Observation of mixed fatty acid monolayer at the air-water interface using phase contrast microscopy

We observed stearic and lignoceric acids, and their mixed fatty acid monolayers at the air-water interface in situ using phase contrast microscopy. Tt was found that these monolayers exhibit quite different cohesion processes depending on the surface pressure. For stearic acid, the domains of the monolayer on the subphase have smooth surfaces and change their shapes to cover free areas with increasing surface pressure. In contrast; the domains of lignoceric acid divide into a number of smaller domains to cover free areas as surface pressure increased. We also found that the domains of lignoceric acid monolayers have internal substructures (i.e., triangular and inhomogeneous). For the mixed fatty acid monolayers, we clearly observed that some domains have the same internal substructures as those of single component monolayers. These facts demonstrate that a phase contrast microscope is a powerful tool for investigating the cohesion process of fatty acid monolayer sat the air-water interface without the need for any probe material.