soap
Summary
A soap is the sodium or potassium salt of a long-chain fatty acid, made by base hydrolysis (saponification) of triglycerides. Its charged head and nonpolar tail make it amphipathic, so it forms micelles that trap grease in a nonpolar core and carry it away in water.
Detail
Meaning: A soap is an amphipathic molecule, RCOO⁻ Na⁺ (or K⁺). The carboxylate head is charged and hydrophilic. The long hydrocarbon tail is hydrophobic. Above a threshold concentration in water, soap molecules form micelles, with tails pointing inward and heads facing the water. Nonpolar grease dissolves in the tail core, so it can be rinsed away. Bile salts emulsify dietary fat in a similar way.
Equation: triglyceride + 3 NaOH → glycerol + 3 RCOO⁻Na⁺. Here R is the fatty acid hydrocarbon chain and NaOH is the strong base. Amounts are in mol. This reaction is saponification, the base-promoted hydrolysis of ester bonds.
On the MCAT: Expect questions that ask you to name the products of saponification, label the head and tail, or draw a micelle cross-section. You may also predict what happens when conditions change. Adding acid protonates the carboxylate to a neutral fatty acid, which is much less soluble and a poor cleaner. In hard water, Ca²⁺ and Mg²⁺ form insoluble salts with soap, which appear as scum.
Common confusion: Students often say soap forms a bilayer. Single-tailed soaps usually form micelles, while two-tailed phospholipids usually form bilayers. Also, saponification is effectively irreversible because the carboxylate is deprotonated in base. Acid-catalyzed ester hydrolysis is reversible.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.