Hard rods in a cylindrical pore: the nematic-to-smectic phase transition.

The effect of cylindrical confinement on the phase behaviour of a system of parallel hard rods is studied using Onsager's second-virial theory. The hard rods are represented as hard cylinders of diameter D and length L, while the cylindrical pore is infinite with diameter W. The interaction between the wall and the rods is hard repulsive, and it is assumed that molecules are parallel to the surface of the pore (planar anchoring). In very narrow pores (D < W < 2D), the structure is homogeneous and the system behaves as a one-dimensional Tonks gas. For wider pores, inhomogeneous fluid structures emerge because of the lowering of the average excluded volume due to the wall-particle interaction. The bulk nematic-smectic A phase transition is replaced by a transition between inhomogeneous nematic and smectic A phases. The smectic is destabilized with respect to the nematic for decreasing pore width; this effect becomes substantial for W < 10D. For W > 100D, results for bulk and confined fluids agree well due to the short range effect of the wall (∼ 3-4D).