We extend the generalized Smoluchowski equation to descrbe the diffusional relaxation of position and orientation in a suspension of interacting spherical colloid particles. Considering a tracer particle which interacts with other particles through spherically symmetric pair potentials and with an external field we obtain a cluster expansion representation of the orientational time correlation functions for the tracer. The one and two body cluster contributions are studied explicitly at short times. Working to first order in volume fraction φ we show that the initial slope of the time correlation functions is described by a modified diffusion coefficient Dr = Dr0(1 −Crφ) where Cr is a number determined by hydrodynamic and potential interactions. We evaluate Cr numerically for spheres with slip-stick hydrodynamic boundary conditions and also for permeable spheres.
