First of every – this is a correct observation, together this problem does without doubt exist: washing plastic dishes from fat, in contrast to glass plates, is an ext tedious and also often needs several to wash cycles. In fact, even several together washes might not suffice come rid the plastic of the "oily" feeling, together if it"s still extended by a thin layer that fat.
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In order to understand why this happens one should understand exactly how soaps (or detergents/surface active agents) work. Generally speaking, soaps eliminate dirt and also fats by make them dissolve in water.
For a product to dissolve it have to share specific chemical properties through the solvent, particularly with regard come its polarity (separation of electrical charge): polar compounds have tendency to dissolve in polar solvents (for example common table salt, i m sorry is a very polar compound composed of ions, or fee atoms, is very soluble in water, i beg your pardon is also very polar). Opposing is additionally true: non-polar link dissolve in non-polar solvents (for example facial makeup, which has mostly non-polar necessary compounds that perform not dissolve in water yet are extremely soluble in liquid paraffin, i m sorry is a mixture that non-polar compounds. So the is an extremely easy to remove makeup using liquid paraffin). For more information on this topic you re welcome refer to "What identify the solubility that materials?".
Water and also oil are an extremely different in their polarity and also are as such insoluble in every other. Soaps are unique compounds because soap molecule contain a tiny polar finish (known together the polar head) and a long non-polar tail:
Because of the two various parts that the molecule, a soap molecule is soluble in water and at the very same time can dissolve fats. The fats space attracted to the non-polar tail component of the soap while the polar head renders the whole facility (soap + fat molecules) dissolve in water.
Cleaning plastic from fat is a much harder job for soaps, due to the fact that plastics space chemically similar to fats. The main difference is the fats contain chains of around 20 carbon atom bound to every other, if in plastics these chains space longer. Similar to in the rule of solubility, comparable molecules have solid attractive forces between them. This results in a sort of tug-of-war situation between the soap and also the plastic, through the plastic trying to keep its organize on the fats and the soap trying to dissolve the in the water. This is why removing fats from plastic requires much harder work contrasted to glass or metal. Glass, i beg your pardon is covered with polar oxides that silicon, and also the metallic surface kind much weaker attractive forces with fats.
Right: the comparable molecular frameworks of soap, fat and plastic (the soap and also the plastic compete for the fat). Left: a tug of war challenge in the United says Navy (from Wikipedia).
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By the way, Teflon coating was developed based on the rule of chemistry described above. Teflon, or the generic name polytetrafluoroethylene, is chemically different from both water and oil. It is coated v fluorine atom which room chemically inert, and also that is why practically nothing sticks to it.