Question

Draw a saturated fatty acid salt that could participate in micelle formation. Indicate the hydrophilic head and the hydrophobic tail. Which end will be on the interior of the micelle and which end will be on the exterior of the micelle?

Short answer

In a micelle, the hydrophilic head is on the exterior, and the hydrophobic tail is on the interior.

Step-by-step solution

Understanding Saturated Fatty Acid Salt

A saturated fatty acid salt consists of a carboxylate group (the hydrophilic head) and a long hydrocarbon chain (the hydrophobic tail). The salt form indicates that the carboxylic acid group has lost a proton (H⁺), leaving a negative charge that is often balanced by a sodium (Na⁺) or potassium (K⁺) ion.

Drawing the Structure

Draw a long straight line representing the hydrocarbon chain with no double bonds for the hydrophobic tail. At one end, draw a carboxylate group (COO⁻) for the hydrophilic head and include a counterion like Na⁺ to make it a salt.

Assigning Hydrophilic and Hydrophobic Ends

Label the carboxylate group (COO⁻) as the hydrophilic head, and the hydrocarbon chain as the hydrophobic tail. The hydrophilic head is attracted to water, while the hydrophobic tail repels water.

Arranging in a Micelle

In a micelle formation, the hydrophobic tails aggregate together, pointing inward, away from water. The hydrophilic heads face outward toward the water, forming the exterior surface of the micelle and allowing interaction with the aqueous environment.

Key concepts

Saturated Fatty Acid Salt

Saturated fatty acid salts are important in the formation of structures like micelles.
They consist of two main components: a carboxylate group and a long hydrocarbon chain.
The carboxylate group is the functional part derived from a carboxylic acid. In its salt form, it has lost a proton to become negatively charged. This negative charge is typically balanced by a positively charged counter ion, like sodium (Na⁺) or potassium (K⁺).

• The carboxylate group is also referred to as the hydrophilic head, due to its affinity for water.
• The hydrocarbon chain is saturated, meaning it contains only single bonds, which results in a straight structure.
• The straight structure leads to the term 'tail,' due to its long, chain-like appearance and its tendency to repel water (hydrophobic).

Hydrophilic Head

The hydrophilic head of a saturated fatty acid salt is the carboxylate group.
This part of the molecule is vital for its ability to form micelles. The term 'hydrophilic' means 'water-loving'.

• The carboxylate group can interact with water molecules through ionic and hydrogen bonding.
• This interaction is crucial because in micelle formation, these heads face outward towards the surrounding water.

These interactions ensure that the micelle remains stable and can be suspended or dissolved in water. The negative charge of the carboxylate group attracts the positive ends of water molecules, helping the formation of a structured outer layer around the micelle.

Hydrophobic Tail

The hydrophobic tail is the long hydrocarbon chain found in saturated fatty acid salts.
'Hydrophobic' means 'water-fearing,' indicating it repels water.

• In a micelle, the hydrophobic tails cluster together in the center, shielded from water by the hydrophilic heads.
• This arrangement minimizes their exposure to water, which is energetically favorable.

The tail's length and saturation (lack of double bonds) contribute to the stable structure of the micelle. The clustering of these tails forms the core of the micelle, away from the aqueous environment, creating a non-polar center where other non-polar substances can be trapped and transported.