Phase Separation Behavior of (Palmitic Acid/Lignoceric Acid) Mixed Monolayer Based on a π-A Isotherm Measurement and a Scanning Probe Microscopic Observation

The aggregation state of a mixed monolayer of palmitic acid (C16) and lignoceric acid (C24) was investigated on the basis of a π-A isotherm measurement and a scanning probe microscopic observation. The independence of the plateau and collapse pressures on the monolayer composition and the additivity of the average molecular area exhibited that the C16 and C24 molecules were immiscible in the monolayer, indirectly suggesting the presence of phase-separated domains in the system. Two kinds of regions with different heights were apparently observed on an atomic force microscope image of the mixed monolayers. The change in the area fraction with the composition, the film thickness and the difference in the lateral force of the two regions revealed that the (C16/C24) mixed monolayer was in a phase-separated state. This phase separation is probably due to an enthalpic contribution based on the difference in cohesive energy of each alkyl chain.