Fatty alcohol monolayers at the air-water interface : a spin one model

An anomalous behavior is observed in the transitions at very low molecular area (≤ 20 Å2/molecule) for fatty alcohol monolayers spread on aqueous substrate. We propose a spin model treated with a mean field Bragg-Williams approximation incorporating the possibility of modifications in the aqueous substrate in the vicinity of the monolayer when the temperature changes. This model allows us to give a good qualitative representation of the experimentally observed isotherm crossings which are not as yet well understood and often simply considered as an experimental artifact. The temperature evolution of the alcohol monolayer is explained in terms of hydrogen bonds with water molecules at the interface whose strength depends upon temperature