Molecular dynamics simulation of hard-disc mixtures

The equation of state has been determined for several binary mixtures of hard discs by molecular dynamics simulation. Excess compressibility factors are compared with the predictions of first-order perturbation theory and their counterparts in one and three dimensions. For a non-additive mixture of discs of the same size (Δ12=0·2) good agreement is obtained with either a pure fluid or mixture reference. For mixtures of discs of different sizes, a two-dimensional ‘one-fluid’ van der Waals approximation underestimates the pressure. THe structure of the mixed two-dimensional fluid, as indicated by the radial distribution functions, is significantly different from that of the pure hard-disc fluid at the same density. First-order perturbation theory gives moderate agreement with the simulated distribution functions in a non-additive mixture of discs of the same size, and is useful in predicting the phase behaviour of the symmetric ternary mixture. The application of the hard-disc-mixture model to monolayers at liquid and solid surfaces is briefly discussed.