Phase transitions in fatty acid monolayers containing a single double bond in the fatty acid tail

Abstract

The effect on π- A isotherms of introducing a single double bond ( trans) into the tail of a fatty acid has been studied. When the double bond is introduced at the 11 position the monolayer becomes liquid expanded. Moving the trans double bond to the 6 position (nearer the carboxyl head group) in the fatty acid tail yields monolayers that show a sharp discontinuity in the slope of the π- A isotherms. A careful study of the equilibrium state of the monolayer was made. It was found that above the equilibrium spreading pressure ( π e) the film is unstable while below π e the film is stable. The surface pressure for the discontinuity in the π- A curve ( π t ) is compared with π e. When π t > π e the sharp discontinuity in the slope of the π- A curve disappears. Both solvent spread and crystal spread films were examined.