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 m onolayers at the air-water interface in situ using phase contrast micr oscopy. Tt was found that these monolayers exhibit quite different coh esion processes depending on the surface pressure. For stearic acid, t he domains of the monolayer on the subphase have smooth surfaces and c hange their shapes to cover free areas with increasing surface pressur e. 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 intern al substructures (i.e., triangular and inhomogeneous). For the mixed f atty acid monolayers, we clearly observed that some domains have the s ame internal substructures as those of single component monolayers. Th ese facts demonstrate that a phase contrast microscope is a powerful t ool for investigating the cohesion process of fatty acid monolayer sat the air-water interface without the need for any probe material.