| Abstract: | A statistical mechanical theory is presented for the behavior of monolayers of aliphatic carboxylic acids at the air/water interface. A lattice fluid model is introduced with monomer states to represent molecules in the all-trans conformation directed perpendicular to the interface, dimer states to represent rotational isomers, and vacant sites or “holes.” Using a Flory–Huggins-type approximation, the temperature and chain length dependences of the monolayer behavior are satisfactorily reproduced for a certain range of parameters. These parameters are compared with those intermolecular energies calculated using the methods of computational quantum chemistry. The agreement thus obtained is fairly satisfactory indicating that the higher-order nature of fatty acid monolayer transitions is due to the onset of the rotational ordering of pairs of chains about their longitudinal axes, while the sensitive temperature dependence is attributed to the flexible nature of fatty acid molecules. Some possible effects due to the presence of the substrate are discussed. |
