Fractionation of polydisperse systems: multi-phase coexistence

The width of the distribution of species in a polydisperse system is employed in a small-variable expansion, to obtain a well-controlled and compact scheme by which to calculate phase equilibria in multi-phase systems. General and universal relations are derived, which determine the partitioning of the fluid components among the phases. The analysis applies to mixtures of arbitrarily many slightly-polydisperse components. An explicit solution is approximated for hard spheres.Comment: 4 pages, 1 figur

Universal law of fractionation for slightly polydisperse systems

By perturbing about a general monodisperse system, we provide a complete description of two-phase equilibria in any system which is slightly polydisperse in some property (e.g., particle size, charge, etc.). We derive a universal law of fractionation which is corroborated by comprehensive experiments on a model colloid-polymer mixture. We furthermore predict that phase separation is an effective method of reducing polydispersity only for systems with a skewed distribution of the polydisperse property

Polydisperse star polymer solutions

We analyze the effect of polydispersity in the arm number on the effective interactions, structural correlations and the 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.Comment: 14 pages, 7 figure

Phase transitions in simple fluids: an application of a one-phase entropic criterion to Lennard-Jones and point Yukawa fluids

A recently proposed entropic criterion [P.V. Giaquinta and G. Guinta, Physica A 187, 145 (1992)] for the determination of phase transitions in simple fluids is applied to two-fluid models, a purely repulsive point Yukawa fluid, and a 6-12 Lennard-Jones system. Both the gas-liquid and the freezing transitions are investigated by means of integral equation theory, and assessed with simulation data available in the literature. Our results indicate that the entropic criterion is a reasonable tool for predicting the freezing transition at low temperatures, in particular for purely repulsive potentials. Comparison with other melting rules is less favorable when there is an important attractive component in the interaction. On the other hand, the determination of the gas-liquid critical point and the liquid side of the gas-liquid coexistence curve is merely qualitative. Our results, however, show the existence of a correlation between the gas-liquid transition and the location of one of the inflection points of the density-dependent excess residual entropy