Monolayer Isotherms on Water Surfaces

Lipids with a suitable hydrophilic-lipophilic balance (HLB) are known to spread on the surface of water to form monolayer films. It is obvious that if the lipid-like molecule is highly soluble in water, it will disappear into the bulk phase (as observed for SDS). Thus, the criteria for a monolayer formation are that it exhibits very low solubility in water. The alkyl part of the lipid points away from the water surface. The polar group is attracted to the water molecules and is inside this phase at the surface. This means that the solid crystal, when placed on the surface of water, is in equilibrium with the him spread on the surface. A detailed analysis of this equilibrium has been given in the literature (Gaines, 1966 Adamson and Gast, 1997 Birdi, 2009). The thermodynamics allows one to obtain extensive physical data on this system. It is thus apparent that, by studying only one monolayer of the substance, the effect of temperature can be very evident. 

The most fascinating characteristic some amphiphile molecules exhibit is that, when mixed with water, they form self-assembly structures. This was already discussed in Chapter 2 on micelle formation. Since most of the biological lipids also exhibit self-assembly structure formation, this subject has been given much attention in the literature (Birdi, 1999). Lipid monolayer studies thus provide a very useful method to obtain information about SAM formation, both concerning technical systems and cell bilayer structures. 

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