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 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 sub-phase 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) 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.

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► Langmuir films of stearic acid can be stabilized by increasing salt concentration and/or pH of the sub-phase.
► 3D solid phase is formed under acidic conditions.
► Dissolution of molecules into sub-phase occurs only at high pH and low salt concentration.
► The influence of multicomponent salt is a combination of ionic strength and specific ion effects.