibility, low toxicity, and environmental friendliness. The properties of the solid particles, such as size, shape, polymorphism and wettability, have an important role in stabilising Pickering emulsions. Curcumin was chosen in this work for its interesting interfacial properties, which enable curcumin particles to act as a stabilizer in emulsions. The present research focused on the production and characterization of two curcumin polymorphs, Form I and Form III. The aim was to produce different curcumin polymorphs of comparable sizes and shape distribution in a reproducible manner, using a crystal engineering approach, and test the two different forms as Pickering stabilizers. Two distinct crystallization techniques were employed to obtain the target polymorphs: Form I was generated via cooling, meanwhile Form III was obtained through antisolvent addition. The potential influence of polymers (carboxymethyl cellulose and k-carrageenan) on the crystallization process was also investigated to determine if these molecules had any effect on the interfacial properties of the crystals. Initial tests were performed with the Crystal 16 system and High-Performance Liquid Chromatography measurements to determine curcumin solubility in ethanol/water mixtures; additionally, the curcumin solubility was evaluated in aqueous solutions at different pH. The obtained particles underwent a comprehensive characterization: Raman spectroscopy and Powder X-ray Diffraction (PXRD) were used to identify the actual polymorphic form; Differential Scanning Calorimetry (DSC) to assess the effects of temperature; Optical Microscopy and Scanning Electron Microscopy (SEM) to examine particle shape and size distribution (PSD) of the crystals and finally contact angle measurements were conducted to evaluate the wettability of the produced powder. After that, water-in-oil (W/O) emulsions were prepared with different batches of the powders previously obtained with varying water content (range 5 – 20wt%). These emulsions were characterised by Droplet Size Distribution, and their stability was assessed. This study confirms the relationship between the properties of curcumin particles, particularly polymorphism, and their effectiveness as Pickering stabilizers. It has been shown that particle size should be an order of magnitude different to the droplets in the dispersed phase to ensure system stability and that crystal shape determines the particle's disposition at the interface. The two chosen crystallization techniques successfully led to obtaining the two polymorphic forms, demonstrating that crystal engineering can be used to achieve the desired crystalline structures. It also revealed the impact of polymers on particle size.