PHASE-TRANSITIONS IN MOLECULAR FLUIDS - THE RESTRICTED ORIENTATION MODEL

The structure of a confined fluid of hard circular cylinders confined to a slit is studied using a simple theory in the context of a restric ted orientation model; previous work is extended to higher densities. It is shown that, as the density increases, the fluid within the centr al region of the slit undergoes phase transitions which depend on the ratio, l/d, of the length to the diameter of the cylinders, and these transitions are interpreted in terms of the bulk behaviour of the flui d. When l/d is 0.25, the bulk fluid first undergoes a first-order isot ropic-nematic transition as the density increases and then makes a tra nsition to a spatially ordered phase. On the other hand when l/d is 0. 8, the fluid undergoes a transition to a spatially ordered phase and d oes not pass through a nematic phase at all. These results are qualita tively in agreement with results obtained by Frenkel and Mulder from t he simulation of hard ellipsoids. The limitations of the model are dis cussed and suggestions made for modifications to improve upon the quan titative predictions of the model at high densities.