Fatty acid monolayer dissociation and collapse: Effect of pH and cations

Surface pressure versus specific molecular area ( π– A) isotherms of stearic acid monolayers have been studied by varying the pH of the aqueous subphase containing monovalent (Na) or different divalent (Mg, Ca, Ba, Zn and Cd) cations. The lowering of the tilted to untilted phase transition pressure ( π t) with increasing pH of the subphase is correlated with an increase of the headgroup dissociation of the acid monolayer. On pure water, the surface pressure drops after the collapse point ( π c) to a lower pressure value ( π l). The pressure-gap Δ π = π c − π l changes with increasing dissociation, depending upon the particular cation. After a certain pH, no pressure gap is observed and π remains constant or increases slowly after π c. This transition from constant area collapse to constant pressure collapse of the monolayer is observed for all monovalent and divalent cations near total monolayer dissociation.