  Partial pair-correlation functions of colloidal suspensions with continuous
polydispersity can be challenging to characterize from optical microscopy or
computer simulation data due to inadequate sampling. As a result, it is common
to adopt an effective one-component description of the structure that ignores
the differences between particle types. Unfortunately, whether this kind of
simplified description preserves or averages out information important for
understanding the behavior of the fluid depends on the degree of polydispersity
and can be difficult to assess, especially when the corresponding
multicomponent description of the pair correlations is unavailable for
comparison. Here, we present a computer simulation study that examines the
implications of adopting an effective one-component structural description of a
polydisperse fluid. The square-well model that we investigate mimics key
aspects of the experimental behavior of suspended colloids with short-range,
polymer-mediated attractions. To characterize the partial pair-correlation
functions and thermodynamic excess entropy of this system, we introduce a Monte
Carlo sampling strategy appropriate for fluids with a large number of
pseudo-components. The data from our simulations at high particle
concentrations, as well as exact theoretical results for dilute systems, show
how qualitatively different trends between structural order and particle
attractions emerge from the multicomponent and effective one-component
treatments, even with systems characterized by moderate polydispersity. We
examine consequences of these differences for excess-entropy based scalings of
shear viscosity, and we discuss how use of the multicomponent treatment reveals
similarities between the corresponding dynamic scaling behaviors of attractive
colloids and liquid water that the effective one-component analysis does not
capture.
