Towards a self-mixing salad dressing

A collaboration between researchers from ECFP, the University of Edinburgh, Nottingham Trent University, and Loughborough University studying the behaviour of a popular alcoholic drink has led to new insights into the process of spontaneous emulsification. The work has implications for low-energy production of a whole range of emulsion products which currently must be mixed vigorously.

15 May 2024

Emulsions are dispersions of micron-sized liquid droplets within another liquid. They are found across a wide range of industries, including food and drink (milk, salad dressings, mayonnaise), paints and coatings (emulsion paints, many glues), and pharmaceuticals (topical creams, emulsion adjuvants in vaccines). The liquids used to make emulsions are normally immiscible and energy is needed to create interfaces between the droplets and the bulk liquid phase. Think of a salad dressing that you must shake before serving. Without added stabilisers, it will quickly separate into two distinct layers. However, some systems exhibit spontaneous emulsification where no mixing is needed and the emulsion remains stable for long periods of time. Understanding this phenomenon could help industries develop novel low-energy, surfactant-free emulsification methods.

There is a table of food and a bottle of raki with two glasses.
Vartieties of ouzo are found in different countries: ouzo in Greece, sambuca in Italy and arak in Lebanon. The bottle here contains the clear spirit raki from Turkey, which, in the right-hand glass, has been added to water forming a cloudy emulsion.

The spontaneous emulsification of Ouzo: from clear to cloudy

Ouzo is a clear, aniseed-flavoured spirit, consisting of roughly 60% water, 40% ethanol, and a very small amount of anise oil. Both oil and water are soluble in ethanol, provided it is present in sufficient quantities. When the neat spirit is diluted with water, the drink will turn cloudy due to oil-rich microdroplets precipitating out of the solution. This is commonly known as the ouzo effect and is an example of spontaneous emulsification. Interestingly (and perplexingly), the emulsion exhibits long term stability and stays cloudy for weeks, an important feature for many industrial emulsions. To fully understand and predict the behaviour of such a system, it is crucial to determine its complete equilibrium phase diagram. To date, this has not been done for ouzo.

A video of a droplet of ouzo that has just been diluted with water, taken using Optical Coherence Tomography (OCT). The emulsion droplets appear bright white and can be seen to swirl around due to differences in density and surface tension (Marangoni forces).

An equilibrium phase diagram for Ouzo

The team took an experimental approach to building a phase diagram for a model system very similar to the commercial ouzo spirit, measuring the phase boundary along with the compositions and physical properties of coexisting phases, including the interfacial tension and density. This experimental approach was verified via the development of a theoretical model using lattice density functional theory, the predictions of which closely matched the experimental findings.

A representation of the experimentally determined phase diagram for Ouzo. The curved black line represents the binodal: mixtures above this line are stable, whereas those below will separate into two phases, with thin slanting lines indicating which phases coexist. The coloured circles indicate where density measurements were taken. Compositions of various phases are indicated by the coloured balls: blue is water, black oil and red alcohol.

Exploring non-equilibrium phenomena

The experimental phase diagram and predictive model developed by the researchers can be taken forward in the future to study the non-equilibrium behaviour of the system, for example its stability. Understanding the mechanism by which this surfactant-free system remains stable could help develop other surfactant-free emulsions. Recently this has been an area of increasing focus for formulation scientists, as industries look to move towards more sustainable formulations and away from the use of petroleum-derived surfactants.


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