4 - Monolayer materials

Summary

Fatty acids and related compounds

A simple long-chain fatty acid such as n-octadecanoic acid (stearic acid) consists of a linear chain (C_nH_2n+1) — an alkyl chain — terminating in a carboxylic acid group (COOH). The polar acid head confers water solubility while the hydrocarbon chain prevents it (section 2.2). It is the balance between these two opposing forces that results in the formation of an insoluble monolayer at the air/water interface. Any change in the nature of either the alkyl chain or the polar end group will affect the monolayer properties.

The solubility of fatty acids in water decreases as the length of the alkyl chain is increased. To obtain an insoluble monolayer of a nonionized fatty acid (i.e., the situation at sufficiently low pH values), the molecule must contain at least 12 carbon atoms. For example, n-dodecanoic acid (lauric acid — C₁₁H₂₃COOH) forms a slightly soluble gaseous monolayer at low temperatures. The addition of two more carbon atoms, to form n-tetradecanoic acid (myristic acid), causes the gas phase to condense at low surface pressures and an expanded monolayer phase to be formed (Stenhagen, 1955). If this monolayer is held at a surface pressure of 10 mM m^-1 and a temperature of 20°C, then the loss in monolayer area due to solubility in the water subphase is 0.1% min^-1.