A molecule with two ends

Section 3 of 4

A soap molecule has two very different ends: a charged head that dissolves in water, and a long oily tail that mixes with grease.

Stir sodium palmitate into water and it splits into two ions. The sodium ion, Na⁺, floats off on its own. The other is the palmitate ion: palmitic acid's chain of 16 carbons, with a -⁠COO⁻ group at one end.

A soap molecule: a water-loving head and an oil-loving tail A palmitate ion drawn as a skeletal formula. At the left end, a carbon atom carries a double-bonded oxygen and a negatively charged oxygen, O-. This charged end is shaded red and labelled water-loving head. From it a straight zigzag chain of 15 more carbon atoms runs to the right, shaded yellow and labelled oil-loving tail. A sodium ion, Na+, sits apart from it. Below, the same molecule drawn as a symbol: a red circle with a minus sign for the head, and a zigzag line for the tail. O − O Na⁺ water-loving head oil-loving tail as a symbol: head tail
The palmitate ion: a charged -⁠COO⁻ head (shaded red) on a tail of 15 more carbons (shaded yellow). In water the sodium ion floats free. Below, the symbol for a soap molecule used in section 4.

The head carries a whole negative charge, much bigger than the small partial charges of a polar bond. The positive hydrogen ends of water molecules crowd round it, so the head dissolves well in water. Chemists call it hydrophilic, from the Greek for water-loving.

The tail is a hydrocarbon chain, non-polar like the chains in fats and oils, so it mixes with grease. Water squeezes it out, the way it squeezes out grease, so chemists call it hydrophobic: water-fearing. The two ends belong to one molecule, so wherever the head goes, the tail goes too.

The two ends of a palmitate ion
PartAtomsMixes with
charged head-⁠COO⁻water
non-polar tailC₁₅H₃₁grease

Acid, base and salt

Palmitic acid behaves as an acid: its -⁠COOH group can give away H⁺. Hydroxide ions pick the H⁺ up and make water. So palmitic acid and sodium hydroxide make sodium palmitate and water, and a soap is a salt of a fatty acid. Soap in water has a pH of about 9 to 10, a little on the basic side.

Quick check

Which part of a soap molecule mixes with grease?