Pickering emulsion stabilisation by wax crystals: A multiscale understanding

Water-in-oil (W/O) emulsions are used within a range of industry ranging from food to petroleum. They are liquid-liquid colloidal dispersions that produce optimal base for oil soluble active ingredients. Some of the applications are as follows: Food, Replacement for calorie dense material e.g., fat, Fuel, Transportations and refining process of fuel, Cosmetics, Better texture and natural appearance of the product and prolonged shelf life.

W/O emulsions are thermodynamically incompatible and tend to phase separate upon storage. There are various emulsifiers that can be used to stabilise the emulsions such as polymers, surfactants, solid particles etc. Systems stabilised by solid particles are known as Pickering emulsions and are usually preferred to surfactant-stabilised systems as they provide high resistance to coalescence and long-term stability. There is an increasing interest to incorporate biocompatible particles to stabilise Pickering emulsions to meet consumer expectations of "clean"- or "green"-label products.

In literature it is highlighted that organic crystals are used to stabilise bubbles and mixtures in the air-oil (A/O) and water-oil (W/O) interface. There is evidence of stabilisation of Pickering W/O emulsions stabilised solely by organic fat crystals (Tenorio-Gracia, 2023) The stability is seen via a combination of interfacial i.e., Pickering stabilisation (Xinya C, 2021) as well as bulk stabilisation i.e. the crystal forming network in the continuous phase.

Although the stability is enhanced by the addition of crystals, there is a gap in the understanding of the true stabilisation mechanism. There is a limited understanding in the importance of interfacial stabilisation versus network stabilisation. Our hypothesis is that tuning interfacial organisation of crystals at the droplet surface will enable controlling macroscopic rheological performance. This PhD will focus on gaining understanding of the interfacial stabilisation of water droplets by organic crystals. We will particularly focus on understanding the organic crystal arrangement at the interface using single droplet measurement and study the crystal formation at a single droplet scale using microscopy, rheology and scattering techniques. A biocompatible sustainable crystal that would be used within formulations is wax (sourced from waste by-products e.g., rice and palm oil).

We aim to control the organic crystal formation at interface of a single droplet through varying the crystallisation variables such as wax purity, co-solvents and temperature in order to affect the macroscopic stabilisation of the emulsions and also the material performance.

References:
E. Tenorio-Garcia, A. Araiza-Calahorra, M. Rappolt, E. Simone, A. Sarkar, Pickering Water-in-Oil Emulsions Stabilized Solely by Fat Crystals. Adv. Mater. Interfaces 2023, 10, 2300190. https://doi.org/10.1002/admi.202300190

Xinya Chen, Guangyu Sun, Daiwei Liu, Huihui Zhang, Hao Zhang, Chuanxian Li, Zhiqi Zhao, Two effects of wax crystals on stabilizing water-in-oil emulsions, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 625, 2021, 126884, ISSN 0927-7757, https://doi.org/10.1016/j.colsurfa.2021.126884.