Physical properties of fatty acids

The main physical properties of fatty acids are solubility and melting point, which are determined by the length of their hydrocarbon chains and by their degree of unsaturation.

• Solubility.
• Melting point.

Solubility

Although fatty acids with 4 or 6 carbons are soluble in water, those with 8 carbons or more are practically insoluble, since their carboxyl group (-COOH) ionizes very little and therefore its hydrophilic pole is very weak.

The fatty acids are amphipathic behavior (from the Greek amphi, "both", and ducks, "affection, passion"), ie have:

• The head, a hydrophilic polar zone (the carboxylic end, -COOH) that interacts with water (H₂O).
• The tail, a hydrophobic apolar zone (the hydrocarbon chain) insoluble in water because it does not have polar groups. The longer the chain, the more insoluble it will be in water and the more soluble it will be in polar solvents.

The carboxyl group is linked by hydrogen bonds with other polar molecules, while the hydrocarbon chain is linked by Van der Waals forces with the other chains of other fatty acids. This is the reason why fatty acids, and the lipids formed by them, in water form closed micelles, monolayers and bilayers, the non-polar part being avoided by water.

When the fatty acids come into contact with water, the fatty acids orient the polar heads (-COOH) towards the water, while the nonpolar tails are placed at the opposite end, forming these structures:

• The micelles are formed in aqueous medium. In them, the hydrophobic tails are inwards while the polar heads are on the surface, in contact with the water. They can be considered as a tiny drop of lipid delimited by polar groups in contact with water. Due to the size of the solute, micellar solutions are colloidal solutions.

Micellar solutions are called emulsions, and the molecules that can form them are called emulsifiers or detergents. Non-water soluble lipids are trapped inside the micelle, and the micelle can be carried away by the solution. It is the so-called detergent effect.

• The monolayers are formed in the air-water interface. The hydrophobic tails face the air, while the polar heads face the water.
• In living beings, amphipathic lipids called phospholipids form bilayers, which are very important because they form the basis of membrane structures. A bilayer can be considered as two superimposed monolayers, joined by their hydrophobic zones. The hydrophilic part of the phospholipids of the bilayer flanks the hydrophobic zone on both sides, and prevents its contact with the aqueous medium.

By LadyofHats (File:Phospholipids_aqueous_solution_structures.svg) [Public domain], via Wikimedia Commons

Melting point

Whether a fatty acid is in a liquid or solid state depends on its melting point. Thus, at room temperature, low-melting fatty acids are liquid and high-melting fatty acids are solid.

Fatty acid molecules tend to clump together because hydrogen bonds are established between carboxyl groups and Van der Waals bonds are formed in lipophilic stretches of hydrocarbon chains. If they are in a solid state, to melt them you have to break these bonds to separate their molecules.

• In saturated fatty acids, the higher the number of carbons, the more heat energy is needed to break the Van der Waals bonds that join the chains, so they will have higher melting points.
• In unsaturated fatty acids, as the presence of double and triple bonds makes the linear chains present "elbows", the packing is less, and they are linked by fewer Van der Waals bonds, so it will take less energy to break them, and they will have lower melting points.

In summary, the melting point of fatty acids increases with the length of the chain, since there are a greater number of Van der Waals bonds with other chains. The presence of double bonds creates elbows that lower the melting point by reducing the number of bonds with other chains.