A theoretical study of phase transitions in bidimensional monolayers

We propose a theoretical study of phase transitions in long hydrocarbon chain monolayers, lying on aqueous substrate. We use a model taken from that applied by Di Marzio to liquid crystals: each molecule can occupy several aligned sites on the holder (1 to 5 for us). We also take into account the attractive interactions between the hydrocarbon chains in the framework of a mean field approximation: it gives a first order phase transition in a diagram showing surface pressure variations in terms of the molecular density, noted (π, ρ) diagram. We also introduce an order–disorder transition and we show that it is either of the first order, or of the second order, according to the physical conditions. The (π, ρ) isotherms are in good qualitative agreement with experimental results.