When mixtures of hard-sphere-like colloids do not behave as mixtures of hard spheres

Abstract

The validity of the concept of "hard-sphere-like" particles for mixtures of colloids is questioned from a theoretical point of view. This concerns the class of pseudobinary mixtures in which the nonsteric interactions between the colloids are "residual" (with very small range and moderate strength). It is shown that contrary to common expectation, such interactions may have unexpected consequences on the theoretical phase diagram. The distinction between this situation and true solute-solvent mixtures is emphasized.

Large attractive depletion interactions in soft repulsive–sphere binary mixtures

“…However, in contrast with the results in Ref. 14 , where attractive interactions were considered and depletion interactions of varying nature (attractive or even repulsive) were obtained, in the case of repulsive direct interactions the depletion potentials are always attractive and, at high fluid densities, greatly enhanced with respect to those obtained for HS mixtures. In the light of these results, and considering that in some cases HS mixtures exhibit a metastable fluid-fluid phase separation, one may wonder whether enhanced attractive depletion forces may change the nature of demixing phenomena in mixtures.…”

“…By confronting Eqns. (12) and (11)(12)(13)(14) together, and by exploiting the results just presented, one can conclude that Asakura-Oosawa theory can be used to construct accurate total effective potentials up to density ρ * 2 ≃ 0.05; the inclusion of the quadratic term is necessary for densities in the range 0.05 ρ * 2 0.40, while for larger densities at least the cubic term is to be used. Once we have assessed the density range where the virial expansion is valid, we now turn to explore the dependence of the depletion and total effective potentials obtained from simulation with respect to density, the degree of softness and the temperature.…”

“…For instance, slight degrees of non-additivity in binary HS systems result in depletion potentials which are very different from those calculated for the corresponding additive binary mixtures 13 . Also, residual interactions of short range and moderate strength, such as very short-ranged attractive Yukawa tails, which prima facie would appear irrelevant compared to the dominant hard-body terms, have been shown to significantly affect phase behavior 14 , tending to make the effective interactions less attractive. Moreover, the importance of carefully taking into account the colloidalcolloidal direct interactions has been stressed 15 : the total effective potential is the sum of the direct and indirect (depletion) contributions, and clearly the final shape depends on very fine details of both.…”

Free-energy landscape for cage breaking of three hard disks

“…For any purely soft core potential, the observed discontinuities will disappear. For a potential more representative of colloidal suspensions, such as a hard core potential with short ranged repulsion [64,65], we expect the essence of our results to be valid, but in need of some modification. For example, a short range repulsion characterized by a length δ will not significantly affect the free energy when average particle separations are larger than δ.…”

Self-assembly and crystallisation of indented colloids at a planar wall

“…In the simulations, the size ratio between colloid and depletant is q = 0.125, larger than the experimental case; the volume fractions of depletant used in simulation are also lower. This indicates that the depletion interaction in the experiments is weaker than that predicted by the idealised model used in the simulations, which we attribute primarily to nonideal colloid-depletant interactions [27,28] and (for the indented colloids) an indentation that does not match the perfect spherical shape used in the simulations. However, the qualitative features of the experiments are well captured by the simulations.…”