Stearate anion

Much of what we call greasy dirt is nonpolar. Grease, for example, consists mostly of long-chain hydrocarbons. However, water, the solvent most commonly available to us, is very polar and does not dissolve "greasy dirt." We need to add something to the water that is somehow compatible with both the polar water and the nonpolar grease. Fatty-acid anions are perfect for this role because they have a long nonpolar tail and a polar head. For example, the stearate anion can be represented as... 

The stearate ion has both a hydrophilic—COj" head and a long organophilic CH3(CH2)i6— tail. As a result, stearate anions in water tend to form clusters consisting of as many as 100 anions clustered together, with their hydrocarbon tails on the inside of a spherical colloidal particle and their ionic heads on the surface in contact with water and with Na+ counter-ions. This results in the formation of micelles, as illustrated in Figure 7.12. 

The structure of (a) sodium stearate and (b) an electrostatic potential map of the stearate anion that illustrates the polar nature of the carboxylate ion and nonpolar nature of the hydrocarbon chain. 

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