Hard Water and the Mechanism of Cleansing Action

Soaps and synthetic detergents clean dirty, greasy surfaces by exactly the same underlying mechanism, even though their chemical structures differ. Dirt sits on a surface held there by a film of oily matter, and because oil does not dissolve in water, plain water alone cannot wash it away. Every soap and detergent molecule has two chemically distinct parts: a polar part (the 'head'), which is hydrophilic and dissolves readily in a polar solvent like water, and a long nonpolar carbon-chain part (the 'tail'), which is hydrophobic and dissolves readily in oil, fat or grease instead. When such molecules encounter an oily dirt droplet, they arrange themselves around it so that every nonpolar tail points inward, dissolving into the oily droplet, while every polar head points outward, into the surrounding water -- this arrangement, shown in Fig. 16.2, is called a micelle. Because the micelle's entire outer surface now consists of polar heads facing the water, the whole dirt-loaded micelle is compatible with water and disperses into it, and is carried away during washing/rinsing -- exactly the cleaning action that plain water on its own could not achieve. A related, separate problem is that soap does not work well in hard water: the calcium and magnesium salts dissolved in hard water react with dissolved soap (2 R-COONa (aq) + CaCl2 (aq) -> (R-COO)2Ca (s) + 2 NaCl (aq)) to precipitate an insoluble calcium/magnesium salt of the fatty acid -- a scum -- which sticks to fabric rather than forming the mic …