I noticed when brushing my teeth that toothpaste causes water to spread away from it. If you put a bit of toothpaste water mixture on your finger and touch it to a wet sink you will see the water on the sink travel away from your finger.

My question is what chemical in toothpaste is causing this to happen and in particular what property of the chemical is causing it.

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    $\begingroup$ It's all about surfactants. Soap would do the same, maybe stronger. $\endgroup$ – Ivan Neretin Jul 5 '16 at 9:17

Drops or patches of water in your sink are basically water that is not evenly spread out. The surface tension of the outermost water molecules acts a bit like a "balloon" containing the inner molecules. To a small degree it is thus possible for the water molecules to be stacked on top of each other because the water's surface tension is stronger than the gravitational pull.

Toothpaste contains detergents like sodium dodecyl sulfate which act as a surfactant reducing the surface tension of water. When you coat your finger in a mixture of water and a weak detergent like toothpaste or a stronger one like soap and touch the water drops in the sinks, the water mixes with the detergent. This lowers the surface tension and gravity spreads the drop out. The water is thus not really moving away from your finger, but the drops are being spread out under and around your finger.


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