General behaviour of lipids in water

From a biological standpoint an essential property of lipid molecules is their ability to form aqueous phases, possessing long-range order combined with disorder at molecular distances. A variety of different phases exist for a particular lipid (polymorphism), and a small change in solution conditions is sufficient to cause a transformation from one form or structure to another. 

It is natural to classify lipids as polar or non-polar according to their interaction with water. Non-polar lipids, for example triglyceride oils, do not form aqueous phases, whereas polar lipids do. Except for cholesterol, membrane-forming lipids form aqueous phases and have polar head groups. Within membranes there are also trace amounts of lipids in membranes that do not interact with water, for example diacylglycerols. The structural formulae of two common membrane lipids, phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are shown above. 

The physical properties of polar lipids, such as phospholipids, glycolipids or monoglycerides, are directly related to their association.behaviour. 

Liquid crystalline phases form more commonly in the presence of water (these solvated phases are called lyotropic ). Above a critical hydrocarbon chain melting temperature, water penetrates the polar region, and a lamellar 

The first ideas on the nature of liquid crystals in hpids were derived from X-ray studies by Luzzati [1]. A crucial discovery was his demonstration of the liquid character of the hydrocarbon chains, which are thus space-filling. This was evident after it was foimd that the Upid bilayer thickness decreases with temperature with a large linear thermal coefficient about 10 3/°C. Such an effect is consistent only with a highly disordered chain conformation. Also the X-ray scattering characteristics were found to be very similar to those of liquid paraffins. 

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