ORGANIZED COLLAPSE OF FATTY-ACID MONOLAYERS

We observe that films of some fatty acids, including stearic acid and heptacosa-10,12-diynoic acid, display a ''spike'' in their compression isotherms, followed by a trough of constant surface pressure, as they are compressed beyond monolayers. Studies of pH variations show that this behavior occurs only when all carboxylate groups in the monolayer are neutral. We observe ''spikes'' for C-16, C18, and C20 alkanoic acids, but not for tetracosanoic acid or alkenoic acids. We interpret the isotherms as follows. The ''spike'' is due to buckling of the monolayer, followed by a cooperative folding over to generate an organized trilayer. Growth of the trilayer is facile and semireversible once the first folding occurs, but further compression of a fully-formed trilayer leads to irreversible collapse into three-dimensional structures.