The monolayer behaviors of 2,4-hexadecadienoic acid (HDDA) were studied such as the dissolution of the monolayer film into the subphase, the characteristics of π-A isotherm and the area change with time at a constant surface pressure.
As HDDA has a conjugated double bond, its surface behavior differs from palmitic acid in π-A isotherms. The HDDA monolayer exibited only the expanded phase and large limiting area because of the orientation caused by the conjugation of the diene moiety to the carboxyl group. Apparently, this is attributed to the cohesion between hydrocarbon chains, as palmitic acid has stronger cohesive force than HDDA has.
The monolayer stability is also affected by the degree of ionization in hydrophilic group when pH changes. The π-A isotherms, repetition effects and area change at constant π with pH indicated that the main factor of film loss was the dissolution of HDDA. But the collapse was another factor of film loss at low temperature and pH.
Considering salt effects, $Al^{3+}$ ion improved the monolayer stability by forming a stable surface structure of the film. In the π-A isotherm, the phase transition was observed. Perhaps it is because that diene group works as hydrophobic group by lifting a conjugated double bond chain from the water surface. However $Ba^{2+}$ did not show any effects, and $Cd^{2+}$ accelerated the dissolution of monolayer into the subphase. Therefore the salt effects on the monolayer should be considered in terms of not only ionic value but also chemical characteristic of salt molecule.
In summary, the film loss by the dissolution and collapse, which shows compensative effects, could be reduced by the conditions of optimum temperature and pH. In addition, the using of good salts could play an important role in increasing the stabilities of monolayers.
