GRAND-CANONICAL ENSEMBLE MONTE-CARLO SIMULATIONS OF CONFINED NEMATIC GAY-BERNE FILMS

In grand canonical ensemble Monte Carlo (GCEMC) simulations, we invest igated the microscopic structure of liquid-crystalline films confined between two plane parallel solid surfaces (i.e. walls) consisting of N -s discrete, rigidly fixed atoms distributed across the plane of a wal l according to the (100) structure of the face-centered cubic lattice. Parameters of the film-wall interaction potential are chosen such tha t a homeotropic alignment of film molecules is favored. In the simulat ions the thermodynamic state of the film is determined by the temperat ure T, the chemical potential mu, the distance between the walls s(z), and the film-wall interfacial area A. Thermodynamic states of the fil m are chosen such that a corresponding bulk liquid crystal is nematic. These films are referred to as 'nematic' to emphasize the physical na ture of the bulk phase in thermodynamic equilibrium with the film. To simulate nematic phases in GCEMC we modified the classic Gay-Berne pot ential for the interaction between a pair of film molecules, so that t he isotropic-nematic phase transition in the bulk occurs at sufficient ly low densities. Variations of the microscopic structure with increas ing st are correlated with the normal component of the stress tensor T -zz (s(z)). Our results show that molecules in inner portions of the f ilm undergo a reorganization from an originally planar orientation of their symmetry axes to a perpendicular one with respect to the plane o f a wall. This orientational change is manifested as a periodic sequen ce of shoulders in T-zz(s(z)) where the periodicity length Delta s(z) is close to the larger diameter of the ellipsoidal film molecules. (C) 1998 Elsevier Science S.A. All rights reserved.