Y. Mao , P. Bladon , H. N. W. Lekkerkerker and M.
Cavendish Laboratory, Madingley Road,
Cambridge, CB3 OHE, UK.
Department of Physics and Astronomy,
University of Edinburgh, JCMB King's Buildings,
Mayfield Road, Edinburgh EH9 3JZ, UK.
Van't Hoff Laboratory, University of Utrecht,
Padualaan 8, 3584 Utrecht, The Netherlands.
We study the density profile of mutually avoiding rodlike particles in the space between two parallel plates, held in equilibrium with a bulk phase of isotropic, semidilute rods. We employ a self-consistent integral equation, which becomes exact as the rod aspect ratio . We also use computer simulation to study finite aspect ratio systems. We then use an ``extended Gibbs adsorption isotherm" to express the free energy (as a function of plate separation) in terms of an integral of the surface excess with respect to chemical potential. This allows thermodynamic properties, such as surface tension and the depletion force between plates, to be found. For , the results confirm both the thermodynamic consistency of the integral equation, and the accuracy of our previous work on the depletion force (which was based on calculating only the contact density of rods at the walls). To extract thermodynamic data from simulations, the same Gibbs isotherm method is very efficient, as it utilizes the statistics of the full density profile rather than just the contact density. This allows precise thermodynamic results for confined rod systems to be obtained from simulation for the first time. Those for L/D = 10,20 are shown to be already quite close to the predictions for infinite aspect ratio.