NEUTRON REFLECTION STUDIES OF SPREAD MONOLAYERS OF DOCOSANOIC ACID AND PENTADECANOIC ACID ON WATER

Neutron reflection measurements have been used to study the structure of spread monolayers of pentadecanoic acid and docosanoic acid on aqueous subphases. A detailed theory of reflection from a patchy film has been set out and used to interpret the results. Subphase contrast variation has been used to demonstrate the heterogeneous nature of the film below the plateau of the pentadecanoic acid isotherm. The results also suggest that the prominent first-order transition involves an increase in film thickness, possibly due to loss of cis bonds as the area per molecule decreases. For docosanoic acid monolayer to trilayer collapse has been directly observed in the spread film. The film thickness and area per molecule have been accurately determined in the L2 region of the isotherm. The temporal evolution of the island size at high areas per molecules has been studied.