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+
Here R represents the hydrocarbon chain. The carboxylate ion, bearing a negative charge, is more polar than the undissociated acid. RCOOH can be converted completely to the ion RCOO− by adding an equal number of molecules of a base such as sodium hydroxide (NaOH). This effectively replaces the H+ with Na+ to give the salt of the fatty acid, which is a soap. The very useful detergent property of soaps stems from the fact that the RCOO− anions in water spontaneously form stable, spherical aggregates called micelles. The interior of these structures, formed by the hydrocarbon chains, is an excellent solvent in which grease and hydrophobic dirt of all sorts can be sequestered. The diameter of each micelle is roughly twice the length of the extended fatty acid. Dispersions of micelles in water can be made quite concentrated and exhibit great cleansing power. These dispersions are stable and generally look very much like pure water. Bubbles and foams on the surface of soap dispersions are the result of the spontaneous adsorption of RCOO− ions at the interface between the aqueous dispersion and air, with the result that the air-water interfaces are energetically stabilized and can therefore be mechanically expanded.
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