It is difficult to wash an oil spot out of clothing with plain water, because oil is a hydrocarbon that does not dissolve in water.   Oil and water actually repel one another, so that oil adheres even more strongly to clothing in the presence of water.  The addition of soap or detergent to water changes the situation; soapy water can dissolve oil from clothing and rinse it away.   What is special about the structure of soaps that makes them effective cleaning agents for oils and greases?

Most soaps are soluble sodium or potassium salts of carboxylic acids.  The most common commercial soap is sodium stearate, Na[C₁₇H₃₅CO₂].    It dissolves in water, forming the sodium and stearate ions.  Even though most of the stearate ion is a hydrocarbon chain, it dissolves in water because of the carboxylate group.  The carboxylate end is called hydrophilic (water-loving), and the hydrocarbon tail is called hydrophobic (water-fearing).

It is the long hydrocarbon chains of the stearate anions that dissolve the oils and greases.  If water containing dissolved soap is mixed with oil, the hydrocarbon chains strongly attract the oil, while the ionic ends keep the soap dissolved into water.    The oil spot is broken up into small droplets and dispersed into the water.    The "tails" of many soap anions are needed to remove each oil droplet.

While the sodium salt of stearate ions and the anions of other soaps are soluble in water, the calcium and magnesium salts are not.  Hard water contains these metal cations, so the metal salts precipitate, reducing the oil-dissolving efficiency of the soap.  "Bathtub ring" originates from the precipitation of soap by hard water.  Thus, soaps do not clean well in hard water until most of the metal cations have been precipitated by reacting with the soap.  In recent years, this problem has been solved by replacing soaps with detergents, which are generally compounds with long hydrophobic tails and the charged sulfate group such as sodium dodecyl sulfate, Na[CH₃(CH₂)₁₁OSO₃].    The calcium and magnesium salts of detergents generally remain soluble in water.

Reger/Goode/Mercer:  Chemistry Principles and Practice,  2/e,  p. 995