Collapse of Langmuir monolayer formed by the mixture of short- and long-tailed fatty acid molecules

Abstract Surface pressure versus molecular area (π-A) isotherms of stearic acid (SA), behenic acid (BA) and their mixtures are studied at high subphase pH (≈ 9.5) in presence of Ba2+ ions. It is found that the collapse pressure (πC) and dip pressure (πD) points of binary fatty acid monolayer varies depending on the relative amount of the two components and nearly follow the Langmuir growth model. Out-of-plane structures obtained from the X-ray reflectivity and atomic force microscopy analysis imply that SA shows multi-layered structure that gradually decreases with the decrease in SA proportion in the mixed film. The transition from constant pressure to constant area collapse is observed when the weightage of BA becomes ≥66% for the mixed film. The change in the collapse behaviour and collapsed structure is explained using the two-dimensional percolated networks formed by the tail-tail hydrophobic interactions of relatively more hydrophobic long-chained BA molecules.