Physical properties

Pure fatty acids form crystals that consist of stacked layers of molecules, with each layer the thickness of two extended molecules. The molecules in a layer are arranged so that the hydrophobic (water-fearing) hydrocarbon chains form the interior of the layer and the hydrophilic (water-loving) carboxylic acid groups form the two faces. For a specific fatty acid the details of the molecular packing may vary, giving rise to different crystal forms known as polymorphs.

The melting temperatures of saturated fatty acids of biological interest are above 27 °C (81 °F) and rise with increasing length of the hydrocarbon chain. Monounsaturated and polyunsaturated molecules melt at substantially lower temperatures than do their saturated analogs, with the lowest melting temperatures occurring when the carbon-carbon double bonds are located near the centre of the hydrocarbon chain, as they are in most biological molecules. As a result, these molecules form viscous liquids at room temperature.

The hydrophobic character of the hydrocarbon chain of most biological fatty acids exceeds the hydrophilic nature of the carboxylic acid group, making the water solubility of these molecules very low. For example, at 25 °C (77 °F) the solubility in grams of fatty acid per gram of solution is 3 × 10⁻⁶. Water solubility decreases exponentially with the addition of each carbon atom to the hydrocarbon chain. This relationship reflects the energy required to transfer the molecule from a pure hydrocarbon solvent to water. With each CH₂ group, for instance, more energy is required to order water molecules around the hydrocarbon chain of the fatty acid, which results in the hydrophobic effect.

In pure water the carboxylate group can dissociate a positively charged hydrogen ion to only a very small degree thus:R―COOH → RCOO⁻ + H⁺.