  We use the Enskog theory for the description of the self-diffusion
coefficient of hard sphere fluids in disordered porous media. Using the scaled
particle theory previously developed by us for the description of thermodynamic
properties of hard sphere fluids, simple analytical expressions for the contact
values of the fluid-fluid and fluid-matrix pair distribution functions are
obtained and used as the input of Enskog theory. The expressions obtained for
the contact values are described only by the geometric porosity and do not
include the dependence on other types of porosity that are important for the
description of thermodynamic properties. It is shown that the application of
such contact values neglects the effects of trapping of fluid particles by a
matrix and at least the probe particle porosity $\phi$ should be included in
the Enskog theory for a correct description of the matrix influence. In this
paper we extend the Enskog theory by changing the contact values of the
fluid-matrix and the fluid-fluid pair distribution functions with new
properties which include the dependence not only on geometric porosity but also
on probe particle porosity $\phi$. It is shown that such semi-empirical
improvement of the Enskog theory corresponds to SPT2b1 approximation for the
description of thermodynamic properties and it predicts correct trends for the
influence of porous media on the diffusion coefficient of a hard sphere fluid
in disordered porous media. Good agreement with computer simulations is
illustrated. The effects of fluid density, fluid to matrix sphere size ratio,
matrix porosity and matrix morphology on the self-diffusion coefficient of hard
sphere fluids are discussed.
