Monte Carlo Computer Simulation of Spreading Pressure Isotherms of Langmuir Monolayers of Fatty Acid Molecules

We describe an off-lattice model with chemical group resolution for investigating the spreading pressure-area isotherms of Langmuir monolayers of fatty-acid molecules at air–water interfaces. It is shown that a balance of the attractive interactions between the methylene chains and longer-range repulsive interactions between the headgroups determines the form of the isotherms. The model reproduces the experimentally observed dependence of the isotherms on the chain length and unsaturation. At 300 K model palmitic acid chains (C16:0) are shown to form liquid-condensed monolayers at all spreading pressures, while the isotherms of monolayers of myristic acid (C14:0) exhibit a liquid-condensed to liquid-expanded transition in agreement with experiments. Moreover, the simulations show that the introduction of cis-unsaturated segments into the 7–8 positions of the C14 chains depresses the phase transition temperature, so that the monolayers undertake a liquid-expanded structure.