Finite-size scaling and particle-size cutoff effects in phase-separating polydisperse fluids

We study the liquid-vapor phase behavior of a polydisperse fluid using grand canonical simulations and moment free energy calculations. The strongly nonlinear variation of the fractional volume of liquid across the coexistence region prevents naive extrapolation from detecting the cloud point. We describe a finite-size scaling method which, nevertheless, permits accurate determination of cloud points from simulations of a single system size. By varying a particle-size cutoff, we find that the cloud point density is highly sensitive to the presence of rare large particles; this could affect the reproducibility of experimentally measured phase behavior in colloids and polymers.