Stability of stearic acid monolayers on artificial sea water

We studied the formation and stability of Stearic Acid (SA) based films on aqueous sub-phases via Langmuir trough and imaging ellipsometry experiments. The aqueous phase was based on Artificial Sea Water (ASW), a multicomponent salt solution with a total molarity of 0.53. The composition of this 10 solution was varied via dilution (1, 10 and 100 times) and adjustment of the pH (3, 7, 10). Also water sub-phases without the ASW salts were studied. Pressure-area isotherms of the SA monolayers show an enhanced stability of the film against fracture when the pH and/or the salt concentration of the subphase are increased. Isobars of SA measured below the pressure needed for film fracture, indicate distinct mechanisms for loss of material: 1) at low pH and salt concentration, three dimensional (3D) 15 structures are formed at the air/water interface via nucleation and growth, and 2) at high pH and salt concentration, diffusion-controlled dissolution of molecules in the sub-phase occurs. The formation of multilayer structures was corroborated with ellipsometry images of the films after Langmuir-Blodgett transfer. 20