Explenation for the movie: Tahini Water-in-Oil Emulsion Turns Into Oil-in-Water Emulsion

 

Tahini is a real world example of colloidal science. Tahini is an oily past similar to peanut butter that is prepared by grinding toasted sesame seeds. When this paste is mixed with water its texture changes continuously: at first it becomes very viscous and bulky, but as more water is added the paste obtains a creamy texture. The creamy mixture is the Tahini served in restaurants and available as a salad in the supermarket.

In order to understand this change in texture, it is crucial to first understand the chemistry involved, that is, what is happening at the molecular level of this food. The sesame seed, like many other natural products contains oil, water, and some amphiphilic molecules (surfactants, molecules similar to soap that can bind with water on one side- hydrophilic, and can bind with oil on the other side- lipophilic). As water and oil don’t mix, when mixing them together, the water is dispersed within the sesame oil.  This system is called an emulsion, and in essence there are droplets of oil or water that are coated with the amphiphilic molecules, and are surrounded by the other liquid phase. If the droplets contain water, and are surrounded by oil, this configuration is called a water-in-oil emulsion. On the other hand, if the droplets contain oil, and they are surrounded by water the system is called an oil-in-water emulsion. The type of emulsion is controlled by the ratio of water and oil, type of the surfactants and other parameters. Each type of Emulsion system has a different texture.

When the sesame seeds are first grinded a water-in-oil system is formed, as the seeds contain only a very small fraction of water and a large fraction of oil. When water is added to the paste, the water droplets become larger and there are more droplets, so the past becomes thicker. But, when more water is added, the water becomes the most abundant phase, and so there is a transition (called Inversion) from a water-in-oil system to an oil-in-water system. This transition explains the change in texture. In this video, we follow the addition of water to the Tahini, and we show both the change in texture, and that after the inversion point the conductivity of the paste jumps. The reason is because the water phase (that contains salt that we added) is more electrically conductive than the oil phase. At the beginning, the non-conducting oil phase is the continuous phase, so conductivity is low. After the inversion point the water with the salt becomes the continuous phase, and so the entire past becomes more electrically conductive.   

To sum this up:

When	Oil vs Water	System	Texture	Paste conductivity
Beginning	Mostly oil	Water in oil	Thick	Very Low
With some water	Mostly oil	Water in oil	Thicker and bulky	Very Low
With more water	Mostly water	Oil in water	Creamy	High

 

In our lab, we use emulsion systems for novel functions, including for 3D printing of new materials, for cosmetics and for improving drugs performance.