Strongly interacting binary mixtures of superparamagnetic colloidal particles confined to a twodimensional water-air interface are examined by theory, computer simulation and experiment. The mixture exhibits a partial clustering in equilibrium: in the voids of the matrix of unclustered big particles, the small particles form subclusters with a sponge-like topology which is accompanied by a characteristic small-wave vector peak in the small-small structure factor. This partial clustering is a general phenomenon occurring for strongly coupled negatively non-additive mixtures.

Abstract. We discuss recent developments and present new findings in the colloidal description of soft polymeric macromolecular aggregates. For various macromolecular architectures, such as linear chains, star polymers, dendrimers and polyelectrolyte stars, the effective interactions between suitably chosen coordinates are shown to be ultrasoft, i.e., they either remain finite or diverge very slowly at zero separation. As a consequence, the fluid phases have unusual characteristics, including anomalous pair correlations and mean-field like thermodynamic behaviour. The solid phases can exhibit exotic, strongly anisotropic as well as open crystal structures. For example, the diamond and the A15-phase are shown to be stable at sufficiently high concentrations. Reentrant melting and clustering transitions are additional features displayed by such systems, resulting in phase diagrams with a very rich topology. We emphasise that many of these effects are fundamentally different from the usual archetypal hard sphere paradigm. Instead, we propose that these fluids fall into the class of mean-field fluids.

Polyelectrolyte brushes are essential in many aspects of surface functionality, particularly for colloidal stabilization and lubrication in biological and materials science applications. It has been shown experimentally that the brushes undergo an abrupt shrinkage in the presence of multivalent counter‐ions. This transition is studied here using a phenomenological mean‐field approach with a model that specifically includes bridging of the polyelectrolyte chains by the multiple charges on the multivalent counter‐ions. Using an energy balance represented by the sum of electrostatic, polymeric and entropic mean‐field terms, additional parameterized phenomenological terms are introduced for counter‐ion condensation and for the attractive interaction between adjacent polyelectrolyte chains to account for the bridging effect. The free energy is minimized with respect to the counter‐ion populations and the brush height. In agreement with experimental observations, increasing the concentration of multivalent ions leads to a sharp collapse of the polyelectrolyte brush height. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 284–291

Suspensions of super-paramagnetic colloids trapped in the water-air interface are an excellent model system for the study of two-dimensional systems. Here, we investigate the structural behaviour of two-component mixtures of such particles. The interparticle interactions are tunable through the application of a magnetic field perpendicular to the air-water surface. Further, they can be influenced by the choice of the relative magnetic susceptibilities of the two colloidal species. We have performed integral equation theory and computer simulations to study the static properties of such a binary superparamagnetic system. For all susceptibility ratios studied, no macroscopic phase separation takes place; the fluid phase remains macroscopically homogeneous but microphase structuring occurs: the interactions with the large particles lead to a clustering of the smaller particles. We combine structural information in reciprocal space together with morphological measures in real space to characterize the ordering of the two species in the binary mixture.

Using the recently developed effective interaction potentials between polyelectrolyte stars, we examine the structure and phase behavior of solutions of the same. The effective interaction is ultrasoft and density dependent, owing to the integration of the counterionic degrees of freedom. The latter contribute extensive volume terms that must be taken into account in drawing the phase diagram of the system. The structural behavior of the uniform fluid is characterized by anomalous structure factors, akin to those found previously for solutions of uncharged star polymers. The phase diagram of the system is very rich, featuring a fluid phase at low arm numbers of the stars, two reentrant melting regions, as well as a variety of crystal structures with unusual symmetry. The physical origin of these features can be traced back to the ultrasoft nature of the effective interaction potential.