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We examine in full generality the phase behavior of systems whose constituent particles interact by means of potentials that do not diverge at the origin, are free of attractive parts, and decay fast enough to zero as the interparticle separation r goes to infinity. By employing a mean field-density functional theory which is shown to become exact at high temperatures and/or densities, we establish a criterion that determines whether a given system will freeze at all temperatures or it will display reentrant melting and an upper freezing temperature.

We analyze the effect of polydispersity in the arm number on the effective interactions, structural correlations, and phase behavior of star polymers in a good solvent. The effective interaction potential between two star polymers with different arm numbers is derived using scaling theory. The resulting expression is tested against monomer-resolved molecular dynamics simulations. We find that the theoretical pair potential is in agreement with the simulation data in a much wider polydispersity range than other proposed potentials. We then use this pair potential as an input in a many-body theory to investigate polydispersity effects on the structural correlations and the phase diagram of dense star polymer solutions. In particular, we find that a polydispersity of 10%, which is typical in experimental samples, does not significantly alter previous findings for the phase diagram of monodisperse solutions.

Penetrable spheres have been the object of recent extensive investigations as a prototype for intermicellar interactions in a solvent, and as representing a class of bounded potentials allowing complete interpenetrability of the particles. Here we compare density-functional and simulation results for the pair-correlation functions in a bulk fluid of penetrable spheres, as a stringent test for the approximation of "universality" of the bridge functional. Considering either a fundamental-measure functional for penetrable spheres or a perturbative treatment using a fundamental-measure hard-sphere functional, we conclude that hard-sphere-type bridge functionals are applicable also for bounded potentials with high penetrability.

Theoretical calculations for colloidal charge-stabilized suspensions and hard-sphere suspensions show that hydrodynamic interactions yield a qualitatively different particle concentration dependence of the short-time self-diffusion coefficient. The effect, however, is numerically small and hardly accessible by conventional light-scattering experiments. By applying multiple-scattering decorrelation equipment and a careful data analysis we show that the theoretical prediction for charged particles is in agreement with our experimental results from aqueous polystyrene latex suspensions.

We report on calculations of the reduced sedimentation velocity U/U0 in homogenous suspensions of strongly and weakly charged colloidal spheres as a function of particle volume fraction φ. For dilute suspensions of strongly charged spheres at low salinity, U/U0 is well represented by the parametric form 1 - pφalpha with a fractional exponent alpha = 13 and a parameter p approximately 1.8, which is essentially independent from the macroion charge Z. This nonlinear volume fraction dependence can be quantitatively understood in terms of a model of effective hard spheres with φ-dependent diameter. For weakly charged spheres in a deionized solvent, we show that the exponent alpha can be equal to 12, if an expression for U/U0 given by Petsev and Denkov (1992, J. Colloid Interface Sci. 149, 329) is employed. We further show that the range of validity of this expression is limited to very small values of φ and Z, which are probably not accessible in sedimentation experiments. The presented results might also hold for other systems such as spherical proteins or ionic micelles. Copyright 1999 Academic Press.