A simple model for ordering in adsorbed layers

Abstract

Order-disorder phase transitions in adsorbed and confined fluids with directional interactions are studied using lattice density functional theory. A new model is developed that is capable of predicting both order-disorder and condensation phase transitions. For systems with weak interactions, the results of this model are compared with both lattice Monte Carlo simulation data and simple isotherm theories that are commonly used to fit experimental data. The results show that these simple isotherms are incapable of duplicating complex behaviour exhibited by anisotropic molecules. When sufficiently strong interactions are present, confined fluids with directional interactions may spontaneously form ordered structures. It is shown that the ordering predicted by this model can result in the assembly of long chains. Such ordering has been observed experimentally in magnetorheological fluids in a magnetic field. It is shown that the orientation of the chains predicted by this model can be controlled by adjusting the molecule-surface interaction. It may be possible to create nanoscale devices that exploit this type of molecular switching.