Theory of the effect of unsaturation on the main-chain transition.

We calculate the temperature of the main transition of lipids as a function of chain length for saturated lipids, and as a function of location of a cis double bond for lipids with one or two monounsaturated chains. We employ an extension of Marcelja's self-consistent field method which contains only two interaction parameters, and include many more chain orientations. Agreement with experiment on both the variation with chain length and with location of the double bond is satisfactory.