Soap is an ancient invention that probably has many origins, with its first recorded use by the ancient Babylonians around 2800 BC.
A recipe has been discovered on a cuneiform tablet written in Girsu 4500 years ago, around the time of the construction of the Great Pyramid of Giza. It describes its use in washing and dyeing of wool, which weavers used - and still do - to wash freshly sheared wool to remove lanolin.
The basic problem is that oil likes dirt, but it doesn't like water. You can see that because, when you add oil to water, it forms globules instead of mixing.
Dirt, on the other hand (or on your shirt), sticks very nicely to oil. To separate them, we need soap.
While you can make a crude form of soap by mixing wet ash with animal grease, it's a complicated family of products.
Their most important attribute is that they wrap oil molecules in a tiny cluster called a "micelle". The exterior of the micelle is attracted to water, while the interior dissolves grease. This gives soap the ability to disperse oil.
Because soaps are a cheap and effective emulsifier, they are used extensively to break up oil slicks such as in the Deepwater Horizon disaster.
At home, soaps attach to the fatty membranes on the outside of bacteria and viruses, lifting them off and even breaking them apart, allowing them to be washed away.
While soap is an immensely useful product, it can have an environmental impact.
Since soap is a surfactant, insects such as water striders and spiders that rely on surface tension can sink and drown.
Even at low levels, soap can destroy mucus layers that protect gills and other surfaces such as eyes and skin. If that happens, it reduces their ability to capture oxygen from the water.
Some studies have shown that concentrations of oil as low as 0.5 per cent in the water can be lethal to animals such as cockles and oysters, while fish and invertebrate eggs are also at risk.
There are quite a few types of soap. Black soap is made in West African countries such as Ghana, and goes by names such as anago, alata simena and dudu-osun.
It's made from the ash of locally harvested plants including cocoa pods or palm tree leaves, which gives it its characteristic black colour.
The first liquid soap was patented in 1865 by William Shepphard, and the first popular variety was made by Palmolive in 1898 by B.J. Johnson.
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